kvitt/SPEC.md

Kvitt: specification

Version 0.1 (2026-10-17). Schema version 1. Sync protocol version 1. Push payload version 1.

This file is the single source of truth for Kvitt. It is written for implementation agents that have no other context. If code, a plan or CLAUDE.md disagrees with this file, this file is right. Words in code are literal identifiers, paths, enum values or error codes and must be used exactly as written. Anything not decided here is listed in §12 as TODO(spec); do not invent a value for it.

Conventions used throughout:

• "Owner" is Jacob, the admin user. "Partner" is the other user. "User" is either of the two.
• "Öre" is 1/100 SEK. All money is an integer number of öre.
• "bp" is basis points. 10000 bp is 100 %.
• Timestamps are RFC 3339 UTC strings in exactly this form: 2026-10-17T06:00:00.000Z (millisecond precision, literal Z). This form sorts correctly as a string.
• Dates without time are ISO 8601 YYYY-MM-DD. Months ("periods") are YYYY-MM. ISO weeks are YYYY-Www with the ISO week-numbering year (week starts Monday).
• Ids are UUIDv7, lowercase, hyphenated, 36 characters, stored and sent as strings.
• JSON keys are snake_case. Enum values are lowercase snake_case strings.
• "Must", "must not" and "may" are binding.

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1. Overview & goals

Kvitt is a shared-expense app for one household of exactly two people living in Sweden: the owner and his spouse. It replaces Splitwise for them. It tracks who paid for what, who owes whom, and how the household spends money over time.

It consists of:

• A native Android app (Kotlin, Jetpack Compose) on two Samsung phones (Galaxy S26 and Galaxy S26+, Android 16). The app keeps a full copy of the household's data in a local Room database and works fully offline for expenses and settlements.
• A small Rust server (axum, SQLite) in Docker on a Raspberry Pi 3 at the owner's home, reachable at https://icehack.asuscomm.com:8443. The server stores the authoritative copy, syncs the phones, creates recurring expenses, sends push notifications, keeps an audit log and hosts the APK for in-app updates.

Goals for v1:

1. G-1 Correct money. Balances are exact to the öre and computed identically on phone and server, guaranteed by shared test vectors.
2. G-2 Offline first. Creating, editing and deleting expenses and settlements never waits for the network. Two weeks offline followed by a sync loses nothing.
3. G-3 Flexible splitting. Per-account default ratios (50/50 to start, configurable such as 60/40), custom ratios, exact amounts, or 100 % on one person.
4. G-4 Clear settling. One global balance, a per-account breakdown, and a settle-up flow that proposes how a payment is spread over accounts.
5. G-5 Insight. Monthly and weekly spending by category and by account, computed on the phone.
6. G-6 Trust. Every change to an expense or settlement is visible as a timeline of who did what.
7. G-7 Low-maintenance hosting. One docker-compose stack on a 1 GB Raspberry Pi, nightly off-site backups, no cloud services except Firebase Cloud Messaging and Google Drive for backups.
8. G-8 Bilingual. Every user-visible string exists in Swedish and English. The phone's locale decides: Swedish when the device language is Swedish, English otherwise.
9. G-9 History. Six years of Splitwise history can be imported once, repeatably without duplicates.

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2. Non-goals

Not in v1. Agents must not build any of these, even partially:

• Receipt photos or receipt scanning. Profile pictures are the only file upload.
• Any card, bank, Spendwise or other financial-institution integration.
• Multiple currencies or exchange rates. All amounts are SEK. (The importer has a one-off conversion option, §4 FR-17.)
• Joint accounts, shared cards or "paid from the shared pot" as a payer. The payer is always one of the two users.
• More than two users, more than one household, user deletion.
• Passwords, password reset, email, SMS, OAuth sign-in.
• Play Store distribution.
• CSV or any other export.
• A web client. The only HTML the server serves is the invite landing page (§4 FR-2).
• Server-side analytics or reporting endpoints.
• Debt simplification (meaningless with two users) and netting several accounts in opposite directions inside one settlement.
• Editing accounts, categories, recurring templates, profiles or invites while offline.
• Tablet, foldable or landscape-specific layouts. Portrait phone only; landscape must not crash.
• Widgets, Wear OS, iOS.
• In-app language picker. The phone locale decides.
• Comments or chat on expenses.

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3. Users & core user stories

3.1 Roles

Role	Who	Count	Extra rights
admin	The owner	exactly 1	Creates invites (partner and device), cancels invites, revokes any device
member	The partner	0 or 1	Revokes own devices other than the one in use

Everything else (expenses, settlements, accounts, categories, recurring templates) is open to both users with identical rights. Either user may edit or delete anything the other created. Each user edits only their own display name and picture.

A user has one or more devices. v1 expects one phone each, but the model allows any number of devices per user without change.

3.2 User stories

• US-1 As the owner I set up the server once from the command line and pair my phone with a one-time code.
• US-2 As the owner I invite my partner with a link; my partner installs the app from the link, picks a name and a picture, and is paired.
• US-3 As a user I add an expense in a few taps: amount, description, category, account; the split defaults to the account's ratio and the payer defaults to me.
• US-4 As a user I record an expense my partner paid, so they do not have to.
• US-5 As a user I split an expense differently: a custom ratio, exact amounts, or the whole amount on one of us.
• US-6 As a user I see one number saying who owes whom overall, and the same per account.
• US-7 As a user I settle up: I enter what I paid my partner, the app proposes which accounts it settles (oldest first), I adjust and confirm.
• US-8 As a user I settle one account from that account's screen.
• US-9 As a user I create expense accounts ("Vardag", "Huset", "Japan 2027"), give each a default ratio, and archive them when done.
• US-10 As a user I set up a recurring expense (rent, streaming) that appears automatically each month.
• US-11 As a user I see how much we spent per month or week, by category or by account, as a household total or as my share.
• US-12 As a user I add and edit expenses on a plane or abroad without a connection, and everything syncs when I am back.
• US-13 As a user I get a notification when my partner adds, changes or deletes something, and I can see in the expense who changed what.
• US-14 As a user I am told when a new app version exists and install it from inside the app.
• US-15 As the owner I import our Splitwise history once.
• US-16 As the owner I can restore everything from a backup if the Pi dies.
• US-17 As the owner I can remove a lost phone's access.

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4. Functional requirements (v1)

Each requirement has acceptance criteria (AC). An AC is satisfied only when a test (unit, integration, screenshot or instrumented) or a documented manual check in the milestone's gate demonstrates it. Limits and rules referenced as §9.x are stated only there.

FR-1 Server bootstrap

The server starts with an empty database. The household is created from the command line on the Pi.

• AC-1.1 kvitt-server bootstrap on a database with no users creates an invite of kind admin_setup, prints its code (format §8.3), its landing link https://icehack.asuscomm.com:8443/i/<CODE> (the base URL from KVITT_PUBLIC_URL), its expiry, and exits 0.
• AC-1.2 kvitt-server bootstrap on a database that has users prints already initialized to stderr and exits 2. It creates nothing.
• AC-1.3 Running kvitt-server bootstrap again before the first code is used cancels the previous admin_setup invite and prints a new one.
• AC-1.4 Accepting the admin_setup invite (FR-2) creates the admin user with the given display name and the first device, and returns a device token.
• AC-1.5 GET /setup/status returns {"initialized": false} before AC-1.4 and {"initialized": true} after.
• AC-1.6 kvitt-server pair-device --user <user_id> creates a device invite for an existing user and prints it like AC-1.1. This is the recovery path when the owner has lost every paired phone.

FR-2 Invites and pairing

• AC-2.1 The admin creates a partner invite from Settings. It is refused with household_full when two users exist. Creating a new partner invite cancels any pending one.
• AC-2.2 The admin creates a device invite for either user from Settings (to pair a second phone).
• AC-2.3 An invite is single-use and expires 72 hours after creation. An expired, cancelled, used or unknown code is refused with invite_invalid; the response does not reveal which.
• AC-2.4 Accepting an invite takes the code, a device name and (for admin_setup and partner) a display name. It returns a bearer token, the user, the device and the household's users. For a partner invite it creates the member user.
• AC-2.5 GET /i/<CODE> serves an HTML landing page in Swedish if the request's Accept-Language prefers sv, English otherwise. It shows: the app name, "Download Kvitt" linking to the latest APK (FR-16), an "Open in Kvitt" button linking to kvitt://invite?server=<urlencoded KVITT_PUBLIC_URL>&code=<CODE>, the code in groups of 4, and one sentence of instructions. It does not reveal whether the code is valid and contains no personal data.
• AC-2.6 The app opens from kvitt://invite?server=…&code=… and from https://icehack.asuscomm.com:8443/i/<CODE> (unverified link, the user may get a chooser) directly into the pairing screen with both fields filled.
• AC-2.7 The pairing screen also accepts manual entry: a server URL (prefilled with https://icehack.asuscomm.com:8443 in release builds) and the code. Codes are normalised per §8.3 before sending.
• AC-2.8 Release builds accept only https:// server URLs. Debug builds also accept http:// for localhost, 10.0.2.2 and private LAN addresses (10.*, 172.16.*-172.31.*, 192.168.*).
• AC-2.9 After pairing, the app runs a full sync (§14.4) and then asks for the notification permission (Android POST_NOTIFICATIONS).
• AC-2.10 A partner pairing continues to a profile step (display name prefilled from the accept call, optional picture) before the Home screen.
• AC-2.11 The admin sees pending invites (kind, created, expiry) in Settings and can cancel each.
• AC-2.12 More than 10 failed invite accepts from one client IP within 60 minutes yield rate_limited for the rest of that window.

FR-3 Devices and revocation

• AC-3.1 Settings lists devices: the admin sees all devices of both users; a member sees their own. Each row shows device name, owner, paired date, last seen date, app version, and "This device" for the current one.
• AC-3.2 The admin can revoke any device except the one in use (cannot_revoke_current_device). A member can revoke their own devices except the one in use.
• AC-3.3 A revoked device's next request gets HTTP 401 device_revoked. The app then deletes its token, its local database and its cached pictures, and shows the "Device removed" screen with a "Pair again" button.
• AC-3.4 A revoked device never receives pushes again.
• AC-3.5 Devices are never deleted from the database; revocation sets revoked_at.

FR-4 Profiles

• AC-4.1 Each user has a display name (limits §9.6) and an optional profile picture.
• AC-4.2 A user edits their own display name in Settings (online only). The change reaches the other phone through sync.
• AC-4.3 A user sets their own picture from the Android photo picker (no camera capture in v1) or removes it. Upload is online only.
• AC-4.4 The server accepts JPEG, PNG and WebP up to 5 MiB and at most 8000 × 8000 pixels, applies EXIF orientation, centre-crops to a square, re-encodes as JPEG quality 85 at 512 × 512 and 128 × 128, strips all metadata, and increments the user's picture_version. Other input is refused with image_invalid, unsupported_media_type or payload_too_large.
• AC-4.5 Where no picture exists, the app shows the user's initials (first letter of the first two words of the display name, uppercase) on a colour derived from the user id (UX sprint defines the two colours).
• AC-4.6 The app caches pictures on disk (Coil) keyed by user id and picture_version, so they show offline.

FR-5 Household gate

• AC-5.1 Until the partner has joined, the admin's Home screen shows "Waiting for your partner" with the "Invite partner" action, and creating accounts, expenses, settlements and recurring templates is not offered.
• AC-5.2 The server refuses account creation, recurring template creation and any expense or settlement mutation with household_incomplete while fewer than two users exist. (The rule exists because every split refers to both users.)

FR-6 Expense accounts

• AC-6.1 Either user creates an account (online) with a name and a default split ratio. The editor starts at 50/50. Ratios are entered as percent with up to two decimals per user, the second user's value always being 100 minus the first; stored as bp (§9.1).
• AC-6.2 Account names are unique among non-deleted accounts, compared case-insensitively after trimming (Unicode lowercase). A duplicate is refused with name_taken.
• AC-6.3 Either user renames an account or changes its default ratio (online). A ratio change affects only expenses created afterwards; existing expenses keep the shares they were saved with.
• AC-6.4 Either user archives or unarchives an account (online), even with a non-zero balance. Archived accounts are hidden from the account pickers for new expenses and recurring templates, shown in a collapsed "Archived" section on Home, still count toward the global balance, still appear in analytics and can still be settled.
• AC-6.5 Either user deletes an account (online) only when no non-deleted expense, no allocation of a non-deleted settlement and no non-deleted recurring template references it. Otherwise account_not_empty. Deletion is a soft delete (tombstone) so phones learn of it through sync.
• AC-6.6 Accounts are listed by created_at ascending, the order used by the settlement proposal (§9.4).
• AC-6.7 The account create and edit actions are disabled with a "Needs connection" hint when the phone is offline (§9.9).

FR-7 Categories

• AC-7.1 One global, editable category list. A category has a name, an icon token (§9.7) and a colour token (§9.7).
• AC-7.2 The database is seeded with exactly these ten default categories (fixed ids, §14.1), each shown with its localised default name until a user renames it:

default_key	English name	Swedish name	icon	color	sort_order
groceries	Groceries	Mat & hushåll	shopping_cart	green	10
restaurants	Restaurants	Restaurang & café	restaurant	orange	20
home	Home	Hem	home	brown	30
utilities	Utilities	Räkningar	bolt	yellow	40
transport	Transport	Transport	directions_car	blue	50
travel	Travel	Resor	flight	teal	60
entertainment	Entertainment	Nöje	movie	purple	70
health	Health	Hälsa	medical_services	red	80
shopping	Shopping	Shopping	shopping_bag	pink	90
other	Other	Övrigt	category	grey	100

• AC-7.3 Either user creates a category (online) with a name, icon and colour. New categories get sort_order = current maximum + 10.
• AC-7.4 Either user renames, re-icons, recolours or reorders a category (online). Renaming a default category stores the custom name; "Reset name" (only for default categories) clears it so the localised name shows again.
• AC-7.5 Either user archives or unarchives a category. Archived categories are not offered for new expenses or templates but stay on existing ones and in analytics.
• AC-7.6 A category is deletable only if no non-deleted expense or non-deleted recurring template uses it (category_in_use otherwise). Default categories (with a default_key) cannot be deleted (category_is_default); they can be archived. Deletion is a soft delete.
• AC-7.7 Category names have no uniqueness rule.

FR-8 Expenses

• AC-8.1 An expense has: account, amount (öre), description, category, date, payer, split, notes, created-by, last-modified-by, timestamps. Limits in §9.6.
• AC-8.2 Either user creates, edits or deletes any expense, online or offline. The id is a client-generated UUIDv7.
• AC-8.3 The payer is either user. The editor defaults the payer to the current user.
• AC-8.4 The date is any date within §9.6 limits, past or future. It defaults to today in the phone's time zone. This date is the only date used in analytics and lists.
• AC-8.5 Split modes (enum §9.1): default (the account's default ratio, copied into the expense when saved), ratio (a custom ratio), exact (exact öre per user), full (the whole amount borne by one chosen user, who may be the payer or not). Share computation per §9.1.
• AC-8.6 The editor defaults: account = the account last used on this phone if still active, else the oldest active account; category = the category last used in that account on this phone if still active, else other; split mode default.
• AC-8.7 Exact mode: the user types one user's share; the other user's share is the amount minus it and is shown read-only. A typed share larger than the amount blocks saving with an inline error.
• AC-8.8 Changing the account of an expense in mode default re-copies the new account's ratio. In other modes the split is kept.
• AC-8.9 Deleting asks for confirmation and is a soft delete. Deleted expenses disappear from lists, balances and analytics on both phones after sync.
• AC-8.10 Expense lists show, per row: category icon and colour, description, date, amount, and a one-line effect from the viewer's side ("You lent 205,00 kr", "You borrowed 205,00 kr", "Not involved" for full on the payer, matching §13 copy). Rows are ordered by date descending, then created_at descending.
• AC-8.11 An expense created from a recurring template is an ordinary expense in every way (editable, deletable) and shows "Recurring" with the template name in the detail view.

FR-9 Recurring expenses

• AC-9.1 Either user creates, edits, pauses, resumes or deletes a recurring template (online). A template holds the expense fields (account, amount, description, category, payer, split mode and shares, notes) plus day_of_month (1-31), start_period and an optional end_period.
• AC-9.2 The server creates one expense per template per period at 06:00 Europe/Stockholm on the template's day. Days 29, 30 and 31 clamp to the last day of shorter months (day 31 in April creates on 30 April; day 29 in February 2027 creates on 28 February).
• AC-9.3 Generation rules (catch-up after downtime, idempotency, split resolution) are in §9.5.
• AC-9.4 Editing a template changes only expenses generated afterwards.
• AC-9.5 A generated expense that a user deletes is not recreated.
• AC-9.6 Each generated expense writes an audit entry with source = recurring and sends recurring_expense_created to every device of both users.
• AC-9.7 The template list shows each template's description, amount, account, day of month, paused state and next occurrence date (computed by the server, next_occurrence_date).
• AC-9.8 The editor's default start_period is the current month if day_of_month is today or later in the phone's time zone, else the next month. The user may choose a start_period up to 12 months in the past (backfill) and any month in the future.

FR-10 Balances

• AC-10.1 Home shows one global balance between the two users from the viewer's side: " owes you X", "You owe X" or "All settled up".
• AC-10.2 Home lists every active account with its balance in the same wording, and archived accounts in a collapsed section.
• AC-10.3 The account screen shows that account's balance and its expenses and settlements interleaved by date.
• AC-10.4 Balances are computed on the phone from the local database with the domain function of §14.5, include pending (unsynced) local changes, exclude deleted rows, and update immediately after a local write.
• AC-10.5 The global balance equals the sum of the per-account balances, archived accounts included.

FR-11 Settlements

• AC-11.1 A settlement is a payment from one user to the other with an amount, a date, an optional note and one or more per-account allocations. Limits in §9.6.
• AC-11.2 The allocations' sum must equal the settlement amount. Each allocation is positive. An account appears at most once.
• AC-11.3 Global settle-up (from Home): the direction defaults to "the user who owes pays", the amount defaults to the absolute global balance (or empty when it is zero). The app computes the proposal with propose_allocation (§9.4, §14.5) and shows one editable row per proposed account plus "Add account" to add any other account (active or archived). The confirm button is enabled only when the allocations sum to the amount; the remaining difference is shown live.
• AC-11.4 Per-account settle-up (from the account screen) creates a settlement with exactly one allocation for that account; the amount defaults to the absolute account balance and the direction to "the user who owes in this account pays".
• AC-11.5 An allocation larger than the payer's current debt in that account is allowed (over-settling flips the account's sign) and is marked with a warning per row ("This settles more than is owed in "). The confirm dialog repeats the warning.
• AC-11.6 Settlements can be created, edited and deleted (soft delete) offline, by either user, like expenses. When editing, the proposal and warnings are computed with the edited settlement excluded from the balances.
• AC-11.7 A settlement row in lists shows " paid X" and the account count when more than one allocation exists.
• AC-11.8 The server's POST /settlements/proposal returns exactly what the phone's propose_allocation returns for the same server state (verified by shared vectors). The phone always computes locally; the endpoint exists for parity tests and server-side tooling.

FR-12 Audit log and timeline

• AC-12.1 The server appends an audit entry for every accepted create, update and delete of an expense or settlement, from the app, the scheduler or the importer. Entries are never updated or deleted.
• AC-12.2 An entry records entity type, entity id, action, source, actor user, actor device (null for scheduler and importer), client time of the change, server time, and a field diff (§14.4.7).
• AC-12.3 Audit entries reach both phones through sync pull.
• AC-12.4 The expense and settlement detail views show a timeline, newest last, one line per entry, such as "Anna created this · 3 Oct 14:02", "You changed the amount from 400,00 kr to 412,00 kr · 3 Oct 18:30", "Anna deleted this". Diffs show field names in the UI language; ids are resolved to names (accounts, categories, users).
• AC-12.5 A pending local change not yet synced shows as a last timeline line "Your change is waiting to sync".

FR-13 Push notifications

• AC-13.1 When the server accepts a create, update or delete of an expense or settlement from a device of user U, it sends a push to every non-revoked device of the other user that has an FCM token. Types and payload in §14.6.
• AC-13.2 Recurring generation sends to every non-revoked device of both users.
• AC-13.3 The importer sends no pushes.
• AC-13.4 One POST /sync that produces more than 3 notifiable changes sends one sync_batch push instead of one per change.
• AC-13.5 The app shows the notification only after it has synced the change, built from local data in the phone's language (title and body copy in §13). If that sync fails within 20 seconds, it shows a generic text without amounts or descriptions.
• AC-13.6 Tapping a notification opens the detail view of the changed expense or settlement, or Home for sync_batch and deletions.
• AC-13.7 The app registers its FCM token after pairing and whenever Firebase rotates it (PUT /devices/current/push-token).
• AC-13.8 No personal content (names, amounts, descriptions, notes) is placed in the FCM payload.

FR-14 Analytics

• AC-14.1 Analytics are computed on the phone from Room with the pure function of §14.5.4; the server has no reporting endpoint. They work offline and include pending local changes.
• AC-14.2 Range presets: this_month, last_3_months, last_6_months, last_12_months, this_year, all_time, custom (start and end date pickers). "Last N months" means the current month and the N-1 months before it, whole months. Default last_6_months.
• AC-14.3 Granularity: month or week (ISO, Monday start). Default month.
• AC-14.4 Measure: household (sum of full expense amounts, whoever paid) or my_share (sum of the viewer's shares). Default household.
• AC-14.5 Group by: category or account. Default category.
• AC-14.6 Charts: (1) spend over time, a stacked bar per bucket coloured by group; (2) category breakdown for the range, horizontal bars sorted by amount descending with amount and percent; (3) account breakdown for the range, same form. A total for the range is shown above the charts. Settlements are never counted as spend.
• AC-14.7 Every bucket in the range appears, empty ones as zero. Buckets at the edges of a custom range count only dates inside the range.
• AC-14.8 Tapping a bar or a breakdown row opens a list of the expenses behind that number.
• AC-14.9 Deleted expenses are excluded; archived accounts and categories are included.
• AC-14.10 Percentages are rounded half-up to one decimal and may not sum to exactly 100.

FR-15 Offline and sync

• AC-15.1 The phone holds a full local copy of: users, accounts, categories, recurring templates, expenses, settlements and audit entries (Room, §14.2).
• AC-15.2 Expenses and settlements can be created, edited and deleted offline. Accounts, categories, recurring templates, profile, pictures, invites and devices need the server; their actions are disabled offline with "Needs connection".
• AC-15.3 Sync runs on app start and every return to the foreground, 2 seconds after the last local write (debounced), every 15 minutes in the background through WorkManager (network constraint), and on every FCM data message. Only one sync runs at a time.
• AC-15.4 The app shows a "Last synced" relative time and a pending-changes count on Home, and a sync status screen (§13).
• AC-15.5 Conflicts resolve by last-write-wins per row on the client updated_at, deletes are final, and rejected changes are shown to the user (§14.4).
• AC-15.6 A phone offline for 14 days with 200 local changes syncs all of them on reconnect (integration test with a scripted clock).

FR-16 In-app update

• AC-16.1 The server hosts the latest signed release APK and its metadata (§10.6).
• AC-16.2 Release builds call GET /apk/latest on every cold start and at most once per 6 hours while resumed. When version_code is greater than the installed one, Home shows an "Update available" banner with the version name.
• AC-16.3 "Update" downloads the APK to the app's cache, verifies the size and SHA-256 from the metadata, and installs through PackageInstaller. The first time, Android asks the user to allow "Install unknown apps" for Kvitt; the app explains this beforehand.
• AC-16.4 A verification failure deletes the file and shows "Download failed, try again".
• AC-16.5 Debug builds never check for updates.

FR-17 Splitwise import

• AC-17.1 A one-off CLI kvitt-import (crate kvitt-import) imports Splitwise CSV exports, one file per Splitwise group, each into a Kvitt account named on the command line. Details §10.8.
• AC-17.2 Re-running the same import creates nothing new (stable import key, §10.8).
• AC-17.3 Splitwise categories map to Kvitt categories by the fixed table in §10.8; unknown names map to other and are listed in the summary.
• AC-17.4 Splitwise payments become settlements with one allocation.
• AC-17.5 --dry-run prints the summary (rows read, expenses, settlements, skipped duplicates, unknown categories, non-SEK rows) and writes nothing.
• AC-17.6 Imported rows produce audit entries with source = import and no pushes.

FR-18 Backups and restore

• AC-18.1 Every night at 03:30 Europe/Stockholm the backup sidecar writes a consistent SQLite snapshot and a pictures archive, gzips them and uploads them to the owner's Google Drive folder with rclone. Nothing on Drive is ever deleted or overwritten.
• AC-18.2 A restore drill following §10.9 restores a backup on the Pi and both phones then sync correctly, documented in docs/validation/restore-drill.md.

FR-19 Localisation and formatting

• AC-19.1 Every user-visible string is a resource in values/strings.xml (English, the fallback) and values-sv/strings.xml (Swedish). No string literal appears in feature code. A lint or test in the Android gate fails on a key missing from either file.
• AC-19.2 Money formatting per §9.8, dates per §9.8.
• AC-19.3 Category default names and every enum shown to users have strings in both languages.

FR-20 Health

• AC-20.1 GET /health returns status, server version, schema version and a database check, unauthenticated, in under 100 ms on the Pi.
• AC-20.2 The docker healthcheck of the server service uses it.

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5. Data model

This section explains the entities and their meaning. The exact server DDL is §14.1, the phone's Room entities §14.2, the wire JSON §14.3. Limits are in §9.6.

5.1 Entity overview

Entity	Id origin	Synced to phones	Written by phones	Notes
User	server	yes (pull)	REST only, own profile	Exactly two in a complete household
Device	server	no	REST only	One per paired app install
Invite	server	no	REST only, admin	Single-use pairing code
Account	server	yes (pull)	REST only	Expense account, like a Splitwise group
Account default share	with account	yes (inside account)	REST only	One row per user, sums to 10000 bp
Category	server (defaults seeded with fixed ids)	yes (pull)	REST only	Global list
Recurring template	server	yes (pull)	REST only	Monthly generator
Recurring template split	with template	yes (inside template)	REST only	Shares for modes ratio, exact, full
Expense	client (UUIDv7)	yes (pull and push)	sync push	Soft delete
Expense split	with expense	yes (inside expense)	sync push	One row per user
Settlement	client (UUIDv7)	yes (pull and push)	sync push	Soft delete
Settlement allocation	with settlement	yes (inside settlement)	sync push	One row per account
Audit entry	server	yes (pull)	never	Append-only
Sync mutation record	server	no	never	Idempotency of pushes
Sync sequence, meta	server	no (epoch is sent in pull)	never	Global server_version counter, sync_epoch

Local-only phone tables: sync_state and sync_rejections (§14.2).

5.2 Ownership of fields

• server_version: an integer from one global, strictly increasing sequence. Every insert or update of a synced row (users, accounts, categories, recurring templates, expenses, settlements, audit entries) takes the next value. Child rows (shares, splits, allocations) have none; a change to a child bumps its parent. Phones never set it.
• created_at, updated_at of expenses and settlements are set by the phone's clock at the time of the local write and are the last-write-wins key. For server-written rows (all others, and expenses created by the scheduler or importer) they are the server clock.
• created_by_user_id is set by the server from the authenticated device's user when it first stores the row, never taken from the client. updated_by_user_id and updated_by_device_id are set by the server on every accepted write.
• deleted_at marks a tombstone. Tombstones are kept forever on both server and phone and are synced like any other row.
• archived_at (accounts, categories) hides an item from pickers; it never affects money.

5.3 Money

• Amounts are integer öre in signed 64-bit integers (Kotlin Long, Rust i64, SQLite INTEGER).
• An expense's shares, one per user, always sum exactly to its amount.
• Ratios are integer bp per user summing to 10000.
• A user's net position in an account is: what they paid for expenses, minus their shares, plus settlement allocations they paid, minus settlement allocations they received. The two users' net positions always sum to zero. A positive net means the other user owes them that much. Formal definition §9.3.

5.4 Relationships

• An expense belongs to one account and one category, has one payer and exactly two splits (one per user), and optionally references the recurring template and period it was generated for.
• A settlement has a payer (from_user_id), a recipient (to_user_id) and one or more allocations, each to one account.
• An audit entry references one expense or settlement by type and id.
• A device belongs to one user. An invite of kind device targets one user.
• References from synced rows point only at rows that are never hard-deleted, so a reference always resolves on the server. On the phone a reference may be briefly unresolved during a pull (§14.2); the UI shows such a row only after its references resolve.

5.5 What is not stored

• No balances, totals or analytics are stored anywhere; they are always computed from rows.
• No exchange rates, no currencies (SEK is implicit).
• No passwords. Tokens and invite codes are stored only as SHA-256 hashes.

---

6. Architecture & stack

6.1 System view

 Samsung S26 (owner)            Samsung S26+ (partner)
 ┌──────────────────┐           ┌──────────────────┐
 │ Kvitt app        │           │ Kvitt app        │
 │ Compose UI       │           │                  │
 │ domain (pure)    │           │                  │
 │ Room (full copy) │           │                  │
 └───────┬──────────┘           └────────┬─────────┘
         │ HTTPS :8443 (bearer token)    │
         ▼                               ▼
   router: external 8443 ──► Kvitt Pi :8443        (80/443 go to the Home Assistant Pi)
 ┌───────────────────────── Raspberry Pi 3, docker compose ─────────────────────────┐
 │ caddy (TLS, :8443) ──► server (axum, :8080) ──► /data/kvitt.sqlite (WAL)          │
 │ cert-sync (ssh pull from HA Pi nightly, reload caddy)                             │
 │ backup (sqlite .backup + gzip nightly ──► rclone ──► Google Drive)                │
 └───────────────────────────────────────────────────────────────────────────────────┘
         │ FCM HTTP v1 (service account)          ▲ Pi pulls images
         ▼                                        │
   Firebase Cloud Messaging ──► phones      owner's M4 Mac: build linux/arm64 images ──► local registry

6.2 Android app

Concern	Choice	Why
Language	Kotlin 2.x, latest stable	Owner's requirement
UI	Jetpack Compose, Material 3; Material 3 Expressive components where they are stable	Native, modern, screenshot-testable
Min / target SDK	minSdk 36 (Android 16); targetSdk and compileSdk the latest stable API at implementation time	Both phones run Android 16; no legacy paths
Local DB	Room with KSP, exported schema	Full offline copy; migrations with tests
DI	Hilt	Standard, works with WorkManager and ViewModels
HTTP	Ktor client (OkHttp engine), kotlinx.serialization JSON	Kotlin-first, multiplatform-style DTOs
Background	WorkManager (periodic 15 min sync, expedited sync)	Survives process death, respects Doze
Dates	kotlinx-datetime	Allowed in pure modules (no java.time)
Images	Coil (Compose) with an auth header interceptor	Disk cache for offline avatars
Charts	Vico (Compose). If Vico's current release cannot draw a stacked bar chart with Material 3 theming, hand-roll the three charts with Compose Canvas in :core:design	Mature Compose chart library; fallback is small
Push	Firebase Cloud Messaging, only inside :adapter:fcm	Free tier; vendor isolated per CLAUDE.md
Secrets	Android Keystore: an AES-256-GCM key (non-exportable) encrypts the device token; ciphertext in a private DataStore file	EncryptedSharedPreferences is deprecated
Build	Gradle with version catalog android/gradle/libs.versions.toml, Gradle daemon on JDK 25 (pinned), bytecode target Java 17	Existing pin
Tests	JUnit, Compose screenshot tests (Roborazzi or the Android Gradle Plugin's Compose Preview Screenshot Testing, whichever is stable at implementation time; record the choice in decisions), Room migration tests, Ktor MockEngine	No mocking libraries (CLAUDE.md)

Module graph (directory android/):

Module	Kind	Contents	May depend on
:core:model	pure Kotlin/JVM	Domain types (§14.5.1), *State classes, repository interfaces (§14.2.4)	kotlin, kotlinx
:domain	pure Kotlin/JVM	Split, balance, allocation, analytics functions (§14.5); test-vector tests	:core:model
:core:design	Android library	Theme, tokens, typography, shared components (money text, avatar, category chip, balance header, charts), MoneyFormatter, DateFormatter	:core:model
:core:fakes	Android library, used only by tests, screenshot tests and debug previews	Scenario-driven fakes of every repository interface (they call :domain for computed values so fakes never disagree with the real rules)	:core:model, :domain
:data	Android library	Room DB, DAOs, entities, Ktor API client and DTOs, sync engine, WorkManager workers, token store, repository implementations, APK updater	:core:model, :domain
:adapter:fcm	Android library	FirebaseMessagingService, token registration, maps messages to PushEvent (:core:model)	:core:model
:feature:onboarding	Android library	Pairing, partner profile step, waiting for partner, device removed	:core:model, :core:design
:feature:home	Android library	Home (global balance, accounts), sync status, update banner	:core:model, :core:design
:feature:account	Android library	Account detail, account editor	:core:model, :core:design
:feature:expense	Android library	Expense editor, expense detail and timeline	:core:model, :core:design
:feature:settle	Android library	Settle up (global and per account), settlement detail and timeline	:core:model, :core:design
:feature:analytics	Android library	Analytics screen and drill-down list	:core:model, :core:design
:feature:settings	Android library	Settings hub, profile, categories, recurring templates, devices, invites, about	:core:model, :core:design
:app	Android application	MainActivity, navigation graph, Hilt wiring, deep links, notification channel and building, Application class	everything

Features never call :domain directly: repositories return computed state (balances, previews, proposals), implemented in :data with :domain. The architecture check (Gradle task in check, owned by milestone M2b) fails the build on any other dependency edge and on java.time, java.io or android.* imports in :core:model and :domain.

6.3 Server

Concern	Choice	Why
Language	Rust stable, edition 2024	Owner's requirement; small RSS on a 1 GB Pi
HTTP	axum on tokio (multi-thread runtime, 2 worker threads)	Mature, small
DB	SQLite via sqlx, WAL mode, migrations embedded with sqlx::migrate!	One file, easy backup, ample for two users
Serialization	serde, serde_json	Standard
OpenAPI	utoipa generates the document from code; contracts/openapi.yaml is the committed contract; a test asserts the generated document equals it (§14.3)	One contract, checked by the gate
Logging	tracing, tracing-subscriber, JSON lines to stdout	Docker log rotation; no personal content (§8.7)
Time	chrono with chrono-tz for Europe/Stockholm	DST-correct scheduling
Ids	uuid crate, v7	Same format as phones
Images	image crate (decode JPEG/PNG/WebP, encode JPEG)	Pure Rust, no system libs
HTTP client (FCM)	reqwest with rustls	No OpenSSL on the image
FCM auth	Service-account JWT (RS256, jsonwebtoken crate) exchanged at https://oauth2.googleapis.com/token for an access token with scope https://www.googleapis.com/auth/firebase.messaging, cached until 5 minutes before expiry	HTTP v1 API needs OAuth2
Scheduler	A tokio task inside kvitt-server (no extra crate)	One process, simple

Cargo workspace (directory server/):

Crate	Kind	Contents
kvitt-domain	library, no I/O	Money types, split, balance, allocation, settlement validation (§14.5), test-vector tests. Depends only on serde, serde_json (tests) and uuid
kvitt-server	binary kvitt-server	Config, DB, migrations, HTTP API, auth, sync, audit, push, scheduler, pictures, APK, CLI subcommands (§10.4)
kvitt-import	binary kvitt-import	Splitwise CSV importer (§10.8). Depends on kvitt-domain and reuses kvitt-server as a library for DB access and audit writing (kvitt-server exposes a lib.rs)

6.4 Why these choices

• Phone computes balances and analytics. Offline is a hard requirement and the data set is small (six years of two-person history is a few thousand rows). Computing on the phone keeps the server dumb and the Pi idle.
• SQLite over Postgres. Postgres alone would use a large share of the Pi's 1 GB. SQLite in WAL mode handles two users trivially and backs up as one file.
• Global server_version sequence. One integer cursor per phone is the simplest correct incremental sync. SQLite's single writer, with every write in a BEGIN IMMEDIATE transaction, makes versions commit in order, so a reader never sees a gap that later fills.
• Last-write-wins on client time. Two people rarely edit the same expense concurrently; LWW with an audit trail lets them see and fix the rare collision. Clock-skew protection is in §14.4.
• Push carries ids only. Personal data does not pass through Google; the phone syncs and builds the text locally.
• No Play Store. Two users; a self-hosted APK with PackageInstaller is enough.
• Domain rules twice, one test suite. Kotlin and Rust implement the same pure functions and must pass the same JSON vectors, so phone and server agree to the öre.

---

7. API outline

Base URL: https://icehack.asuscomm.com:8443 (the value of KVITT_PUBLIC_URL). All paths below are relative to it. There is no path prefix and no API version in the path; the protocol version travels in a header (§14.3.1).

#	Method	Path	Auth	Purpose
E-1	GET	/health	none	Liveness and DB check
E-2	GET	/setup/status	none	Whether the household has been bootstrapped
E-3	POST	/invites/accept	none	Pair a device with an invite code (all three invite kinds)
E-4	GET	/i/{code}	none	Invite landing page (HTML)
E-5	GET	/apk/latest	none	Latest APK metadata
E-6	GET	/apk/files/{file_name}	none	APK download
E-7	GET	/me	device	Current user, device and household users
E-8	PATCH	/me	device	Change own display name
E-9	PUT	/me/picture	device	Upload own picture
E-10	DELETE	/me/picture	device	Remove own picture
E-11	GET	/users/{user_id}/picture	device	Download a user's picture
E-12	POST	/invites	admin	Create a partner or device invite
E-13	GET	/invites	admin	List pending invites
E-14	DELETE	/invites/{invite_id}	admin	Cancel an invite
E-15	GET	/devices	device	List devices (admin: all; member: own)
E-16	DELETE	/devices/{device_id}	device	Revoke a device
E-17	PUT	/devices/current/push-token	device	Set or clear this device's FCM token
E-18	POST	/accounts	device	Create account
E-19	PATCH	/accounts/{account_id}	device	Rename, change default ratio
E-20	POST	/accounts/{account_id}/archive	device	Archive
E-21	POST	/accounts/{account_id}/unarchive	device	Unarchive
E-22	DELETE	/accounts/{account_id}	device	Soft delete when empty
E-23	POST	/categories	device	Create category
E-24	PATCH	/categories/{category_id}	device	Rename, icon, colour, order, reset name
E-25	POST	/categories/{category_id}/archive	device	Archive
E-26	POST	/categories/{category_id}/unarchive	device	Unarchive
E-27	DELETE	/categories/{category_id}	device	Soft delete when unused and not default
E-28	POST	/recurring-templates	device	Create template
E-29	PATCH	/recurring-templates/{template_id}	device	Edit, pause, resume
E-30	DELETE	/recurring-templates/{template_id}	device	Soft delete
E-31	GET	/sync	device	Pull changes since a cursor
E-32	POST	/sync	device	Push local expense and settlement changes
E-33	POST	/settlements/proposal	device	Server-side allocation proposal

Reads of accounts, categories, templates, expenses, settlements and audit entries happen only through GET /sync. Every REST write that changes a synced row returns that row in its wire form (§14.3.3), with its new server_version, so the phone can store it immediately.

Request and response shapes, error codes and status codes: §14.3. The OpenAPI 3.1 file contracts/openapi.yaml is written by milestone M1 from §14.3, which is binding.

---

8. Auth & security

8.1 Threat model

Two trusted users. The server is on the public internet on port 8443. Threats in scope: random internet scanners, a stolen or lost phone, leaked invite links, a compromised backup location, accidental personal data in logs or push payloads. Out of scope: a malicious partner, an attacker with physical access to the Pi.

8.2 Device tokens

• On a successful invite accept the server generates 32 random bytes from the OS CSPRNG and returns the token kvt_ + base64url without padding of those bytes (47 characters total).
• The server stores only token_hash = lowercase hex SHA-256 of the full token string.
• Tokens do not expire. They stop working only on revocation.
• Every authenticated request sends Authorization: Bearer <token>. The server hashes it, looks up a device with that hash and revoked_at IS NULL, and loads the user. Missing or unknown token: 401 unauthorized. Revoked: 401 device_revoked.
• The server updates devices.last_seen_at and devices.app_version_code at most once per hour per device.
• On the phone the token is encrypted with an Android Keystore AES-256-GCM key (alias kvitt_token_key, setUserAuthenticationRequired(false), no export) and the ciphertext and IV are stored in DataStore file auth.preferences_pb. The server base URL, user id and device id are stored next to it in plain text.
• The token is never logged, never put in a URL, never included in a crash report or screenshot.

8.3 Invite codes

• 12 characters from the Crockford base32 alphabet 0123456789ABCDEFGHJKMNPQRSTVWXYZ (60 bits), generated from the CSPRNG.
• Displayed as three groups of four separated by -, as 7K3M-QX9D-2HPA. URLs carry the 12 characters without hyphens.
• Normalisation before hashing or sending: uppercase; remove - and spaces; replace O with 0 and I and L with 1. A normalised code that is not exactly 12 alphabet characters is invite_invalid.
• Stored as code_hash = lowercase hex SHA-256 of the normalised code.
• Single-use, 72-hour expiry, cancellable. Rate limit in AC-2.12.

8.4 Authorization rules

Action	admin	member
All expense, settlement, account, category, recurring template operations	yes	yes
Edit own display name and picture	yes	yes
Edit the other user's display name or picture	no	no
Create, list, cancel invites	yes	no (forbidden)
List devices	all	own only
Revoke a device	any except current	own except current

8.5 Transport

• TLS terminates in Caddy on port 8443 with the Let's Encrypt certificate for icehack.asuscomm.com (§10.5). TLS 1.2 minimum, Caddy defaults otherwise.
• Release builds' network security config: cleartext disabled, system trust anchors only, unless the fallback of §10.5.3 is active, in which case the pinned Caddy root replaces the system anchors for icehack.asuscomm.com only.
• Debug builds additionally allow cleartext to the hosts listed in AC-2.8.
• The server trusts X-Forwarded-For only from the Caddy container (the request's peer address on the compose network) to get the client IP for rate limiting.

8.6 Rate limiting and input limits

• Unauthenticated endpoints: at most 60 requests per minute per client IP, then 429 rate_limited with a Retry-After header in seconds.
• Authenticated endpoints: at most 600 requests per minute per device.
• Invite accept failures: AC-2.12.
• Request bodies: 1 MiB maximum, except PUT /me/picture at 5 MiB. Larger: 413 payload_too_large.
• JSON strings are validated for length after trimming leading and trailing whitespace; control characters other than newline in notes are refused with validation_failed.
• The rate limiter is in memory (resets on restart); no table.

8.7 Privacy and logging

• Server logs contain: timestamp, level, request method, route template (not the raw path with codes), status, latency, device id, error code. They never contain tokens, invite codes, display names, descriptions, notes, amounts or picture bytes.
• The phone never logs personal content, debug builds included (CLAUDE.md). Log tags use the prefix Kvitt.
• FCM payloads carry ids and types only (§14.6).
• Backups on Google Drive are not encrypted in v1; they go to the owner's own Drive folder, reachable only by his Google account.

8.8 Phone hardening

• android:allowBackup="false" and android:dataExtractionRules excluding everything, so neither Google backup nor device transfer copies the token or database.
• Exported components: only MainActivity (launcher and deep-link intent filters) and the FCM service as Firebase requires.
• On device_revoked the app wipes local data as in AC-3.3.

8.9 Server hardening

• The server container runs as uid/gid 10001, with a read-only root filesystem, /data as the only writable volume, /secrets read-only, no Linux capabilities, no-new-privileges.
• Only Caddy publishes a port (8443). The server, Caddy's admin API and the sidecars are reachable only on the internal compose network.
• SQLite PRAGMA foreign_keys = ON, journal_mode = WAL, synchronous = NORMAL, busy_timeout = 5000 on every connection.

---

9. Edge cases & business rules

9.1 Splits

Split modes:

split_mode	Meaning	Stored share_bp per user	Stored share_ore per user
default	The account's default ratio at the time of saving	copied from the account	computed by the ratio rule
ratio	A custom ratio for this expense	as entered	computed by the ratio rule
exact	Exact amounts for this expense	null	as entered
full	One user bears the whole amount	bearer 10000, other 0	bearer = amount, other 0

Ratio rule (modes default, ratio, full; exactly two users P = payer, N = non-payer):

share_ore(N) = floor(amount_ore * share_bp(N) / 10000)
share_ore(P) = amount_ore - share_ore(N)

The non-payer's exact share is rounded down, so the payer absorbs the remainder: the payer's share is 0 or 1 öre more than their exact share. Intermediate products are computed in 64-bit integers (the maximum, 1 000 000 000 × 10000, fits). Examples (also test vectors):

amount_ore	payer	bp payer / non-payer	share payer	share non-payer
1001	A	5000 / 5000	501	500
1	A	5000 / 5000	1	0
100	B	4000 / 6000	40	60
999	A	3333 / 6667	333	666
999	B	6667 / 3333	667	332
50000	A	0 / 10000	0	50000
50000	A	10000 / 0	50000	0

Exact rule: both shares are given, each 0 or more, and they must sum exactly to the amount, otherwise exact_sum_mismatch.

Validity: exactly two share entries, one for each household user, no other user ids; bp each 0 to 10000 and summing to 10000.

Server check: for every pushed expense the server recomputes the shares from split_mode, the amount, the payer and the given bp (or exact öre) with kvitt-domain and rejects the mutation with reason validation_failed if the pushed share_ore values differ. For mode default the server does not compare the bp with the account's current default (the phone may have copied an older default while offline); it only checks they sum to 10000.

9.2 Effect line from the viewer's side (lists)

For viewer V and an expense with payer P:

Case	Line
V = P and the other's share > 0	"You lent <other's share>"
V ≠ P and V's share > 0	"You borrowed <V's share>"
the non-payer's share = 0	"Not involved" when V ≠ P; "Paid for yourself" when V = P

9.3 Balances

For an account and user u, over non-deleted expenses and non-deleted settlements:

net(u) =   Σ expenses where payer = u            : amount_ore
         − Σ expenses                             : share_ore(u)
         + Σ settlement allocations where from = u : allocation.amount_ore
         − Σ settlement allocations where to = u   : allocation.amount_ore

• net(A) + net(B) = 0 always.
• The account balance shown to viewer V is net(V): positive " owes you net(V)", negative "You owe −net(V)", zero "All settled up" (account screens say "Settled up").
• The global balance is the sum over all accounts, archived included, deleted excluded (a deleted account has no non-deleted rows by AC-6.5).
• Balances include pending local changes.

9.4 Settlement allocation proposal

Inputs: payer user id, amount (öre, > 0), and for every non-deleted account: id, created_at, archived flag, and the payer's current net in it (computed with §9.3, excluding the settlement being edited if any).

Algorithm (normative; §14.5 has the signature):

1. Owing accounts = accounts where payer's net < 0. Sort by created_at ascending, then by id ascending.
2. remaining = amount. For each owing account in order while remaining > 0: a = min(remaining, −net), emit allocation (account, a), remaining −= a.
3. If remaining > 0 (the amount exceeds the total debt) and at least one owing account exists, add remaining to the last emitted allocation and report that account as over-settled.
4. If no owing account exists: the target is the oldest non-archived account (same sort); if every account is archived, the oldest account. Emit one allocation of the whole amount to it and report it as over-settled.
5. If there are no accounts: error no_accounts.

Output: ordered allocations, excess_ore (the amount added in steps 3 or 4, 0 otherwise), and over_settled_account_ids.

Consequences, stated so nobody "fixes" them:

• A global settle-up of exactly the global debt can leave individual accounts non-zero when the payer is owed in some account and owes in another (allocations are never negative). The global balance is then zero. Users settle individual accounts with per-account settle-ups in either direction.
• The proposal is only a suggestion; the user may edit any allocation, add accounts or remove rows.

Over-settle warning for a user-edited allocation list: allocation (account, a) is warned when a > max(0, −net_payer(account)).

Settlement validity (phone before saving, server on push): amount within §9.6; from_user_id ≠ to_user_id, both household users; 1 to 50 allocations; each amount_ore ≥ 1; account ids distinct and referencing existing non-deleted accounts; allocations sum to the amount. Error tokens in §14.5.3.

9.5 Recurring generation

• The scheduler runs once at server start and then daily at 06:00 Europe/Stockholm (DST-aware: always local 06:00).
• For each template with deleted_at IS NULL and paused = 0, the candidate periods are every month p with start_period ≤ p, p ≤ end_period (if set), p > last_generated_period (if set), and whose occurrence instant (the clamped date at 06:00 Europe/Stockholm) is at or before now.
• For each candidate in ascending order the server, in one transaction: creates the expense (server-generated UUIDv7 id; expense_date = clamped date; fields copied from the template; created_by_user_id and updated_by_user_id = the template's created_by_user_id; updated_by_device_id null; created_at = updated_at = server now; recurring_template_id and recurring_period set), writes its splits (mode default resolves the account's current default ratio at generation time; other modes use the template's splits, with ratio/full recomputed by the ratio rule for the template amount), writes an audit entry (create, source = recurring), sets the template's last_generated_period (bumping its server_version), then enqueues the push (AC-9.6).
• The unique index on (recurring_template_id, recurring_period) makes generation idempotent; a conflict means the period was already generated and is skipped silently.
• Pausing stops generation. Resuming does not backfill periods whose occurrence passed while paused: on resume the server sets last_generated_period to the latest period whose occurrence instant is before now, if that is later than the current value.
• Changing day_of_month or start_period never regenerates past periods.
• next_occurrence_date (in the wire form) is the clamped date of the first candidate period after last_generated_period that is ≥ start_period and ≤ end_period; null when paused, deleted or past end_period.
• A template's account may be archived (generation continues). Account deletion is blocked while a non-deleted template references it (AC-6.5).

9.6 Limits and validation

All string lengths are counted in Unicode code points after trimming. One table, used by phone validation and server validation alike:

Field	Rule	Error (REST) / reason (sync)
amount_ore (expense, settlement, template)	integer 1 to 1 000 000 000 (10 000 000,00 kr)	validation_failed
allocation amount_ore	integer ≥ 1	validation_failed
allocations per settlement	1 to 50	validation_failed
share_bp	integer 0 to 10000; the two sum to 10000	validation_failed
share_ore	integer ≥ 0; the two sum to the amount	validation_failed
expense description, template description	1 to 100 code points	validation_failed
expense notes, template notes	0 to 1000 code points; newlines allowed	validation_failed
settlement note	0 to 200 code points	validation_failed
expense_date, settlement_date	valid date, from 2000-01-01 to server today (Europe/Stockholm) + 365 days	validation_failed
account name	1 to 60 code points; unique (AC-6.2)	validation_failed, name_taken
category name	1 to 40 code points, or null for a default category	validation_failed
user display_name	1 to 40 code points	validation_failed
device name	1 to 60 code points	validation_failed
day_of_month	integer 1 to 31	validation_failed
start_period	YYYY-MM, not earlier than 12 months before the current month	validation_failed
end_period	null or YYYY-MM ≥ start_period	validation_failed
updated_at in a push	≤ server now + 10 minutes	clock_skew
mutations per POST /sync	1 to 100	batch_too_large
GET /sync limit	1 to 1000, default 500	invalid_request
picture upload	≤ 5 MiB, ≤ 8000 × 8000 px, JPEG/PNG/WebP	payload_too_large, image_invalid, unsupported_media_type

The phone input field for amounts accepts digits, one decimal separator (, or .) and at most two decimals, without sign, and converts to öre exactly (string parsing, never floating point).

9.7 Icon and colour tokens

Category icon tokens are Material Symbols names. The allowed set (exhaustive, v1):

shopping_cart, restaurant, local_cafe, home, bolt, water_drop, wifi, directions_car, train, local_gas_station, flight, hotel, luggage, movie, music_note, sports_esports, fitness_center, medical_services, medication, shopping_bag, checkroom, pets, child_care, school, card_giftcard, celebration, build, chair, subscriptions, category.

Colour tokens (exhaustive, v1) with provisional values; the UX sprint may change the hex values and must keep the token names:

Token	Light	Dark
green	#2E7D32	#81C784
orange	#E65100	#FFB74D
brown	#6D4C41	#BCAAA4
yellow	#F9A825	#FFF176
blue	#1565C0	#90CAF9
teal	#00796B	#80CBC4
purple	#6A1B9A	#CE93D8
red	#C62828	#EF9A9A
pink	#AD1457	#F48FB1
grey	#546E7A	#B0BEC5
indigo	#283593	#9FA8DA
lime	#827717	#DCE775

The server validates icon and colour against these lists (validation_failed).

9.8 Formatting

• Money, Swedish locale: 1 234,50 kr (U+00A0 no-break space as the thousands separator and before kr; comma decimal; always two decimals).
• Money, any other locale: SEK 1,234.50 (U+00A0 after SEK; comma thousands; point decimal; always two decimals).
• Money is never shown with a minus sign; direction is in words (§9.2, §9.3).
• Dates in lists: Swedish 3 okt., English 3 Oct; with year when not the current year (3 okt. 2025, 3 Oct 2025). Detail views: Swedish lördag 3 oktober 2026, English Saturday, 3 October 2026. Timeline times: 24-hour 14:02 in both languages.
• Relative "last synced": "just now" (< 1 minute), "N min ago" (< 60 minutes), "N h ago" (< 24 hours), otherwise the list date and time. Swedish: "nyss", "för N min sedan", "för N h sedan".
• Formatting lives in :core:design (MoneyFormatter, DateFormatter), never in :domain.

9.9 Online-only actions

When the phone has no validated network (Android NET_CAPABILITY_VALIDATED) or the last request failed with a network error less than 30 seconds ago, these actions are disabled with "Needs connection": create, edit, archive, unarchive, delete of accounts, categories and recurring templates; profile name and picture; invites; device revoke; in-app update. A REST call that fails with a network error shows "Could not reach the server" and changes nothing locally.

9.10 Other edge cases

Situation	Rule
Both users edit the same expense offline	Last updated_at wins; the loser's phone receives stale with the winner's row, overwrites locally and lists it under "Changes not synced" (§14.4.6). Both edits are in the audit log only if both were accepted, so only the winning edit is logged.
One user deletes, the other edits the same expense offline	Delete wins regardless of arrival order and timestamps: a pushed delete is always applied, and any non-delete write to a tombstoned row is rejected deleted (§14.4.4).
An expense is created offline in an account that was deleted meanwhile	Rejected reference_not_found; the row is removed locally and listed under "Changes not synced". Cannot happen for archived accounts (writes to archived accounts are accepted).
An expense uses a category that was archived meanwhile	Accepted.
Phone clock far in the future	Pushes rejected clock_skew; the app shows "Your phone's clock is wrong" on the sync status screen and keeps the changes pending.
Phone clock behind	Its edits may lose LWW against older real edits. Accepted risk.
Server restored from an older backup	The server's sync_epoch changes (§10.9); phones detect it and re-push everything they hold (§14.4.8), so changes made after the backup are recovered from the phones.
Partner never joins	Admin stays in the waiting state (AC-5.1).
Admin loses phone	kvitt-server pair-device --user <admin id> on the Pi (AC-1.6); then revoke the lost device from the new phone.
Both users' devices revoked	Same recovery as above for each user.
An account is renamed to a name used by a deleted account	Allowed (uniqueness ignores deleted accounts).
Recurring template's category deleted	Cannot happen: category deletion is blocked while a template uses it.
Expense amount edited in mode exact	The editor keeps the typed user's share and recomputes the other; if the typed share now exceeds the amount, saving is blocked (AC-8.7).
Settlement edited so that an allocation's account was deleted meanwhile	Cannot happen: an account with allocations cannot be deleted.
Same user on two devices edits offline	Same as two users: LWW per row.
Time zones	Dates (expense_date, settlement_date) are calendar dates chosen by the user; analytics never convert them. Only "today" defaults use the phone's zone; server validation and scheduling use Europe/Stockholm.
A day with a DST change	Scheduling uses local 06:00, which exists on every day in Europe/Stockholm.

---

10. Deployment & ops

All infrastructure files live in infra/ (ownership §14.7). Secrets live in infra/secrets/ on the Mac (git-ignored) and in /opt/kvitt/secrets/ on the Pi; neither is ever committed.

10.1 Hosts and network

Host	Role	Facts
Kvitt Pi	Runs the compose stack	Raspberry Pi 3, 1 GB RAM, 64-bit Raspberry Pi OS Lite (Debian bookworm or later), Docker Engine with the compose plugin, static LAN address. LAN address and SSH user: TODO(spec) OQ-3
Home Assistant Pi	Owns ports 80/443 and the Let's Encrypt certificate (certbot) for icehack.asuscomm.com	Reached by cert-sync over SSH. Address, SSH user and certificate paths: TODO(spec) OQ-3
Owner's M4 Mac	Builds images and APKs, runs the local registry	Apple Silicon, so linux/arm64 builds run natively in Docker
Router (ASUS)	icehack.asuscomm.com DDNS	Forwards external TCP 8443 to Kvitt Pi TCP 8443 (HUMAN step)
Local container registry	Holds Kvitt images	Address: TODO(spec) OQ-1

The Pi never builds anything. It only pulls images and runs them.

10.2 Compose stack

File infra/compose.yaml, deployed to /opt/kvitt/compose.yaml on the Pi with /opt/kvitt/.env (from infra/env.example). Services:

Service	Image	Ports / volumes	Memory limit	Healthcheck
caddy	official caddy image, pinned by version and digest	publishes 8443:8443; ./Caddyfile:/etc/caddy/Caddyfile:ro; certs:/certs:ro; caddy_data:/data	64 MiB	wget -qO- http://localhost:2019/config/ >/dev/null
server	${KVITT_REGISTRY}/kvitt/server:${KVITT_TAG}	data:/data; /opt/kvitt/secrets/fcm-service-account.json:/secrets/fcm-service-account.json:ro; no published port (listens on 8080 internally)	160 MiB (budget: RSS under 100 MB in steady state)	GET http://localhost:8080/health via the binary's own kvitt-server healthcheck subcommand (no curl in the image)
cert-sync	${KVITT_REGISTRY}/kvitt/cert-sync:${KVITT_TAG}	certs:/certs; ./Caddyfile:/etc/caddy/Caddyfile:ro; /opt/kvitt/secrets/ha_ssh_key:/secrets/ha_ssh_key:ro; /opt/kvitt/secrets/ha_known_hosts:/secrets/known_hosts:ro	32 MiB	both cert files exist and are non-empty
backup	${KVITT_REGISTRY}/kvitt/backup:${KVITT_TAG}	data:/data; backups:/backups; /opt/kvitt/secrets/rclone.conf:/config/rclone/rclone.conf:ro	96 MiB	last successful run marker newer than 26 hours, or no run yet since start

• Named volumes: data (SQLite, pictures, APKs), certs, caddy_data, backups.
• caddy depends_on cert-sync with condition: service_healthy and server with condition: service_started.
• Every service: restart: unless-stopped, logging driver json-file with max-size: 10m, max-file: 3, TZ=Europe/Stockholm.
• One internal bridge network kvitt.

.env keys (exhaustive): KVITT_REGISTRY, KVITT_TAG, HA_HOST, HA_PORT, HA_USER, HA_CERT_DIR, RCLONE_REMOTE (default gdrive:), RCLONE_FOLDER_ID.

10.3 Images, registry and build

• Script infra/build-images.sh [tag] on the Mac: docker buildx build --platform linux/arm64 --push for the three Kvitt images, tag defaults to the short git SHA of HEAD, also tags latest. Refuses to run with a dirty working tree.
• server image: multi-stage. Stage 1 rust:<stable>-bookworm builds kvitt-server and kvitt-import in release mode with --locked for aarch64-unknown-linux-gnu (native on the M4; no target-cpu=native, the Pi 3 is a Cortex-A53). Stage 2 gcr.io/distroless/cc-debian12:nonroot (arm64) with the two binaries in /usr/local/bin/, user 10001, entrypoint kvitt-server, command serve.
• cert-sync image: alpine with openssh-client, curl and a shell script infra/cert-sync/sync.sh run by crond.
• backup image: alpine with sqlite, gzip, tar, rclone and infra/backup/backup.sh run by crond.
• Script infra/deploy.sh <tag> on the Mac: copies compose.yaml and Caddyfile to the Pi over SSH, sets KVITT_TAG in /opt/kvitt/.env, runs docker compose pull && docker compose up -d, waits for server to be healthy, prints GET /health.
• The Pi's Docker daemon must trust the registry (insecure-registries if it is plain HTTP on the LAN); the infra plan writes the exact step once OQ-1 is answered.

10.4 Server configuration and CLI

Environment variables (exhaustive):

Variable	Default	Meaning
KVITT_DATA_DIR	/data	Holds kvitt.sqlite, pictures/, apk/
KVITT_BIND	0.0.0.0:8080	Listen address
KVITT_PUBLIC_URL	https://icehack.asuscomm.com:8443	Used in invite links and the landing page
KVITT_FCM_CREDENTIALS	/secrets/fcm-service-account.json	Service-account key; if the file is missing, push is disabled and a warning is logged at start
KVITT_FCM_PROJECT_ID	kvitt-1337	FCM project
KVITT_TRUSTED_PROXY	caddy	Host name whose resolved address may set X-Forwarded-For
RUST_LOG	info	tracing filter

The time zone Europe/Stockholm is a constant in code, not configuration.

Files under KVITT_DATA_DIR:

• kvitt.sqlite (+ -wal, -shm)
• pictures/<user_id>/<picture_version>-512.jpg and -128.jpg; older versions are deleted after a successful upload
• apk/latest.json and apk/kvitt-<version_code>.apk

kvitt-server subcommands (exhaustive):

Command	Effect	Exit codes
serve	Run migrations, start scheduler, push sender and HTTP server	0 on clean shutdown (SIGTERM drains in-flight requests up to 10 s)
bootstrap	FR-1	0 ok, 2 already initialized
pair-device --user <user_id>	AC-1.6	0 ok, 3 unknown user
rotate-epoch	Sets a new random sync_epoch (§10.9)	0
healthcheck	Calls http://127.0.0.1:8080/health, exit 0 if 200	0 or 1
migrate	Run migrations only	0

Run them on the Pi with docker compose exec server kvitt-server <command> (or docker compose run --rm server <command> when server is stopped).

10.5 TLS

10.5.1 Certificate sync

• The Home Assistant Pi renews the Let's Encrypt certificate for icehack.asuscomm.com (its own certbot; Kvitt does not touch it).
• cert-sync runs sync.sh at container start and daily at 04:00 Europe/Stockholm. It connects to ${HA_USER}@${HA_HOST}:${HA_PORT} with the key /secrets/ha_ssh_key and StrictHostKeyChecking=yes against /secrets/known_hosts, and copies ${HA_CERT_DIR}/fullchain.pem and ${HA_CERT_DIR}/privkey.pem into a temp directory.
• The key is dedicated to Kvitt and, where the Home Assistant SSH setup allows authorized_keys options, restricted with restrict,command="tar -C <cert dir> -cf - fullchain.pem privkey.pem" (the script then reads a tar stream instead of using scp). The infra plan records which variant the Home Assistant SSH add-on supports.
• If both files parse as PEM, the certificate's not-after is in the future, and the SHA-256 of either differs from /certs, the script moves them into /certs (mode 0640, group readable by Caddy) and reloads Caddy: curl -fsS -X POST http://caddy:2019/load -H 'Content-Type: text/caddyfile' -H 'Cache-Control: must-revalidate' --data-binary @/etc/caddy/Caddyfile.
• Any failure logs one line and keeps the existing certificate. The healthcheck stays green while valid files exist.

10.5.2 Caddyfile

{
	admin 0.0.0.0:2019
	auto_https off
}

icehack.asuscomm.com:8443 {
	tls /certs/fullchain.pem /certs/privkey.pem
	request_body {
		max_size 6MB
	}
	reverse_proxy server:8080
	log {
		output stdout
		format json
	}
}

Caddy logs must not include the Authorization header (Caddy redacts it by default; the infra plan verifies).

10.5.3 Fallback: Caddy internal CA

Used only if the certificate sync cannot be made to work. Documented in infra/README.md, not deployed by default.

• Caddyfile site block uses tls internal instead of the file pair.
• The Caddy root certificate (/data/caddy/pki/authorities/local/root.crt in caddy_data) is copied once to android/app/src/release/res/raw/kvitt_root_ca.crt. This file is a public certificate and is committed.
• The release network security config gets a domain-config for icehack.asuscomm.com whose trust anchors are only @raw/kvitt_root_ca. Switching is a build property kvitt.tls.pinnedRoot=true in android/gradle.properties.
• Caddy's internal root is valid 10 years; intermediates renew automatically.

10.6 APK hosting and release

• version_code = git rev-list --count HEAD on the commit being released; version_name = YYYY.MM.DD-<short sha> of that commit. Both are passed to Gradle as properties kvitt.versionCode and kvitt.versionName by the release script; debug builds use 1 and dev.
• The release keystore is created once by the owner (HUMAN, §11) at android/keystore/release.jks (git-ignored); its passwords and alias are in android/local.properties keys kvitt.release.storeFile, kvitt.release.storePassword, kvitt.release.keyAlias, kvitt.release.keyPassword. Losing it means the phones must uninstall and reinstall (data comes back by sync). The owner keeps a copy in their password manager.
• infra/publish-apk.sh on the Mac: builds :app:assembleRelease through tools/gradle-serial.sh, computes SHA-256 and size, copies the APK to the Pi's data volume as apk/kvitt-<version_code>.apk, writes apk/latest.json atomically (write temp, rename):

{
  "version_code": 412,
  "version_name": "2026.11.02-1a2b3c4",
  "file_name": "kvitt-412.apk",
  "size_bytes": 18234567,
  "sha256": "9f2c…64 hex chars…",
  "released_at": "2026-11-02T19:40:00.000Z"
}

• The server reads latest.json on each GET /apk/latest (no cache) and serves GET /apk/files/{file_name} only for a file_name matching ^kvitt-[0-9]+\.apk$ that exists, with Content-Type: application/vnd.android.package-archive.
• Older APKs stay on the Pi; the publish script keeps the newest 5 and deletes older ones.

10.7 Firebase setup (infra plan, with HUMAN sign-ins)

1. HUMAN: gcloud auth login and firebase login on the Mac with the owner's Google account.
2. gcloud config set project kvitt-1337; gcloud services enable fcm.googleapis.com firebase.googleapis.com.
3. firebase projects:addfirebase kvitt-1337 (skip if already a Firebase project).
4. firebase apps:create ANDROID "Kvitt" --package-name se.hagstedt.kvitt and firebase apps:create ANDROID "Kvitt debug" --package-name se.hagstedt.kvitt.debug.
5. firebase apps:sdkconfig ANDROID <app id> for each, merged into one android/app/google-services.json (git-ignored; the file holds both clients).
6. gcloud iam service-accounts create kvitt-fcm --display-name "Kvitt FCM sender"; grant roles/firebasecloudmessaging.admin on the project; gcloud iam service-accounts keys create infra/secrets/fcm-service-account.json --iam-account kvitt-fcm@kvitt-1337.iam.gserviceaccount.com.
7. Copy the key to /opt/kvitt/secrets/fcm-service-account.json on the Pi (mode 0440, group 10001).
8. If an organisation policy blocks service-account key creation, stop and report; do not change org policies.

The FCM free tier is sufficient; no billing account is required.

10.8 Splitwise import

Command (run on the Pi with the server stopped, or on the Mac against a copy of the database):

kvitt-import --db /data/kvitt.sqlite --file <csv> --account "<Kvitt account name>" \
  --user "<Splitwise name of the owner>=<owner user id>" \
  --user "<Splitwise name of the partner>=<partner user id>" \
  [--fx <CUR>=<SEK per unit>]... [--dry-run]

• One CSV file per Splitwise group (Splitwise's group "Export as spreadsheet"). The account is created with default 50/50 if no non-deleted account with that name exists; otherwise rows go into the existing one.
• Expected columns, in order: Date, Description, Category, Cost, Currency, then one column per group member named by the member's Splitwise name, holding that member's net effect for the row (positive = paid more than their share). A final row whose Description is Total balance is ignored, as are blank rows. To be confirmed against the owner's real file: TODO(spec) OQ-2. The importer must fail with a clear message, writing nothing, if the header differs.
• Date is YYYY-MM-DD; Cost and member columns are decimal numbers with . as separator, converted to öre exactly by string parsing.
• For each expense row with members A and B and cost C (öre): the payer is the member with positive net. If net_A > 0: payer A, share_B = net_A, share_A = C − net_A, mode exact. Symmetric for B. If both nets are 0: payer is the owner, mode full on the owner. If net_A + net_B ≠ 0 or a resulting share is negative, the row is reported as invalid and the import aborts (nothing written).
• Rows whose Category is Payment become settlements: from = the member with positive net, to = the other, amount = that net, one allocation to the account, note = the Splitwise description truncated to 200 code points.
• Rows whose Currency is not SEK abort the import unless a --fx <CUR>=<rate> is given for that currency; then every amount is converted with round_half_up(amount_ore × rate) per value, and for expenses the exact shares are recomputed as share_payer = C' − share_nonpayer'.
• import_key = lowercase hex SHA-256 of splitwise|v1|<account name>|<Date>|<Description>|<Category>|<Cost>|<Currency>|<member nets in header order, joined with |>|<n> where <n> is the 0-based occurrence index of an identical line earlier in the same file. Rows whose key exists are skipped.
• Every created row: created_by_user_id = updated_by_user_id = the owner; updated_by_device_id null; created_at = updated_at = the import time; one audit entry create with source = import; new server_versions from the global sequence; no pushes.
• Description is truncated to 100 code points; the full original goes into notes when truncated.
• Category mapping (case-insensitive exact match on the Category column):

Splitwise category	Kvitt default_key
Groceries, Food and drink - Other, Liquor	groceries
Dining out	restaurants
Electronics, Furniture, Household supplies, Maintenance, Mortgage, Rent, Services, Pets, Home - Other	home
Electricity, Heat/gas, Water, Trash, TV/Phone/Internet, Cleaning, Utilities - Other	utilities
Bicycle, Bus/train, Car, Gas/fuel, Parking, Taxi, Transportation - Other	transport
Hotel, Plane	travel
Games, Movies, Music, Sports, Entertainment - Other	entertainment
Medical expenses	health
Clothing, Gifts	shopping
General, Childcare, Education, Insurance, Taxes, Life - Other, Other, any other value	other

• The summary lists each unmapped category name with its row count.

10.9 Backups and restore

Backup (infra/backup/backup.sh, daily 03:30 Europe/Stockholm, also runnable as docker compose exec backup /backup.sh):

1. sqlite3 /data/kvitt.sqlite ".backup /backups/staging/kvitt.sqlite" (online, consistent).
2. sqlite3 /backups/staging/kvitt.sqlite "PRAGMA integrity_check" must print ok, else abort with a log line.
3. gzip -9 to /backups/kvitt-<UTC yyyyMMddTHHmmssZ>.sqlite.gz; tar -czf /backups/pictures-<same stamp>.tar.gz -C /data pictures.
4. rclone copy /backups/<both files> "${RCLONE_REMOTE}" --drive-root-folder-id "${RCLONE_FOLDER_ID}" --immutable (copy only; never sync, never delete, never overwrite).
5. On success write /backups/last-success with the stamp. Local files older than 30 days are deleted from the Pi's backups volume only after they are confirmed on Drive (rclone lsf); nothing on Drive is ever deleted.

rclone setup (HUMAN, once): on the owner's laptop run rclone authorize "drive" and sign in; the infra agent writes infra/secrets/rclone.conf with a [gdrive] remote (type = drive, scope = drive, the token from the authorize step) and copies it to the Pi. The owner chooses the Drive folder and gives its folder id (TODO(spec) OQ-4).

Restore (documented step by step in infra/RESTORE.md):

1. docker compose stop server.
2. Download the chosen kvitt-*.sqlite.gz and pictures-*.tar.gz from Drive (rclone or browser) to the Pi.
3. gunzip; sqlite3 <file> "PRAGMA integrity_check" must print ok.
4. Move the current /data/kvitt.sqlite* aside (never delete), place the restored file as /data/kvitt.sqlite, extract pictures into /data/pictures.
5. docker compose run --rm server rotate-epoch.
6. docker compose start server; check /health.
7. Open the app on both phones; they detect the new epoch and re-push their data (§14.4.8). Check the balances match on both phones.

10.10 Operations

• Logs: docker compose logs <service>.
• Disk: SQLite is a few MB; pictures under 1 MB; APKs about 20 MB each, 5 kept.
• Upgrades: build, push, infra/deploy.sh <tag>. Migrations run on start; a failed migration exits non-zero, the container restarts in a loop, and the previous tag is redeployed with infra/deploy.sh <previous tag> (migrations are forward-only, so the infra plan's runbook says to restore the pre-upgrade backup if a migration had partially applied; sqlx runs each migration in a transaction, so this should not happen).
• Before every deploy that includes a migration, infra/deploy.sh triggers a backup run and waits for last-success to update.
• Memory check: docker stats --no-stream after the release milestone; server RSS must be under 100 MB with both phones syncing.

---

11. Milestones

Each milestone is one plan (PLAN-<id>.md, written by the orchestrator's plans agent from this section) run by one Opus agent in its own worktree, unless the row says otherwise. "Gate" is what must pass before landing. The Android gate is tools/gradle-serial.sh check; the server gate is cargo fmt --check && cargo clippy -- -D warnings && cargo test in server/; "both gates" means both. HUMAN steps are what only the owner can do.

11.1 Dependency graph

M0 spec ──┬─► U1 UX directions ──(owner picks)──► U2 UX design spec ──► M6 design system ──► M7a..M7f features ─┐
          │                                                                                                     │
          ├─► M1 contracts + vectors ─┬─► M2a server skeleton + domain ─┬─► M3 server core API ───────────┐       │
          │                           │                                 ├─► M4 server sync/audit/push ─┬──┤       │
          │                           │                                 │                              └──┼─► M9 importer
          │                           └─► M2b android skeleton + domain ┴─► M5 android data layer ───────┼───────┤
          │                                                                                               │       ▼
          └─► M2c infra base (Pi, compose, TLS, registry, Firebase, backups) ───────────────────────────┴──► M8 app integration
                                                                                                                  │
                                                                              M10 first release ◄─────────────────┘
                                                                              M11 import real history ◄── M9, M10

After M1, M2a and M2b land, these run in parallel without touching each other's files (§14.7): M3, M4, M5, M2c, M6 (once U2 is approved), M7a to M7f (once M6 lands), M9 (once M4 lands).

11.2 Milestone table

Id	Name	Deliverable	Gate	HUMAN	Depends on
M0	Spec v0.1	SPEC.md, filled CLAUDE.md, repository bootstrap	none (docs only)	Review the spec; answer §12	none
U1	UX directions	§13.4 phase 1 files in docs/ux/	Files exist and open; every screen of §13.3 appears in each direction; Swedish copy, no placeholder text	Review 2-3 directions, pick one, give comments	M0
U2	UX design spec	§13.4 phase 2 files in docs/ux/	Every string key has en and sv; every screen has all states of §13.3	Approve the design spec	U1 and the owner's pick
M1	Contracts and vectors	contracts/openapi.yaml, contracts/schema/server-v1.sql, contracts/test-vectors/*.json, contracts/examples/**/*.json, tools/contracts-check.py	python3 tools/contracts-check.py passes: all JSON parses, every vector file matches §14.5.5, a stdlib-only Python reference of §9.1, §9.3, §9.4 reproduces every expected value, every example validates against the shapes of §14.3 by key set	none	M0
M2a	Server skeleton and kvitt-domain	Cargo workspace, three crates, config, DB pool, migration 0001_init.sql byte-identical to contracts/schema/server-v1.sql, error type with every §14.3.2 code, auth extractor (§8.2), all routes of §7 registered with handlers returning 501 not_implemented, utoipa document equal to contracts/openapi.yaml, GET /health and GET /setup/status implemented, kvitt-domain passing all vectors, server/Dockerfile	Server gate (includes the OpenAPI match test, the migration identity test, the vector tests)	none	M1
M2b	Android skeleton and :domain	Gradle project in android/ with every module of §6.2, version catalog, architecture check, :core:model types and repository interfaces of §14.2.4, :domain passing all vectors (including analytics), :core:fakes with an empty scenario for each repository, :app launching to an empty themed screen, string resource files for both languages, lint rule for missing translations	Android gate	Plug the phone in once to confirm installDebug works	M1
M2c	Infra base	infra/ compose stack, Caddyfile, cert-sync and backup images, build, deploy and publish scripts, infra/README.md (Pi provisioning, router forwarding, registry trust), infra/RESTORE.md, Firebase project and apps, service-account key in infra/secrets/, google-services.json in android/app/	docker compose -f infra/compose.yaml config succeeds; shellcheck infra/**/*.sh clean if shellcheck is installed; on the Pi: cert-sync fetched a valid certificate, https://icehack.asuscomm.com:8443/health answers (with the M2a image) from the phone on mobile data; one backup file appears in the Drive folder	Answer OQ-1, OQ-3, OQ-4; install Docker on the Pi if missing; set the router forward 8443 → Kvitt Pi; add the cert-sync public key to the Home Assistant SSH add-on; gcloud auth login, firebase login; rclone authorize "drive"	M0 (uses the M2a image for the health check when available)
M3	Server core API	E-2 to E-30 and E-33 implemented (§14.3), invite landing page, pictures, rate limiting, bootstrap, pair-device	Server gate; integration tests per endpoint and per error code listed for it	none	M2a
M4	Server sync, audit, push, scheduler	E-31, E-32, audit writer and diff, sync_mutations pruning, FCM sender, scheduler, rotate-epoch	Server gate; integration tests for every sync rule of §14.4 and every case of §9.5; FCM sender tested against a local stub HTTP server	none	M2a
M5	Android data layer	Room DB v1 (§14.2) with exported schema, DAOs, Ktor client and DTOs, token store, sync engine and workers, real repositories, APK updater	Android gate; DTO round-trip tests over every file in contracts/examples/; sync engine tests with Ktor MockEngine covering §14.4 including the 14-day offline case (AC-15.6)	none	M1, M2b
M6	Design system	:core:design theme, tokens, typography, shared components, charts, formatters (§9.8) per docs/ux/design-spec.md, screenshot tests of every component in light and dark, Swedish and English	Android gate	Look at the component screenshots the agent opens and posts (yes/no)	U2, M2b
M7a	Feature: onboarding	:feature:onboarding screens S-1, S-2, S-3, S-18	Android gate with screenshot tests of every state, sv and en, light and dark	none	M6
M7b	Feature: home and account	:feature:home, :feature:account screens S-4, S-5, S-6, S-16, S-17	as M7a	none	M6
M7c	Feature: expense	:feature:expense screens S-7, S-8	as M7a	none	M6
M7d	Feature: settle	:feature:settle screens S-9, S-10	as M7a	none	M6
M7e	Feature: analytics	:feature:analytics screens S-11, S-12	as M7a	none	M6
M7f	Feature: settings	:feature:settings screens S-13, S-14, S-15, S-19, S-20, S-21, S-22	as M7a	none	M6
M8	App integration	:app navigation and deep links, Hilt bindings from interfaces to :data and :domain, :adapter:fcm, notification building (AC-13.5), update banner wiring, revoke wipe	Android gate; on the owner's phone against the real server: pair, add an expense offline, reconnect, see it synced	Tap through the pairing and the offline test as relayed by the orchestrator	M3, M4, M5, M7a-M7f, M2c (Firebase)
M9	Splitwise importer	kvitt-import per §10.8 with tests over synthetic CSV fixtures	Server gate	Provide the CSV files (kept outside git) when OQ-2 is answered	M2a, M4
M10	First release	Release build signed, published, both phones paired, push working both ways, nightly backup on Drive, restore drill report, memory check	docs/validation/first-release.md and docs/validation/restore-drill.md complete; server RSS under 100 MB	Create the release keystore; install the APK on both phones from the landing page; allow "Install unknown apps"; partner pairs with the invite; confirm notifications arrive	M2c, M3, M4, M8
M11	Import history	Real import run, docs/validation/splitwise-import.md with per-account totals compared to Splitwise's balances (aggregates only)	Imported balances equal Splitwise's "Total balance" rows per group	Provide the CSVs; confirm the balances on the phone match Splitwise	M9, M10

11.3 Notes per milestone

• M1 writes the OpenAPI file by hand from §14.3 (no code exists yet). M2a then annotates handlers so the generated document matches it exactly; M2a may only change contracts/openapi.yaml for representational differences that do not change meaning (key order, $ref naming), recorded in the plan report. Any semantic change follows the contract-change rule (§14.8).
• M2a owns the router and registers every route up front so that M3 and M4 only fill in handler bodies in their own files. The same for the utoipa ApiDoc.
• M2b writes the *State classes and repository interfaces of §14.2.4 so that M5 and M7 work against the same interfaces in parallel.
• M7x features use :core:fakes only; they never depend on :data. Each feature agent adds its fake scenarios in android/core/fakes/src/main/kotlin/se/hagstedt/kvitt/fakes/<feature>/ and nowhere else in that module.
• M8 is the only plan that edits :app after M2b.
• M10: the orchestrator relays every HUMAN step in chat.

---

12. Open questions

Only things that cannot be decided without the owner. Everything else in this spec is decided.

Id	Question	Needed by	Default if unanswered
OQ-1	TODO(spec) Address of the owner's local container registry (host:port), whether it is HTTP or HTTPS, and whether it needs credentials.	M2c	none; M2c blocks on it
OQ-2	TODO(spec) Confirm the Splitwise CSV export format against a real file: the exact header line (expected Date,Description,Category,Cost,Currency,<member 1>,<member 2>), the date format, the decimal separator, the name of the payment category (expected Payment), the trailing Total balance row, and the two member names as they appear in the header. Also: which groups exist and what Kvitt account name each should become; whether any group has non-SEK rows and which rates to use.	M9	M9 builds against the expected format with synthetic fixtures and makes the header check strict
OQ-3	TODO(spec) LAN address and SSH user of the Kvitt Pi; LAN address, SSH port and user of the Home Assistant Pi; the directory holding fullchain.pem and privkey.pem there (expected /ssl on Home Assistant OS); which SSH add-on is used.	M2c	none; M2c blocks on it
OQ-4	TODO(spec) Which Google Drive folder receives backups (its folder id).	M2c	none; M2c blocks on the backup part only

---

13. UX sprint

A brief for one UX agent (Opus) who has read this spec and nothing else. It runs right after M0, in parallel with M1, and before any UI work (M6, M7). It produces proposals; the owner chooses; the UX agent then writes the binding design spec. The UX agent writes no Kotlin.

13.1 Context the UX agent must design for

• Two adults in Sweden, one household, using Samsung Galaxy S26 (about 411 × 891 dp viewport, 6.3") and S26+ (about 411 × 891 dp at a higher density, 6.7"). Portrait only. Android 16, One UI on top; the app uses Material 3 (Expressive where stable) and must not imitate One UI.
• Most sessions are 10 seconds: add an expense right after paying. The add flow is the most important flow; it must need at most: open app, tap add, type amount, type description, tap save (category and account defaulted, AC-8.6).
• The second most important view is "who owes whom" on Home.
• Swedish is the primary language (both users' phones are Swedish), English must fit too. Swedish words are long ("Utgiftskonto", "Återkommande"); designs must survive them at 200 % font scale.
• Light and dark theme follow the system. Dynamic colour (Material You wallpaper colours) is offered as an option per direction only if the direction still works with it; the default is the direction's own palette.
• Money must be legible at a glance: amounts in tabular figures, direction ("owes you", "you owe") always in words plus colour, never colour alone.
• Accessibility: touch targets ≥ 48 dp, contrast ≥ 4.5:1 for text, TalkBack labels for every icon-only control, nothing conveyed by colour only, layouts that reflow at 200 % font scale.
• Category icons are Material Symbols from the set in §9.7; category colours are the tokens of §9.7 (the UX agent may change hex values, not token names, and must give light and dark values with contrast checked against both surfaces).
• Users are shown by avatar (picture or initials, AC-4.5) and display name. The viewer is always "you" / "du".

13.2 Navigation model (binding unless a direction proposes a better one with reasons)

• Bottom navigation with three destinations: Home (Översikt), Analytics (Statistik), Settings (Inställningar).
• A floating action button "Add expense" (Lägg till utgift) on Home and on Account detail.
• Account detail, expense editor, expense detail, settle up, settlement detail and all settings sub-screens are pushed screens with a top app bar and back.
• The expense editor is a full-screen screen, not a bottom sheet (the numeric keyboard needs the room).

13.3 Screens and what each must show

Each screen must be designed in these states where they apply: loaded, empty, offline (with pending changes), loading (first sync only; afterwards data is local), error (a failed online action), and, for lists, long content (long Swedish names, 7-digit amounts).

System notifications (copy needed in both languages, built per AC-13.5):

The UX agent writes the Swedish versions.

13.4 Deliverables

Phase 1 (U1): directions. In docs/ux/directions/:

• 2 or 3 visual directions, each a self-contained HTML file docs/ux/directions/<n>-<slug>/index.html (no external requests except Google Fonts, if a direction uses a font whose licence allows bundling in an app; the font must then also be bundleable as an Android font resource) rendering at a 411 dp wide phone frame: S-4 Home, S-7 Expense editor, S-5 Account detail, S-9 Settle up, S-11 Analytics, S-8 Expense detail with timeline, each in light and dark, in Swedish, with realistic household data (names "Jacob" and "Anna", Swedish shops and amounts; no lorem ipsum).
• Each direction states: name and one-line idea; colour palette (primary, secondary, tertiary, surfaces, error, "owed" and "owing" semantic colours, the 12 category colours) in light and dark with contrast ratios; typography (Material 3 type scale with chosen typeface and why; tabular figures for amounts); shape (corner radii per component size); elevation and motion principles; iconography (Material Symbols style: outlined, rounded or sharp, weight, fill rules); how the balance header looks in its three states; how it handles 200 % font scale (one screenshot).
• docs/ux/directions/README.md: a comparison table of the directions (character, strengths, risks, implementation cost in Compose), and the UX agent's recommendation with reasons.
• PNG exports of every screen of every direction (docs/ux/directions/<n>-<slug>/png/), so the orchestrator can show them to the owner without a browser.

Suggested starting points the UX agent may use, replace or merge (it must deliver at least two clearly different ones):

1. "Kvitto": receipt-paper calm. Off-white surfaces, ink-black text, one accent colour, monospaced tabular amounts, dashed dividers like a till receipt. Expressive only in motion.
2. "Skärgård": Material 3 Expressive with a cool sea-and-granite palette, large rounded shapes, bold balance header, playful but quiet.
3. "Kontor": dense, utilitarian, data-forward. Neutral greys, strong typographic hierarchy, charts as the hero on Analytics, minimal decoration.

Phase 2 (U2): design spec for the chosen direction. After the owner picks (and comments), in docs/ux/:

• design-spec.md: final tokens (colour roles light and dark, category colour hex values, typography scale, shapes, spacing scale in dp, elevation, motion durations and easing), component inventory (each with anatomy, states, sizes), and for every screen S-1 to S-22: layout description, all states of §13.3, interaction details, TalkBack labels.
• strings.csv: every user-visible string of the app with columns key,en,sv,notes, keys in snake_case prefixed by screen (home_balance_owes_you), placeholders as Android format arguments (%1$s), plurals marked in notes with their quantities. This becomes values/strings.xml and values-sv/strings.xml in M6 and M7; feature agents add no strings that are not in this file without a decisions entry.
• screens/<S-id>-<slug>.png for every screen and state, light and dark, Swedish.
• Updated colour hex values for §9.7, proposed as a spec change (decisions entry, spec version bump) by the orchestrator.

13.5 Out of scope for the UX sprint

New features, changes to flows required by §4, changes to enums or limits, app icon production (a direction may sketch one), animations beyond descriptions, Kotlin code.

---

14. Contracts

Contracts are what lets agents work in parallel. Each subsection is binding. The files that M1 writes from this section are listed in §14.8. A change to any contract follows §14.8.

14.1 SQLite schema v1 (server)

This DDL is the content of contracts/schema/server-v1.sql and of the server's first migration server/crates/kvitt-server/migrations/0001_init.sql, byte for byte (a test asserts equality). Conventions: ids TEXT (UUIDv7, see the conventions at the top of this file); money INTEGER öre; timestamps TEXT in the fixed RFC 3339 form; booleans INTEGER 0/1; server_version from sync_sequence. Enum and token lists that may grow (icons, colours) are checked in code, not in DDL. Every connection sets the pragmas of §8.9.

-- Kvitt server schema v1. Do not edit; add a new migration and bump the schema version (SPEC §14.8).

CREATE TABLE meta (
    key   TEXT PRIMARY KEY NOT NULL,
    value TEXT NOT NULL
) STRICT;

-- One global, strictly increasing counter for server_version. Exactly one row.
CREATE TABLE sync_sequence (
    id    INTEGER PRIMARY KEY CHECK (id = 1),
    value INTEGER NOT NULL
) STRICT;

CREATE TABLE users (
    id              TEXT PRIMARY KEY NOT NULL,
    role            TEXT NOT NULL CHECK (role IN ('admin', 'member')),
    display_name    TEXT NOT NULL CHECK (length(display_name) BETWEEN 1 AND 40),
    picture_version INTEGER CHECK (picture_version IS NULL OR picture_version >= 1),
    created_at      TEXT NOT NULL,
    updated_at      TEXT NOT NULL,
    server_version  INTEGER NOT NULL UNIQUE
) STRICT;
CREATE UNIQUE INDEX users_single_admin ON users (role) WHERE role = 'admin';

CREATE TABLE devices (
    id                 TEXT PRIMARY KEY NOT NULL,
    user_id            TEXT NOT NULL REFERENCES users (id),
    name               TEXT NOT NULL CHECK (length(name) BETWEEN 1 AND 60),
    token_hash         TEXT NOT NULL UNIQUE,
    fcm_token          TEXT,
    app_version_code   INTEGER,
    created_at         TEXT NOT NULL,
    last_seen_at       TEXT NOT NULL,
    revoked_at         TEXT,
    revoked_by_user_id TEXT REFERENCES users (id)
) STRICT;
CREATE INDEX devices_user ON devices (user_id);

CREATE TABLE invites (
    id                 TEXT PRIMARY KEY NOT NULL,
    kind               TEXT NOT NULL CHECK (kind IN ('admin_setup', 'partner', 'device')),
    code_hash          TEXT NOT NULL UNIQUE,
    target_user_id     TEXT REFERENCES users (id),
    created_by_user_id TEXT REFERENCES users (id),
    created_at         TEXT NOT NULL,
    expires_at         TEXT NOT NULL,
    accepted_at        TEXT,
    accepted_device_id TEXT REFERENCES devices (id),
    cancelled_at       TEXT,
    CHECK ((kind = 'device') = (target_user_id IS NOT NULL)),
    CHECK ((kind = 'admin_setup') = (created_by_user_id IS NULL) OR kind = 'device')
) STRICT;

CREATE TABLE accounts (
    id                 TEXT PRIMARY KEY NOT NULL,
    name               TEXT NOT NULL CHECK (length(name) BETWEEN 1 AND 60),
    name_key           TEXT NOT NULL,
    created_by_user_id TEXT NOT NULL REFERENCES users (id),
    created_at         TEXT NOT NULL,
    updated_at         TEXT NOT NULL,
    archived_at        TEXT,
    deleted_at         TEXT,
    server_version     INTEGER NOT NULL UNIQUE
) STRICT;
-- name_key = Unicode-lowercased, trimmed name, computed in Rust.
CREATE UNIQUE INDEX accounts_name_live ON accounts (name_key) WHERE deleted_at IS NULL;

CREATE TABLE account_default_shares (
    account_id TEXT NOT NULL REFERENCES accounts (id),
    user_id    TEXT NOT NULL REFERENCES users (id),
    share_bp   INTEGER NOT NULL CHECK (share_bp BETWEEN 0 AND 10000),
    PRIMARY KEY (account_id, user_id)
) STRICT, WITHOUT ROWID;

CREATE TABLE categories (
    id             TEXT PRIMARY KEY NOT NULL,
    default_key    TEXT UNIQUE CHECK (default_key IS NULL OR default_key IN
                       ('groceries', 'restaurants', 'home', 'utilities', 'transport',
                        'travel', 'entertainment', 'health', 'shopping', 'other')),
    name           TEXT CHECK (name IS NULL OR length(name) BETWEEN 1 AND 40),
    icon           TEXT NOT NULL,
    color          TEXT NOT NULL,
    sort_order     INTEGER NOT NULL,
    created_at     TEXT NOT NULL,
    updated_at     TEXT NOT NULL,
    archived_at    TEXT,
    deleted_at     TEXT,
    server_version INTEGER NOT NULL UNIQUE,
    CHECK (name IS NOT NULL OR default_key IS NOT NULL)
) STRICT;

CREATE TABLE recurring_templates (
    id                    TEXT PRIMARY KEY NOT NULL,
    account_id            TEXT NOT NULL REFERENCES accounts (id),
    category_id           TEXT NOT NULL REFERENCES categories (id),
    amount_ore            INTEGER NOT NULL CHECK (amount_ore BETWEEN 1 AND 1000000000),
    description           TEXT NOT NULL CHECK (length(description) BETWEEN 1 AND 100),
    notes                 TEXT NOT NULL DEFAULT '' CHECK (length(notes) <= 1000),
    payer_user_id         TEXT NOT NULL REFERENCES users (id),
    split_mode            TEXT NOT NULL CHECK (split_mode IN ('default', 'ratio', 'exact', 'full')),
    day_of_month          INTEGER NOT NULL CHECK (day_of_month BETWEEN 1 AND 31),
    start_period          TEXT NOT NULL,
    end_period            TEXT,
    paused                INTEGER NOT NULL DEFAULT 0 CHECK (paused IN (0, 1)),
    last_generated_period TEXT,
    created_by_user_id    TEXT NOT NULL REFERENCES users (id),
    updated_by_user_id    TEXT NOT NULL REFERENCES users (id),
    created_at            TEXT NOT NULL,
    updated_at            TEXT NOT NULL,
    deleted_at            TEXT,
    server_version        INTEGER NOT NULL UNIQUE
) STRICT;
CREATE INDEX recurring_templates_account ON recurring_templates (account_id);

-- Modes ratio and full: share_bp set, share_ore NULL. Mode exact: share_ore set, share_bp NULL. Mode default: no rows.
CREATE TABLE recurring_template_splits (
    template_id TEXT NOT NULL REFERENCES recurring_templates (id),
    user_id     TEXT NOT NULL REFERENCES users (id),
    share_bp    INTEGER CHECK (share_bp IS NULL OR share_bp BETWEEN 0 AND 10000),
    share_ore   INTEGER CHECK (share_ore IS NULL OR share_ore >= 0),
    PRIMARY KEY (template_id, user_id),
    CHECK ((share_bp IS NULL) <> (share_ore IS NULL))
) STRICT, WITHOUT ROWID;

CREATE TABLE expenses (
    id                    TEXT PRIMARY KEY NOT NULL,
    account_id            TEXT NOT NULL REFERENCES accounts (id),
    category_id           TEXT NOT NULL REFERENCES categories (id),
    amount_ore            INTEGER NOT NULL CHECK (amount_ore BETWEEN 1 AND 1000000000),
    description           TEXT NOT NULL CHECK (length(description) BETWEEN 1 AND 100),
    notes                 TEXT NOT NULL DEFAULT '' CHECK (length(notes) <= 1000),
    expense_date          TEXT NOT NULL,
    payer_user_id         TEXT NOT NULL REFERENCES users (id),
    split_mode            TEXT NOT NULL CHECK (split_mode IN ('default', 'ratio', 'exact', 'full')),
    recurring_template_id TEXT REFERENCES recurring_templates (id),
    recurring_period      TEXT,
    import_key            TEXT UNIQUE,
    created_by_user_id    TEXT NOT NULL REFERENCES users (id),
    updated_by_user_id    TEXT NOT NULL REFERENCES users (id),
    updated_by_device_id  TEXT REFERENCES devices (id),
    created_at            TEXT NOT NULL,
    updated_at            TEXT NOT NULL,
    deleted_at            TEXT,
    server_received_at    TEXT NOT NULL,
    server_version        INTEGER NOT NULL UNIQUE,
    CHECK ((recurring_template_id IS NULL) = (recurring_period IS NULL))
) STRICT;
CREATE UNIQUE INDEX expenses_recurring_once ON expenses (recurring_template_id, recurring_period)
    WHERE recurring_template_id IS NOT NULL;
CREATE INDEX expenses_account ON expenses (account_id);
CREATE INDEX expenses_category ON expenses (category_id);

-- Exactly two rows per expense, one per household user. share_bp NULL for mode exact.
CREATE TABLE expense_splits (
    expense_id TEXT NOT NULL REFERENCES expenses (id),
    user_id    TEXT NOT NULL REFERENCES users (id),
    share_ore  INTEGER NOT NULL CHECK (share_ore >= 0),
    share_bp   INTEGER CHECK (share_bp IS NULL OR share_bp BETWEEN 0 AND 10000),
    PRIMARY KEY (expense_id, user_id)
) STRICT, WITHOUT ROWID;

CREATE TABLE settlements (
    id                   TEXT PRIMARY KEY NOT NULL,
    from_user_id         TEXT NOT NULL REFERENCES users (id),
    to_user_id           TEXT NOT NULL REFERENCES users (id),
    amount_ore           INTEGER NOT NULL CHECK (amount_ore BETWEEN 1 AND 1000000000),
    settlement_date      TEXT NOT NULL,
    note                 TEXT NOT NULL DEFAULT '' CHECK (length(note) <= 200),
    import_key           TEXT UNIQUE,
    created_by_user_id   TEXT NOT NULL REFERENCES users (id),
    updated_by_user_id   TEXT NOT NULL REFERENCES users (id),
    updated_by_device_id TEXT REFERENCES devices (id),
    created_at           TEXT NOT NULL,
    updated_at           TEXT NOT NULL,
    deleted_at           TEXT,
    server_received_at   TEXT NOT NULL,
    server_version       INTEGER NOT NULL UNIQUE,
    CHECK (from_user_id <> to_user_id)
) STRICT;

CREATE TABLE settlement_allocations (
    settlement_id TEXT NOT NULL REFERENCES settlements (id),
    account_id    TEXT NOT NULL REFERENCES accounts (id),
    amount_ore    INTEGER NOT NULL CHECK (amount_ore >= 1),
    PRIMARY KEY (settlement_id, account_id)
) STRICT, WITHOUT ROWID;
CREATE INDEX settlement_allocations_account ON settlement_allocations (account_id);

-- Append-only. changes is a JSON array of {"field","old","new"} (SPEC §14.4.7).
CREATE TABLE audit_log (
    id              TEXT PRIMARY KEY NOT NULL,
    entity_type     TEXT NOT NULL CHECK (entity_type IN ('expense', 'settlement')),
    entity_id       TEXT NOT NULL,
    action          TEXT NOT NULL CHECK (action IN ('create', 'update', 'delete')),
    source          TEXT NOT NULL CHECK (source IN ('app', 'recurring', 'import')),
    actor_user_id   TEXT NOT NULL REFERENCES users (id),
    actor_device_id TEXT REFERENCES devices (id),
    client_at       TEXT NOT NULL,
    server_at       TEXT NOT NULL,
    changes         TEXT NOT NULL,
    server_version  INTEGER NOT NULL UNIQUE
) STRICT;
CREATE INDEX audit_log_entity ON audit_log (entity_type, entity_id);

-- Idempotency of POST /sync. Rows older than 90 days are pruned daily.
CREATE TABLE sync_mutations (
    mutation_id TEXT PRIMARY KEY NOT NULL,
    device_id   TEXT NOT NULL REFERENCES devices (id),
    entity_type TEXT NOT NULL CHECK (entity_type IN ('expense', 'settlement')),
    entity_id   TEXT NOT NULL,
    result      TEXT NOT NULL,
    received_at TEXT NOT NULL
) STRICT;
CREATE INDEX sync_mutations_received ON sync_mutations (received_at);

INSERT INTO sync_sequence (id, value) VALUES (1, 10);
INSERT INTO meta (key, value) VALUES ('schema_version', '1');
INSERT INTO meta (key, value) VALUES ('sync_epoch', lower(hex(randomblob(16))));

INSERT INTO categories (id, default_key, name, icon, color, sort_order, created_at, updated_at, archived_at, deleted_at, server_version) VALUES
    ('01920000-0000-7000-8000-000000000001', 'groceries',     NULL, 'shopping_cart',    'green',  10,  '2026-10-17T00:00:00.000Z', '2026-10-17T00:00:00.000Z', NULL, NULL, 1),
    ('01920000-0000-7000-8000-000000000002', 'restaurants',   NULL, 'restaurant',       'orange', 20,  '2026-10-17T00:00:00.000Z', '2026-10-17T00:00:00.000Z', NULL, NULL, 2),
    ('01920000-0000-7000-8000-000000000003', 'home',          NULL, 'home',             'brown',  30,  '2026-10-17T00:00:00.000Z', '2026-10-17T00:00:00.000Z', NULL, NULL, 3),
    ('01920000-0000-7000-8000-000000000004', 'utilities',     NULL, 'bolt',             'yellow', 40,  '2026-10-17T00:00:00.000Z', '2026-10-17T00:00:00.000Z', NULL, NULL, 4),
    ('01920000-0000-7000-8000-000000000005', 'transport',     NULL, 'directions_car',   'blue',   50,  '2026-10-17T00:00:00.000Z', '2026-10-17T00:00:00.000Z', NULL, NULL, 5),
    ('01920000-0000-7000-8000-000000000006', 'travel',        NULL, 'flight',           'teal',   60,  '2026-10-17T00:00:00.000Z', '2026-10-17T00:00:00.000Z', NULL, NULL, 6),
    ('01920000-0000-7000-8000-000000000007', 'entertainment', NULL, 'movie',            'purple', 70,  '2026-10-17T00:00:00.000Z', '2026-10-17T00:00:00.000Z', NULL, NULL, 7),
    ('01920000-0000-7000-8000-000000000008', 'health',        NULL, 'medical_services', 'red',    80,  '2026-10-17T00:00:00.000Z', '2026-10-17T00:00:00.000Z', NULL, NULL, 8),
    ('01920000-0000-7000-8000-000000000009', 'shopping',      NULL, 'shopping_bag',     'pink',   90,  '2026-10-17T00:00:00.000Z', '2026-10-17T00:00:00.000Z', NULL, NULL, 9),
    ('01920000-0000-7000-8000-00000000000a', 'other',         NULL, 'category',         'grey',   100, '2026-10-17T00:00:00.000Z', '2026-10-17T00:00:00.000Z', NULL, NULL, 10);

Server-side rules on top of the DDL:

• Every write transaction starts with BEGIN IMMEDIATE. Taking a server_version is UPDATE sync_sequence SET value = value + 1 WHERE id = 1 RETURNING value inside that transaction.
• At most two rows in users (checked in code; household_full).
• meta.sync_epoch is replaced by kvitt-server rotate-epoch with a new lower(hex(randomblob(16))).
• No row of users, devices, accounts, categories, recurring_templates, expenses, settlements or audit_log is ever hard-deleted. Child rows (account_default_shares, recurring_template_splits, expense_splits, settlement_allocations) are replaced (delete and insert) when their parent is updated.
• invites rows are never deleted; sync_mutations rows are pruned after 90 days.

14.2 Room entities (phone)

Database class KvittDatabase in :data, file name kvitt.db, version 1, exportSchema = true with schemas in android/data/schemas/ (committed; the exported JSON for version 1 is a contract file once M5 lands). Package se.hagstedt.kvitt.data.db.

Rules:

• Table and column names are identical to the server's (§14.1) where the column exists on both. Kotlin properties are camelCase with @ColumnInfo(name = "snake_case").
• No foreign keys in Room: sync pages may deliver a child before its parent (a parent updated later carries a higher server_version). Queries that feed the UI join and skip rows whose references do not resolve yet.
• Server-only columns are not mirrored: token_hash, fcm_token, name_key, import_key, server_received_at, updated_by_device_id, code_hash. Tables devices, invites, meta, sync_sequence, sync_mutations are not mirrored.
• Timestamps are String in the fixed RFC 3339 form; dates String YYYY-MM-DD; money Long; bp Int; booleans Boolean.
• local_updated_at (Long, epoch milliseconds, phone clock) is set on every local write and on every row written from the network. It orders the push queue and is never sent.

14.2.1 Mirrored tables

Local-only semantics on expenses and settlements:

• pending = true means the row has a local change not yet accepted by the server. Every local create, edit or delete sets pending = true, sets a new UUIDv7 pending_mutation_id, sets updated_at to the phone's current time in the fixed RFC 3339 form (and created_at on create), and sets local_updated_at.
• A local delete sets deleted_at = updated_at = now. The row stays as a tombstone.
• The pending-changes count (AC-15.4) is count(expenses where pending) + count(settlements where pending).

14.2.2 Local-only tables

Per-phone preferences (last used account, last used category per account, "install unknown apps" explanation shown) live in DataStore prefs.preferences_pb, not Room.

14.2.3 Migrations

Version 1 has no migrations. Every later Room version adds a Migration and a migration test in :data (CLAUDE.md), and is a contract change (§14.8).

14.2.4 UI contract: types and repository interfaces in :core:model

Package se.hagstedt.kvitt.model. Features see only these. :data implements them; :core:fakes fakes them. Flows are cold kotlinx.coroutines.flow.Flow; suspend functions are main-safe.

// Ids and enums
@JvmInline value class UserId(val value: String)
@JvmInline value class AccountId(val value: String)
@JvmInline value class CategoryId(val value: String)
@JvmInline value class ExpenseId(val value: String)
@JvmInline value class SettlementId(val value: String)
@JvmInline value class TemplateId(val value: String)
@JvmInline value class DeviceId(val value: String)
@JvmInline value class InviteId(val value: String)

enum class Role { ADMIN, MEMBER }
enum class SplitMode { DEFAULT, RATIO, EXACT, FULL }
enum class InviteKind { ADMIN_SETUP, PARTNER, DEVICE }
enum class EntityType { EXPENSE, SETTLEMENT }
enum class AuditAction { CREATE, UPDATE, DELETE }
enum class AuditSource { APP, RECURRING, IMPORT }
enum class DefaultCategoryKey { GROCERIES, RESTAURANTS, HOME, UTILITIES, TRANSPORT, TRAVEL, ENTERTAINMENT, HEALTH, SHOPPING, OTHER }
enum class ColorToken { GREEN, ORANGE, BROWN, YELLOW, BLUE, TEAL, PURPLE, RED, PINK, GREY, INDIGO, LIME }
enum class RangePreset { THIS_MONTH, LAST_3_MONTHS, LAST_6_MONTHS, LAST_12_MONTHS, THIS_YEAR, ALL_TIME, CUSTOM }
enum class Granularity { MONTH, WEEK }
enum class Measure { HOUSEHOLD, MY_SHARE }
enum class GroupBy { CATEGORY, ACCOUNT }
enum class SyncRejectionReason { STALE, DELETED, VALIDATION_FAILED, REFERENCE_NOT_FOUND, CLOCK_SKEW }
// Wire value of every enum = lowercase name (SplitMode.DEFAULT <-> "default"). Icon tokens are Strings from CategoryIcons.ALL (§9.7).
object CategoryIcons { val ALL: List<String> /* the 30 tokens of §9.7, in that order */ }

// Plain records
data class User(val id: UserId, val role: Role, val displayName: String, val pictureVersion: Int?)
data class Account(val id: AccountId, val name: String, val defaultShares: Map<UserId, Int>, val createdAt: String, val archived: Boolean)
data class Category(val id: CategoryId, val defaultKey: DefaultCategoryKey?, val customName: String?, val icon: String, val color: ColorToken, val sortOrder: Int, val archived: Boolean)
data class Share(val userId: UserId, val shareOre: Long, val shareBp: Int?)

// Results
sealed interface OnlineResult<out T> {
    data class Ok<T>(val value: T) : OnlineResult<T>
    data object Offline : OnlineResult<Nothing>
    data class Failed(val code: String, val field: String?) : OnlineResult<Nothing>   // code from §14.3.2
}
sealed interface SaveResult<out T> {
    data class Saved<T>(val id: T) : SaveResult<T>
    data class Invalid(val errors: List<FieldError>) : SaveResult<Nothing>
}
data class FieldError(val field: String, val error: String)   // error = a domain error token of §14.5.3 or "required"

// Session and onboarding (S-1, S-2, S-3, S-18)
sealed interface SessionState {
    data object Unpaired : SessionState
    data class Paired(val me: User, val partner: User?, val deviceId: DeviceId) : SessionState
    data object Revoked : SessionState
    data object Unrecognised : SessionState   // 401 unauthorized: pair again, data kept (§14.4.8)
}
interface SessionRepository {
    val session: Flow<SessionState>
    suspend fun pair(serverUrl: String, code: String, displayName: String?, deviceName: String): OnlineResult<User>
    suspend fun acknowledgeRevoked()   // wipes local data (AC-3.3) and moves to Unpaired
}

// Balances shared by several screens. netOre is the viewer's net (§9.3): > 0 partner owes viewer.
data class BalanceView(val netOre: Long, val partner: User)

// Home (S-4)
data class AccountSummary(val account: Account, val netOre: Long, val expensesThisMonth: Int)
data class SyncSummary(val lastSuccessAt: String?, val pendingCount: Int, val rejectedCount: Int, val clockSkew: Boolean, val running: Boolean)
data class HomeState(val me: User, val partner: User?, val global: BalanceView?, val active: List<AccountSummary>, val archived: List<AccountSummary>, val sync: SyncSummary, val update: UpdateInfo?)
interface HomeRepository { fun home(): Flow<HomeState> }

// Ledger rows (S-5, S-12)
sealed interface ExpenseEffect {
    data class Lent(val ore: Long) : ExpenseEffect
    data class Borrowed(val ore: Long) : ExpenseEffect
    data object NotInvolved : ExpenseEffect
    data object PaidForYourself : ExpenseEffect
}
sealed interface LedgerRow {
    data class ExpenseRow(val id: ExpenseId, val date: String, val description: String, val category: Category, val amountOre: Long, val effect: ExpenseEffect, val pending: Boolean) : LedgerRow
    data class SettlementRow(val id: SettlementId, val date: String, val from: User, val to: User, val amountOre: Long, val accountCount: Int, val pending: Boolean) : LedgerRow
}
data class LedgerMonth(val period: String, val householdSpendOre: Long, val rows: List<LedgerRow>)

// Accounts (S-5, S-6)
data class AccountDetailState(val account: Account, val balance: BalanceView, val months: List<LedgerMonth>, val deletable: Boolean, val users: List<User>)
interface AccountRepository {
    fun detail(id: AccountId): Flow<AccountDetailState?>
    fun all(): Flow<List<Account>>
    suspend fun create(name: String, defaultShares: Map<UserId, Int>): OnlineResult<AccountId>
    suspend fun update(id: AccountId, name: String, defaultShares: Map<UserId, Int>): OnlineResult<Unit>
    suspend fun setArchived(id: AccountId, archived: Boolean): OnlineResult<Unit>
    suspend fun delete(id: AccountId): OnlineResult<Unit>
}

// Expenses (S-7, S-8)
data class ExpenseDraft(val id: ExpenseId?, val accountId: AccountId?, val categoryId: CategoryId?, val amountOre: Long?, val description: String, val notes: String, val date: String, val payer: UserId, val mode: SplitMode, val ratioBp: Map<UserId, Int>?, val exactOre: Map<UserId, Long>?, val fullOn: UserId?)
data class ExpenseEditorOptions(val me: User, val partner: User, val accounts: List<Account>, val categories: List<Category>, val defaultAccountId: AccountId?, val defaultCategoryFor: Map<AccountId, CategoryId>, val today: String)
data class TimelineLine(val actor: User?, val action: AuditAction, val source: AuditSource, val clientAt: String, val changes: List<FieldChange>, val pendingLocal: Boolean)
data class FieldChange(val field: String, val oldDisplay: String?, val newDisplay: String?)   // ids already resolved to names; amounts and dates raw (öre as String, ISO date), formatted by the UI
data class ExpenseDetailState(val id: ExpenseId, val account: Account, val category: Category, val amountOre: Long, val description: String, val notes: String, val date: String, val payer: User, val mode: SplitMode, val shares: List<Share>, val recurringTemplateName: String?, val deleted: Boolean, val pending: Boolean, val timeline: List<TimelineLine>)
interface ExpenseRepository {
    fun editorOptions(): Flow<ExpenseEditorOptions>
    suspend fun load(id: ExpenseId): ExpenseDraft?
    fun preview(draft: ExpenseDraft): List<Share>?     // null while the draft cannot be computed
    suspend fun save(draft: ExpenseDraft): SaveResult<ExpenseId>
    suspend fun delete(id: ExpenseId)
    fun detail(id: ExpenseId): Flow<ExpenseDetailState?>
}

// Settlements (S-9, S-10)
data class AllocationLine(val account: Account, val payerNetOre: Long, val amountOre: Long, val overSettles: Boolean)
data class SettleState(val payer: User, val recipient: User, val amountOre: Long?, val lines: List<AllocationLine>, val remainingOre: Long, val canConfirm: Boolean, val date: String, val note: String)
data class SettleRequest(val editing: SettlementId?, val payer: UserId, val amountOre: Long?, val fixedAccount: AccountId?, val allocations: Map<AccountId, Long>?, val date: String, val note: String)
data class SettlementDetailState(val id: SettlementId, val from: User, val to: User, val amountOre: Long, val date: String, val note: String, val allocations: List<Pair<Account, Long>>, val deleted: Boolean, val pending: Boolean, val timeline: List<TimelineLine>)
interface SettlementRepository {
    suspend fun start(editing: SettlementId?, fixedAccount: AccountId?): SettleState   // defaults per AC-11.3 / AC-11.4
    suspend fun recompute(request: SettleRequest): SettleState  // allocations == null: propose (§9.4); else validate and warn
    suspend fun save(request: SettleRequest): SaveResult<SettlementId>
    suspend fun delete(id: SettlementId)
    fun detail(id: SettlementId): Flow<SettlementDetailState?>
}

// Analytics (S-11, S-12)
data class AnalyticsFilter(val preset: RangePreset, val customStart: String?, val customEnd: String?, val granularity: Granularity, val measure: Measure, val groupBy: GroupBy)
data class GroupKey(val id: String, val label: String?, val color: ColorToken?)   // label null: UI uses the default category name
data class SpendBucket(val key: String, val start: String, val end: String, val totals: Map<String, Long>, val totalOre: Long)
data class AnalyticsState(val filter: AnalyticsFilter, val start: String, val end: String, val groups: List<GroupKey>, val buckets: List<SpendBucket>, val byCategory: List<Pair<GroupKey, Long>>, val byAccount: List<Pair<GroupKey, Long>>, val totalOre: Long)
data class DrillDownState(val title: String, val totalOre: Long, val rows: List<LedgerRow.ExpenseRow>)
interface AnalyticsRepository {
    fun analytics(filter: AnalyticsFilter): Flow<AnalyticsState>
    fun drillDown(filter: AnalyticsFilter, bucketKey: String?, categoryId: CategoryId?, accountId: AccountId?): Flow<DrillDownState>
}

// Settings (S-13 to S-15, S-19 to S-22)
interface ProfileRepository {
    fun household(): Flow<List<User>>
    suspend fun setDisplayName(name: String): OnlineResult<Unit>
    suspend fun setPicture(bytes: ByteArray, mimeType: String): OnlineResult<Unit>
    suspend fun removePicture(): OnlineResult<Unit>
}
interface CategoryRepository {
    fun all(): Flow<List<Category>>
    suspend fun create(name: String, icon: String, color: ColorToken): OnlineResult<CategoryId>
    suspend fun update(id: CategoryId, name: String?, icon: String, color: ColorToken): OnlineResult<Unit>   // name null = reset default name
    suspend fun reorder(idsInOrder: List<CategoryId>): OnlineResult<Unit>
    suspend fun setArchived(id: CategoryId, archived: Boolean): OnlineResult<Unit>
    suspend fun delete(id: CategoryId): OnlineResult<Unit>
}
data class RecurringTemplate(val id: TemplateId, val draft: ExpenseDraft, val dayOfMonth: Int, val startPeriod: String, val endPeriod: String?, val paused: Boolean, val nextOccurrence: String?)
interface RecurringRepository {
    fun all(): Flow<List<RecurringTemplate>>
    suspend fun create(t: RecurringTemplate): OnlineResult<TemplateId>
    suspend fun update(t: RecurringTemplate): OnlineResult<Unit>
    suspend fun delete(id: TemplateId): OnlineResult<Unit>
}
data class Device(val id: DeviceId, val user: User, val name: String, val pairedAt: String, val lastSeenAt: String, val appVersionCode: Int?, val current: Boolean, val revoked: Boolean)
interface DeviceRepository {
    suspend fun list(): OnlineResult<List<Device>>
    suspend fun revoke(id: DeviceId): OnlineResult<Unit>
}
data class Invite(val id: InviteId, val kind: InviteKind, val targetUser: UserId?, val code: String?, val link: String?, val createdAt: String, val expiresAt: String)   // code and link only right after creation
interface InviteRepository {
    suspend fun list(): OnlineResult<List<Invite>>
    suspend fun create(kind: InviteKind, targetUser: UserId?): OnlineResult<Invite>
    suspend fun cancel(id: InviteId): OnlineResult<Unit>
}
data class Rejection(val mutationId: String, val entityType: EntityType, val entityId: String, val reason: SyncRejectionReason, val summary: String, val rejectedAt: String)
data class SyncStatusState(val summary: SyncSummary, val pending: List<LedgerRow>, val rejections: List<Rejection>)
interface SyncRepository {
    fun status(): Flow<SyncStatusState>
    suspend fun syncNow()
    suspend fun dismiss(mutationId: String)
}
data class UpdateInfo(val versionCode: Int, val versionName: String, val sizeBytes: Long)
sealed interface UpdateState {
    data object None : UpdateState
    data class Available(val info: UpdateInfo) : UpdateState
    data class Downloading(val info: UpdateInfo, val progress: Float) : UpdateState
    data class Failed(val info: UpdateInfo) : UpdateState
    data class NeedsInstallPermission(val info: UpdateInfo) : UpdateState
}
interface UpdateRepository {
    val state: Flow<UpdateState>
    suspend fun check()
    suspend fun install()
}
data class AboutState(val versionName: String, val versionCode: Int, val serverUrl: String, val serverVersion: String?)
interface AboutRepository { fun about(): Flow<AboutState> }
interface ConnectivityRepository { val online: Flow<Boolean> }   // §9.9

Agents may add fields to these types only through a contract change (§14.8). A feature that needs a value not listed asks the orchestrator.

14.3 OpenAPI 3.1 outline

Milestone M1 writes contracts/openapi.yaml (OpenAPI 3.1.0, info.title Kvitt, info.version 1, one server https://icehack.asuscomm.com:8443, security scheme device = HTTP bearer) from this outline. This outline is binding; the YAML file must not add or drop anything.

14.3.1 Common rules

• Content type application/json; charset=utf-8 for every request and response body, except: PUT /me/picture (request image/jpeg, image/png or image/webp, raw bytes), GET /users/{user_id}/picture (response image/jpeg), GET /i/{code} (response text/html; charset=utf-8), GET /apk/files/{file_name} (response application/vnd.android.package-archive).
• Request headers the app sends on every call: X-Kvitt-Protocol: 1, X-Kvitt-App-Version: <versionCode>, Accept-Language: <device locale>, and Authorization: Bearer <token> on authenticated endpoints. A request to an authenticated endpoint without X-Kvitt-Protocol or with a value other than 1 gets 426 protocol_unsupported (the app then shows the update banner and stops syncing until updated).
• Every response carries X-Kvitt-Server-Version: <crate version>.
• Unknown JSON fields in requests are ignored. Missing required fields or wrong types: 400 invalid_request. Values outside §9.6: 422 validation_failed with field.
• Optional fields in responses are present with null, never omitted.
• PATCH bodies: every field optional; absent = unchanged; at least one field required (invalid_request).
• Path ids that are not a UUID: 404 not_found.

14.3.2 Error shape and error codes

{ "code": "validation_failed", "message": "amount_ore must be between 1 and 1000000000", "field": "amount_ore" }

message is English developer text and is never shown to users; the app maps code (and field) to localised strings. field is the JSON key concerned or null.

Exhaustive list:

Each endpoint below lists the codes it can return besides the always-possible unauthorized, device_revoked, protocol_unsupported, rate_limited, internal, unavailable (authenticated) or rate_limited, internal, unavailable (unauthenticated).

14.3.3 Wire objects

All server-produced objects. ? marks nullable (always present). Types: string, int (64-bit), bool, ts (fixed RFC 3339), date (YYYY-MM-DD), period (YYYY-MM), uuid.

// User
{ "id": uuid, "role": "admin" | "member", "display_name": string, "picture_version": int?,
  "created_at": ts, "updated_at": ts, "server_version": int }

// Device (only in /me, /devices, /invites/accept)
{ "id": uuid, "user_id": uuid, "name": string, "app_version_code": int?, "created_at": ts,
  "last_seen_at": ts, "revoked_at": ts?, "is_current": bool }

// Invite (code and link only in the POST /invites response; null in GET /invites)
{ "id": uuid, "kind": "admin_setup" | "partner" | "device", "target_user_id": uuid?,
  "code": string?, "link": string?, "created_at": ts, "expires_at": ts }

// Share (of an account default)
{ "user_id": uuid, "share_bp": int }

// Account
{ "id": uuid, "name": string, "default_shares": [Share, Share], "created_by_user_id": uuid,
  "created_at": ts, "updated_at": ts, "archived_at": ts?, "deleted_at": ts?, "server_version": int }

// Category
{ "id": uuid, "default_key": "groceries" | "restaurants" | "home" | "utilities" | "transport" | "travel" |
  "entertainment" | "health" | "shopping" | "other" | null, "name": string?, "icon": string, "color": string,
  "sort_order": int, "created_at": ts, "updated_at": ts, "archived_at": ts?, "deleted_at": ts?, "server_version": int }

// Split (of an expense): share_bp null for mode exact
{ "user_id": uuid, "share_ore": int, "share_bp": int? }

// TemplateSplit: exactly one of share_bp / share_ore non-null; empty array for mode default
{ "user_id": uuid, "share_bp": int?, "share_ore": int? }

// Expense
{ "id": uuid, "account_id": uuid, "category_id": uuid, "amount_ore": int, "description": string, "notes": string,
  "expense_date": date, "payer_user_id": uuid, "split_mode": "default" | "ratio" | "exact" | "full",
  "splits": [Split, Split], "recurring_template_id": uuid?, "recurring_period": period?,
  "created_by_user_id": uuid, "updated_by_user_id": uuid, "created_at": ts, "updated_at": ts,
  "deleted_at": ts?, "server_version": int }

// Allocation
{ "account_id": uuid, "amount_ore": int }

// Settlement
{ "id": uuid, "from_user_id": uuid, "to_user_id": uuid, "amount_ore": int, "settlement_date": date, "note": string,
  "allocations": [Allocation, ...], "created_by_user_id": uuid, "updated_by_user_id": uuid,
  "created_at": ts, "updated_at": ts, "deleted_at": ts?, "server_version": int }

// RecurringTemplate
{ "id": uuid, "account_id": uuid, "category_id": uuid, "amount_ore": int, "description": string, "notes": string,
  "payer_user_id": uuid, "split_mode": "default" | "ratio" | "exact" | "full", "splits": [TemplateSplit, ...],
  "day_of_month": int, "start_period": period, "end_period": period?, "paused": bool,
  "last_generated_period": period?, "next_occurrence_date": date?, "created_by_user_id": uuid,
  "updated_by_user_id": uuid, "created_at": ts, "updated_at": ts, "deleted_at": ts?, "server_version": int }

// AuditEntry
{ "id": uuid, "entity_type": "expense" | "settlement", "entity_id": uuid, "action": "create" | "update" | "delete",
  "source": "app" | "recurring" | "import", "actor_user_id": uuid, "client_at": ts, "server_at": ts,
  "changes": [ { "field": string, "old": any?, "new": any? }, ... ], "server_version": int }

// Error
{ "code": string, "message": string, "field": string? }

Arrays splits are sorted by user_id, allocations by account_id, default_shares by user_id.

14.3.4 Endpoints

E-1 GET /health (none). 200:

{ "status": "ok", "server_version": "0.1.0", "schema_version": 1, "db_ok": true }

When the database check (SELECT 1) fails: 503 with the same shape, status "degraded", db_ok false.

E-2 GET /setup/status (none). 200 { "initialized": bool }.

E-3 POST /invites/accept (none). Request:

{ "code": "7K3MQX9D2HPA", "device_name": "Galaxy S26", "display_name": "Jacob" }

display_name required for admin_setup and partner (invalid_request if missing), ignored for device. 200:

{ "token": "kvt_…", "user": User, "device": Device, "household": [User, ...] }

Errors: invalid_request, invite_invalid, household_full, validation_failed.

E-4 GET /i/{code} (none). 200 HTML per AC-2.5, for any code string of 1 to 32 characters (validity is not checked). Otherwise 404 HTML.

E-5 GET /apk/latest (none). 200 latest.json content (§10.6). Errors: not_found (nothing published).

E-6 GET /apk/files/{file_name} (none). 200 binary with Content-Length. Errors: not_found.

E-7 GET /me (device). 200:

{ "user": User, "device": Device, "household": [User, ...], "public_url": "https://icehack.asuscomm.com:8443" }

E-8 PATCH /me (device). Request { "display_name": string }. 200 User. Errors: invalid_request, validation_failed.

E-9 PUT /me/picture (device). Raw image body. 200 User (new picture_version). Errors: payload_too_large, unsupported_media_type, image_invalid.

E-10 DELETE /me/picture (device). 200 User (picture_version null). Idempotent.

E-11 GET /users/{user_id}/picture?size=128|512&v=<picture_version> (device). 200 image/jpeg, ETag: "<picture_version>", Cache-Control: private, max-age=31536000, immutable. size defaults to 512; other values invalid_request. v is a cache key only and is ignored. Errors: not_found (unknown user or no picture), invalid_request.

E-12 POST /invites (admin). Request { "kind": "partner" | "device", "target_user_id": uuid? } (target_user_id required for device, must be null for partner). 201 Invite with code (12 characters, no hyphens) and link (<public_url>/i/<code>). Errors: forbidden, invalid_request, household_full, not_found (target user).

E-13 GET /invites (admin). 200 { "invites": [Invite, ...] } (pending only: not accepted, not cancelled, not expired; code and link null). Errors: forbidden.

E-14 DELETE /invites/{invite_id} (admin). 204. Idempotent for already cancelled. Errors: forbidden, not_found.

E-15 GET /devices (device). 200 { "devices": [Device, ...] } ordered by created_at, revoked included (with revoked_at).

E-16 DELETE /devices/{device_id} (device). 204. Revoking an already revoked device is 204. Errors: forbidden, not_found, cannot_revoke_current_device.

E-17 PUT /devices/current/push-token (device). Request { "fcm_token": string? } (null clears). 204. Errors: invalid_request.

E-18 POST /accounts (device). Request:

{ "name": "Vardag", "default_shares": [ { "user_id": "…", "share_bp": 5000 }, { "user_id": "…", "share_bp": 5000 } ] }

201 Account. Errors: invalid_request, validation_failed, name_taken, household_incomplete.

E-19 PATCH /accounts/{account_id} (device). Request { "name"?: string, "default_shares"?: [Share, Share] }. 200 Account. Errors: invalid_request, validation_failed, name_taken, not_found.

E-20 POST /accounts/{account_id}/archive, E-21 POST /accounts/{account_id}/unarchive (device). No body. 200 Account. Idempotent. Errors: not_found.

E-22 DELETE /accounts/{account_id} (device). 200 Account (tombstone). Errors: not_found, account_not_empty.

E-23 POST /categories (device). Request { "name": string, "icon": string, "color": string }. 201 Category. Errors: invalid_request, validation_failed.

E-24 PATCH /categories/{category_id} (device). Request { "name"?: string | null, "icon"?: string, "color"?: string, "sort_order"?: int } (name: null resets a default category's name). 200 Category. Reordering several categories is one PATCH per category whose sort_order changes. Errors: invalid_request, validation_failed, not_found, category_is_default.

E-25 POST /categories/{category_id}/archive, E-26 POST /categories/{category_id}/unarchive (device). 200 Category. Errors: not_found.

E-27 DELETE /categories/{category_id} (device). 200 Category (tombstone). Errors: not_found, category_in_use, category_is_default.

E-28 POST /recurring-templates (device). Request: the RecurringTemplate fields account_id, category_id, amount_ore, description, notes, payer_user_id, split_mode, splits (TemplateSplit, rules of §9.1: none for default; two bp entries for ratio and full; two öre entries summing to the amount for exact), day_of_month, start_period, end_period, paused. 201 RecurringTemplate. If a period is already due (§9.5), the server generates it before responding. Errors: invalid_request, validation_failed (also for an unknown or deleted account_id or category_id, with field), household_incomplete.

E-29 PATCH /recurring-templates/{template_id} (device). Any subset of the E-28 fields; split_mode and splits must be sent together. 200 RecurringTemplate. Errors: invalid_request, validation_failed, not_found.

E-30 DELETE /recurring-templates/{template_id} (device). 200 RecurringTemplate (tombstone). Errors: not_found.

E-31 GET /sync?since=<int>&limit=<int> (device). See §14.4.2. Errors: invalid_request, cursor_ahead.

E-32 POST /sync (device). See §14.4.3. Errors (whole request): invalid_request, batch_too_large, payload_too_large, household_incomplete.

E-33 POST /settlements/proposal (device). Request { "payer_user_id": uuid, "amount_ore": int, "exclude_settlement_id": uuid? }. 200:

{ "allocations": [ { "account_id": "…", "amount_ore": 30000 } ], "excess_ore": 0, "over_settled_account_ids": [] }

Computed with kvitt-domain::propose_allocation over the server's non-deleted rows. Errors: invalid_request, validation_failed, no_accounts.

14.3.5 Examples as contract

M1 writes one JSON example per request and response body above, plus one per error code, in contracts/examples/<E-id>-<name>/{request,response}.json and contracts/examples/errors/<code>.json, and the sync examples of §14.4. The server's tests serialise its responses for the same fixtures and compare; the app's :data tests decode every example into its DTOs and re-encode it, comparing JSON values.

14.4 Sync protocol v1

14.4.1 Principles

• The server is the authority. The phone keeps a full copy and a cursor.
• Every synced row carries server_version from one global sequence (§14.1). The cursor is the highest server_version the phone has applied.
• Phones push only expenses and settlements. Everything else changes through REST and comes back through pull (the REST response is also written locally at once).
• Conflict rule: per row, last write wins on the client updated_at; ties go to the server's copy; deletes are final and always win.
• Pushes are idempotent per mutation_id.
• Tombstones are kept forever on both sides and are synced like any other row.
• A sync cycle is: push all pending rows (in batches), then pull until has_more is false. One cycle at a time per process (a Mutex in the sync engine; WorkManager unique work name kvitt-sync, policy KEEP for periodic and APPEND_OR_REPLACE for one-off).

14.4.2 Pull: GET /sync?since=N&limit=L

Server:

1. If N > current sync_sequence.value: 409 cursor_ahead.
2. Select the L (default 500, max 1000) rows with the lowest server_version > N across users, categories, accounts, recurring_templates, expenses, settlements, audit_log (a UNION ALL of (server_version, table, id) ordered by server_version), including tombstones. An expense with its splits, a settlement with its allocations, an account with its default shares and a template with its splits each count as one row.
3. Respond 200:

{
  "epoch": "30b85fb529aac0016dc91650841005b9",
  "since": 0,
  "next_since": 512,
  "has_more": true,
  "server_time": "2026-10-17T12:00:00.000Z",
  "users": [User],
  "categories": [Category],
  "accounts": [Account],
  "recurring_templates": [RecurringTemplate],
  "expenses": [Expense],
  "settlements": [Settlement],
  "audit_entries": [AuditEntry]
}

next_since is the highest server_version in the page, or N when the page is empty. has_more is true when rows with server_version > next_since exist. Every array is present (possibly empty) and sorted by server_version.

Phone, per page, in one Room transaction:

1. If sync_state.epoch is non-null and differs from epoch: discard the page and run epoch recovery (§14.4.8).
2. Apply arrays in this order: users, categories, accounts, recurring_templates, expenses, settlements, audit_entries.
3. For each row: if the local row exists and has server_version ≥ the incoming one, skip. For expenses and settlements: if the local row is pending, skip (the push will resolve it). Otherwise upsert the row, replace its child rows, set local_updated_at. Audit entries are insert-or-ignore.
4. Set sync_state.cursor = next_since, sync_state.epoch = epoch.

Repeat while has_more. Then set last_success_at.

14.4.3 Push: POST /sync

Request (1 to 100 mutations, body ≤ 1 MiB):

{
  "mutations": [
    {
      "mutation_id": "01929c5e-6c1f-7d3a-9b2e-4f5a6b7c8d9e",
      "entity_type": "expense",
      "entity": {
        "id": "01929c5e-1111-7aaa-8bbb-000000000001",
        "account_id": "…", "category_id": "…", "amount_ore": 41200, "description": "ICA Maxi", "notes": "",
        "expense_date": "2026-10-16", "payer_user_id": "…", "split_mode": "default",
        "splits": [ { "user_id": "…", "share_ore": 20600, "share_bp": 5000 }, { "user_id": "…", "share_ore": 20600, "share_bp": 5000 } ],
        "created_at": "2026-10-16T17:02:11.123Z", "updated_at": "2026-10-16T17:02:11.123Z", "deleted_at": null
      }
    },
    {
      "mutation_id": "…",
      "entity_type": "settlement",
      "entity": {
        "id": "…", "from_user_id": "…", "to_user_id": "…", "amount_ore": 50000, "settlement_date": "2026-10-16", "note": "",
        "allocations": [ { "account_id": "…", "amount_ore": 50000 } ],
        "created_at": "…", "updated_at": "…", "deleted_at": null
      }
    }
  ]
}

The pushed entity contains exactly the client-writable fields: for expenses id, account_id, category_id, amount_ore, description, notes, expense_date, payer_user_id, split_mode, splits, created_at, updated_at, deleted_at; for settlements id, from_user_id, to_user_id, amount_ore, settlement_date, note, allocations, created_at, updated_at, deleted_at. Server-owned fields (server_version, created_by_user_id, updated_by_user_id, recurring_template_id, recurring_period) are ignored if sent. A delete is the full entity with deleted_at set.

Response 200, one result per mutation in request order:

{
  "server_time": "2026-10-17T12:00:00.000Z",
  "results": [
    { "mutation_id": "…", "entity_type": "expense", "entity_id": "…", "status": "accepted", "server_version": 913, "entity": Expense },
    { "mutation_id": "…", "entity_type": "settlement", "entity_id": "…", "status": "rejected", "reason": "stale",
      "message": "a newer version exists", "current": Settlement }
  ]
}

entity and current are full wire objects (§14.3.3) or, for current, null when the server has no row with that id. server_version is the row's version after processing.

14.4.4 Server processing of one mutation

The whole request runs in one BEGIN IMMEDIATE transaction; each mutation in its own SAVEPOINT, rolled back on rejection. Pushes are enqueued after commit. Steps, in order:

1. Replay. If mutation_id exists in sync_mutations, return the stored result unchanged (no other effect).
2. Household. Fewer than two users: the whole request fails with 409 household_incomplete before any mutation.
3. Shape. Missing fields, wrong types, unknown enum values, ids that are not UUIDs: rejected, validation_failed.
4. Clock. updated_at > server now + 10 minutes: rejected, clock_skew.
5. Load the stored row by id.
6. Stored row is a tombstone. If the pushed entity is also a delete: accepted, no change, no audit, no push (idempotent delete). Otherwise rejected, deleted, current = the tombstone.
7. Pushed entity is a delete (deleted_at non-null):
   • Stored row exists: set deleted_at = pushed deleted_at, updated_at = the later of stored and pushed updated_at, keep every other stored field, updated_by_* from the caller, new server_version, audit delete with changes [{"field":"deleted_at","old":null,"new":<deleted_at>}], push *_deleted. accepted. This applies even when the pushed updated_at is older (deletes always win).
   • No stored row (created and deleted before it ever synced): validate references as in step 8; insert the row as a tombstone with a new server_version, no audit, no push. accepted.
8. Validate the entity against §9.6, §9.1 (expenses) or §9.4 settlement validity (settlements), and references: account_id, every allocation account_id, category_id must exist and not be deleted (reference_not_found otherwise; archived is fine); user ids must be household users (validation_failed). Failure: rejected with that reason, current = stored row or null.
9. No stored row: insert with created_by_user_id = updated_by_user_id = caller's user, updated_by_device_id = caller's device, server_received_at = now, new server_version; audit create with every diff field (old null); push *_created. accepted.
10. Stored row exists, pushed updated_at ≤ stored updated_at: rejected, stale, current = stored row.
11. Stored row exists, newer: compute the diff (§14.4.7). Empty diff: change nothing (not even updated_at), no audit, no push, accepted with the stored row and its existing server_version. Otherwise update fields and children, updated_by_* from the caller, new server_version, audit update with the diff, push *_updated. accepted. recurring_template_id, recurring_period and created_by_user_id keep their stored values.
12. Store (mutation_id, device_id, entity_type, entity_id, result JSON, now) in sync_mutations, for accepted and rejected results alike (step 1 then replays it).

14.4.5 Phone push procedure

1. Read pending expenses and settlements ordered by local_updated_at ascending; take up to 100. Build each mutation from the row with mutation_id = pending_mutation_id.
2. POST /sync. On a network error or 5xx: stop the cycle, keep everything pending, let WorkManager retry with exponential backoff starting at 30 seconds. On 401 device_revoked: AC-3.3. On 401 unauthorized: SessionState.Unrecognised (§14.4.8).
3. Apply results in one Room transaction:
   • accepted: if the row's pending_mutation_id still equals the sent mutation_id, overwrite the local row with entity, set pending = false, pending_mutation_id = null. If it differs (edited again during the request), set only server_version, created_by_user_id, updated_by_user_id from entity and leave it pending.
   • rejected with current equal to the pushed entity in every client-writable field: treat as accepted with current (no rejection record). This covers re-pushes of an already stored state.
   • other rejected: if pending_mutation_id still matches, overwrite the local row with current (or delete the local row and its children if current is null), set pending = false, and insert a sync_rejections row with the pushed entity as rejected_payload. If it no longer matches, insert the rejection record only and leave the newer local edit pending. Reason clock_skew: do not overwrite and do not clear pending; set sync_state.clock_skew = true and stop the cycle.
4. Repeat until no pending rows remain or a batch made no progress, then pull.

14.4.6 Rejection reasons

14.4.7 Audit diff

Diff fields and their JSON value types:

• create: one change per field above except deleted_at, old null, in the table's order.
• update: one change per field whose JSON value differs, in the table's order.
• delete: exactly the deleted_at change.
• client_at = the pushed updated_at (for delete, the pushed deleted_at); server_at = server now. Scheduler and importer use server now for both.

The phone renders changes with ids resolved to names (FR-12).

14.4.8 Epoch recovery and re-pairing

• sync_epoch changes only when the owner runs rotate-epoch after a restore (§10.9).
• When a pull returns a different epoch, the phone, in one transaction: sets cursor = 0, stores the new epoch, and marks every local expense and settlement (tombstones included) pending with a new pending_mutation_id, keeping updated_at. It then runs a normal cycle: push everything (rows the restored server lacks are inserted, rows it has with older updated_at are updated, identical rows come back as stale with equal content and are silently accepted per §14.4.5), then a full pull.
• Devices paired after the restored backup do not exist on the restored server; their requests get 401 unauthorized. The app keeps its local data and shows S-1 with "Pair this phone again". After pairing with a new invite: if the user id equals the previous one, the local data is kept and the epoch-recovery push runs; otherwise local data is wiped first.

14.4.9 Limits and timing

With these values a phone offline for 14 days loses nothing: tokens do not expire, cursors stay valid (versions only grow), and idempotency records outlive any realistic retry.

14.5 Domain rules as pure functions

Implemented twice: Kotlin in :domain (package se.hagstedt.kvitt.domain, input and output types in :core:model package se.hagstedt.kvitt.model.money) and Rust in kvitt-domain. Both are pure (no I/O, no clock, no randomness, no locale) and must pass every vector in contracts/test-vectors/. Analytics exists only in Kotlin. The rules themselves are §9.1, §9.3 and §9.4; this section fixes signatures, error tokens and the vector format.

14.5.1 Types

Ids are plain strings in domain inputs (the vectors use short ids like "a", "b", "x"; implementations must not assume UUID format). Money is Long / i64 öre; bp is Int / u32.

Kotlin (:core:model, package se.hagstedt.kvitt.model.money):

sealed interface SplitSpec {
    data class Ratio(val bp: Map<String, Int>) : SplitSpec      // modes default and ratio
    data class Exact(val ore: Map<String, Long>) : SplitSpec    // mode exact
    data class Full(val bearerUserId: String) : SplitSpec       // mode full
}
enum class SplitError { AMOUNT_OUT_OF_RANGE, USERS_INVALID, PAYER_UNKNOWN, BP_OUT_OF_RANGE, BP_SUM_NOT_10000, EXACT_NEGATIVE, EXACT_SUM_MISMATCH, BEARER_UNKNOWN, SHARE_USERS_MISMATCH }
sealed interface SplitResult {
    data class Ok(val shares: Map<String, Long>) : SplitResult
    data class Err(val error: SplitError) : SplitResult
}

data class BalanceExpense(val accountId: String, val amountOre: Long, val payerUserId: String, val shares: Map<String, Long>)
data class BalanceSettlement(val fromUserId: String, val toUserId: String, val allocations: Map<String, Long>)   // accountId -> öre
data class Balances(val perAccount: Map<String, Map<String, Long>>, val global: Map<String, Long>)              // accountId -> userId -> net

data class AccountPosition(val accountId: String, val createdAt: String, val archived: Boolean, val payerNetOre: Long)
enum class AllocationError { AMOUNT_OUT_OF_RANGE, NO_ACCOUNTS }
data class Allocation(val accountId: String, val amountOre: Long)
sealed interface AllocationResult {
    data class Ok(val allocations: List<Allocation>, val excessOre: Long, val overSettledAccountIds: List<String>) : AllocationResult
    data class Err(val error: AllocationError) : AllocationResult
}

enum class SettlementError { AMOUNT_OUT_OF_RANGE, SAME_USER, USER_UNKNOWN, NO_ALLOCATIONS, TOO_MANY_ALLOCATIONS, ALLOCATION_NOT_POSITIVE, DUPLICATE_ACCOUNT, ALLOCATION_SUM_MISMATCH }

Rust (kvitt-domain):

pub enum SplitSpec { Ratio { bp: BTreeMap<String, u32> }, Exact { ore: BTreeMap<String, i64> }, Full { bearer_user_id: String } }
#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)] #[serde(rename_all = "snake_case")]
pub enum SplitError { AmountOutOfRange, UsersInvalid, PayerUnknown, BpOutOfRange, BpSumNot10000, ExactNegative, ExactSumMismatch, BearerUnknown, ShareUsersMismatch }

pub struct BalanceExpense { pub account_id: String, pub amount_ore: i64, pub payer_user_id: String, pub shares: BTreeMap<String, i64> }
pub struct BalanceSettlement { pub from_user_id: String, pub to_user_id: String, pub allocations: BTreeMap<String, i64> }
pub struct Balances { pub per_account: BTreeMap<String, BTreeMap<String, i64>>, pub global: BTreeMap<String, i64> }

pub struct AccountPosition { pub account_id: String, pub created_at: String, pub archived: bool, pub payer_net_ore: i64 }
pub enum AllocationError { AmountOutOfRange, NoAccounts }
pub struct Allocation { pub account_id: String, pub amount_ore: i64 }
pub struct AllocationProposal { pub allocations: Vec<Allocation>, pub excess_ore: i64, pub over_settled_account_ids: Vec<String> }

pub enum SettlementError { AmountOutOfRange, SameUser, UserUnknown, NoAllocations, TooManyAllocations, AllocationNotPositive, DuplicateAccount, AllocationSumMismatch }

14.5.2 Functions

Semantics beyond §9:

• computeSplit: userIds must have exactly 2 distinct entries (users_invalid); payerUserId must be one of them (payer_unknown); the Ratio/Exact map keys must equal userIds as a set (share_users_mismatch); Full.bearerUserId must be one of them (bearer_unknown); amount outside 1 to 1 000 000 000 is amount_out_of_range. Errors are checked in the order of the SplitError enum and the first failure is returned. The result map has both user ids. Full is computed as Ratio with bearer 10000 (so payer remainder rules apply trivially).
• computeBalances: inputs contain only non-deleted rows. perAccount has an entry for every id in accountIds (zeros included) and for any other account id referenced; each inner map has every id of userIds. global sums perAccount. Expenses' and settlements' account ids not in accountIds still count.
• proposeAllocation: accounts are all non-deleted accounts with the payer's net per account (callers compute nets with computeBalances, excluding the settlement being edited). Errors in enum order. Allocations are in the order produced by §9.4. overSettledAccountIds lists the account that received excessOre (empty when excessOre is 0).
• validateSettlement: errors in enum order; null / Ok(()) when valid. Reference existence is not checked here (the server checks it, §14.4.4).
• overSettledAccounts: ids, in allocation order, whose allocation exceeds max(0, −payerNetOre); an allocation for an account not in accounts counts as net 0.

14.5.3 Error tokens on the wire and in vectors

Snake-case of the enum names: amount_out_of_range, users_invalid, payer_unknown, bp_out_of_range, bp_sum_not_10000, exact_negative, exact_sum_mismatch, bearer_unknown, share_users_mismatch, no_accounts, same_user, user_unknown, no_allocations, too_many_allocations, allocation_not_positive, duplicate_account, allocation_sum_mismatch. The server puts the token in the message of validation_failed errors and rejections; the phone uses it as FieldError.error.

14.5.4 Analytics (Kotlin only)

data class AnalyticsExpense(val date: LocalDate, val accountId: String, val categoryId: String, val amountOre: Long, val shares: Map<String, Long>)
data class DateRange(val start: LocalDate, val endInclusive: LocalDate)
enum class AnalyticsGranularity { MONTH, WEEK }
sealed interface AnalyticsMeasure { data object Household : AnalyticsMeasure; data class Share(val userId: String) : AnalyticsMeasure }
enum class AnalyticsGroupBy { CATEGORY, ACCOUNT }
data class Bucket(val key: String, val start: LocalDate, val endInclusive: LocalDate, val totals: Map<String, Long>, val total: Long)
data class SpendSeries(val buckets: List<Bucket>, val groupTotals: Map<String, Long>, val grandTotal: Long)

fun spendSeries(expenses: List<AnalyticsExpense>, range: DateRange, granularity: AnalyticsGranularity, measure: AnalyticsMeasure, groupBy: AnalyticsGroupBy): SpendSeries
fun resolveRange(preset: RangePreset, today: LocalDate, earliestExpense: LocalDate?, customStart: LocalDate?, customEnd: LocalDate?): DateRange

(LocalDate is kotlinx.datetime.LocalDate.)

• spendSeries: expenses outside the range are ignored. Buckets cover the range completely, in order, including empty ones. Month keys YYYY-MM, week keys YYYY-Www (ISO week-numbering year, week 1 contains the first Thursday). A bucket's start/endInclusive are clipped to the range. totals maps group id (category id or account id) to öre, omitting zero groups; total is their sum. Measure Household adds amountOre; Share(u) adds shares[u] (0 if absent). groupTotals sums over all buckets, omitting zeros.
• resolveRange: THIS_MONTH = first of the current month to today; LAST_3_MONTHS, LAST_6_MONTHS, LAST_12_MONTHS = first day of the month N−1 months before the current month to today; THIS_YEAR = 1 January to today; ALL_TIME = earliestExpense (or today when null) to today; CUSTOM = the custom dates, swapped if start > end. Expenses dated after today are therefore excluded from presets; a custom range may include future dates.

14.5.5 Test-vector format

One file per function in contracts/test-vectors/: compute_split.json, compute_balances.json, propose_allocation.json, validate_settlement.json, over_settled_accounts.json, spend_series.json (Kotlin only), resolve_range.json (Kotlin only).

{
  "format": 1,
  "function": "compute_split",
  "cases": [
    {
      "name": "50/50 odd amount, payer gets the extra öre",
      "input": {
        "amount_ore": 1001, "payer_user_id": "a", "user_ids": ["a", "b"],
        "spec": { "mode": "ratio", "bp": { "a": 5000, "b": 5000 } }
      },
      "expected": { "ok": { "a": 501, "b": 500 } }
    },
    {
      "name": "ratio does not sum to 10000",
      "input": {
        "amount_ore": 1000, "payer_user_id": "a", "user_ids": ["a", "b"],
        "spec": { "mode": "ratio", "bp": { "a": 5000, "b": 4000 } }
      },
      "expected": { "error": "bp_sum_not_10000" }
    }
  ]
}

Input keys are the snake_case parameter names of the Rust signatures. spec is {"mode":"ratio","bp":{…}}, {"mode":"exact","ore":{…}} or {"mode":"full","bearer_user_id":"…"}. Outputs: compute_split {"ok": {user: öre}}; compute_balances {"ok": {"per_account": {...}, "global": {...}}}; propose_allocation {"ok": {"allocations": [{"account_id","amount_ore"}], "excess_ore": n, "over_settled_account_ids": [...]}}; validate_settlement {"ok": null}; over_settled_accounts {"ok": [ids]}; spend_series {"ok": {"buckets": [{"key","start","end_inclusive","totals","total"}], "group_totals": {...}, "grand_total": n}} with measure input "household" or {"share": "a"}, and dates as strings; resolve_range {"ok": {"start","end_inclusive"}}. Errors: {"error": "<token>"}.

Minimum cases M1 must include (more are welcome):

• compute_split: every row of the §9.1 example table; full on payer; full on non-payer; exact valid; exact sum mismatch; exact negative; amount 0; amount 1 000 000 001; amount 1 000 000 000 with 3333 bp; bp 10001; three user ids; payer not in users; map keys not matching users; one error case per SplitError value.
• compute_balances: empty; one account with the two expenses (1001, payer a, 50/50) and (100, payer b, a 6000 bp) giving a +440, b −440, then adding settlement b → a 440 giving zeros; two accounts with opposite signs and a global sum; an account in account_ids with no rows (zeros).
• propose_allocation: payer owes in accounts X (created 2025-01-01T00:00:00.000Z, net −30000), Y (2025-02-01…, −20000) and is owed in Z (2024-12-01…, +10000): amount 40000 gives X 30000, Y 10000, excess 0; amount 60000 gives X 30000, Y 30000, excess 10000, over-settled [Y]; amount 5000 with only Z gives Z 5000 over-settled [Z]; all accounts archived and none owing gives the oldest; equal created_at ordered by id; no accounts gives no_accounts; amount 0 gives amount_out_of_range.
• validate_settlement: one case per SettlementError value plus a valid case with 3 allocations and one with 50.
• over_settled_accounts: none, one, an unknown account.
• spend_series: monthly and weekly over a range crossing a year boundary (ISO week 2026-W53 exists: 28 Dec 2026 to 3 Jan 2027), household and share measures, category and account grouping, empty buckets, edge-clipped buckets.
• resolve_range: every preset with today = 2026-10-17, ALL_TIME with and without an earliest expense, CUSTOM swapped.

14.6 Push payload v1

FCM HTTP v1 message the server sends, one per target device (POST https://fcm.googleapis.com/v1/projects/kvitt-1337/messages:send):

{
  "message": {
    "token": "<device fcm_token>",
    "android": { "priority": "high", "ttl": "86400s" },
    "data": {
      "v": "1",
      "type": "expense_created",
      "entity_type": "expense",
      "entity_id": "01929c5e-1111-7aaa-8bbb-000000000001",
      "actor_user_id": "0192…",
      "count": "1"
    }
  }
}

Data-only messages (no notification block), all values strings. Types (exhaustive):

Server sending rules:

• Sending happens on a background task fed by a bounded queue (capacity 100; when full, the oldest message is dropped with a warning log). It never delays an HTTP response.
• Retries: 3 attempts with delays 1 s, 5 s, 25 s on 429, 5xx and network errors. FCM UNREGISTERED or HTTP 404 for a token clears that device's fcm_token.
• Push is disabled (with a start-up warning) when the credentials file is missing.

Phone handling (:adapter:fcm maps the message to PushEvent(type, entityType, entityId, actorUserId, count) in :core:model; :app handles it):

1. Enqueue an expedited sync and wait for it up to 20 seconds.
2. Build the notification from local data per §13.3 copy (or the generic text if the sync failed or the entity is missing). Channel id activity, name "Activity" / "Aktivitet", importance default. Notification id = hash of entity_id (so an update replaces the create's notification); sync_batch uses a fixed id.
3. Tap opens S-8 or S-10 for the entity, Home for deletions and sync_batch.
4. Unknown type or v other than 1: sync only, no notification.

14.7 Module layout and ownership map

Repository layout (paths are binding):

SPEC.md  CLAUDE.md  README-agents.md  PLAN-*.md
contracts/
  openapi.yaml
  schema/server-v1.sql
  test-vectors/*.json
  examples/<E-id>-<name>/{request,response}.json, examples/errors/<code>.json, examples/sync/*.json, examples/push/*.json
docs/  HANDOFF.md  decisions.md  ux/  spikes/  validation/
prompts/
tools/  gradle-serial.sh  clean-builds.sh  install-git-hooks.sh  git-hooks/  dependency-report/  contracts-check.py
server/
  Cargo.toml (workspace)  Cargo.lock  Dockerfile  rust-toolchain.toml
  crates/kvitt-domain/  src/{lib.rs, split.rs, balance.rs, allocation.rs, settlement.rs}  tests/vectors.rs
  crates/kvitt-server/  migrations/0001_init.sql
                        src/{main.rs, lib.rs, config.rs, error.rs, router.rs, openapi.rs, auth.rs, time.rs}
                        src/db/{mod.rs, pool.rs, sequence.rs}
                        src/wire/{mod.rs, user.rs, device.rs, invite.rs, account.rs, category.rs, expense.rs, settlement.rs, recurring.rs, audit.rs, sync.rs}
                        src/api/{mod.rs, health.rs, setup.rs, invites.rs, landing.rs, me.rs, pictures.rs, devices.rs, accounts.rs, categories.rs, recurring.rs, proposal.rs, apk.rs, sync.rs}
                        src/ratelimit.rs
                        src/sync/  src/audit/  src/push/  src/scheduler/
                        src/cli/{mod.rs, bootstrap.rs, pair_device.rs, rotate_epoch.rs, healthcheck.rs}
                        tests/
  crates/kvitt-import/  src/  tests/fixtures/
android/
  settings.gradle.kts  build.gradle.kts  gradle.properties  gradle/{libs.versions.toml, gradle-daemon-jvm.properties, wrapper/}
  build-logic/          (convention plugins, architecture check)
  core/model/  core/design/  core/fakes/  domain/  data/  adapter/fcm/
  feature/{onboarding, home, account, expense, settle, analytics, settings}/
  app/
infra/
  compose.yaml  Caddyfile  env.example  build-images.sh  deploy.sh  publish-apk.sh  README.md  RESTORE.md
  cert-sync/{Dockerfile, sync.sh}  backup/{Dockerfile, backup.sh}  secrets/ (git-ignored)

Ownership (an agent edits only paths its milestone owns; anything else needs the orchestrator):

Strings: each feature module has its own src/main/res/values/strings.xml and values-sv/strings.xml with keys prefixed by the feature name; shared strings live in :core:design. Keys come from docs/ux/strings.csv (§13.4).

14.8 Contract files and the change rule

Contract files (exhaustive): contracts/openapi.yaml, contracts/schema/server-v1.sql and any later server-vN.sql, contracts/test-vectors/*.json, contracts/examples/**, the Room exported schema android/data/schemas/**, android/core/model/** (the UI contract and domain types), the sync protocol (§14.4) and the push payload (§14.6).

Rule: a change to any contract file, or to §14 of this spec, requires in the same change: a spec version bump (in the header and the change log), a docs/decisions.md entry, and the matching implementation changes on both sides, or a plan that schedules them. A schema change also bumps the schema version (meta.schema_version and a new migration on the server; a new Room version and migration on the phone). A sync protocol change bumps X-Kvitt-Protocol; a push payload change bumps v. Numbers come from the orchestrator's brief.

---

15. Change log

Id	Screen	Must show	Actions
S-1	Pairing	App name and one-line purpose; server URL field (prefilled); invite code field (12 characters, shown in groups of 4); explanation of where the code comes from; progress while pairing; errors invite_invalid, network error, rate_limited	Pair; paste code
S-2	Partner profile step	Display name field (prefilled), avatar with "Add picture"; the owner's name and avatar ("You are joining Jacob")	Continue
S-3	Waiting for partner	Explanation that the partner must join first; the pending invite's expiry if any	Invite partner (goes to S-21); Settings
S-4	Home	Global balance header: both avatars, wording per §9.3, amount; "Settle up" button when non-zero; sync line: "Last synced …" and pending count when > 0, tap opens S-16; update banner (S-17) when available; list of active accounts, each with name, balance wording and amount, number of expenses this month; collapsed "Archived (n)" section; empty state when no accounts ("Create your first account")	Add expense (FAB); Settle up; open account; New account
S-5	Account detail	Account name, default split ("50 / 50" with names), balance header; list of expenses and settlements interleaved by date, grouped by month with month totals (household spend), each expense row per AC-8.10, each settlement row per AC-11.7; archived badge if archived; pending-sync marker per row	Add expense; Settle this account; Edit account; Archive / Unarchive; Delete (only if allowed)
S-6	Account editor	Name field; default split control (one slider or stepper plus two percent fields that always sum to 100, two decimals allowed); preview "Of 100,00 kr: Jacob 60,00 kr, Anna 40,00 kr"; "Needs connection" state	Save; Cancel
S-7	Expense editor	Amount entry (large, numeric keyboard, kr); description; account picker (active accounts); category picker (grid of icon + colour + name, active categories); date (today default, date picker); payer toggle (two avatars: "Paid by"); split section: mode chooser (default "Account default 60/40", ratio "Custom ratio", exact "Exact amounts", full "All on one person") and its controls; live preview of each person's share; notes (collapsed by default); validation errors inline; delete in edit mode	Save; Cancel; Delete (edit mode)
S-8	Expense detail	Amount, description, category, account, date, payer, each user's share and the split mode, notes, "Recurring: " when generated; timeline (AC-12.4, AC-12.5); "deleted" state when opened from a notification of a deletion	Edit; Delete	S-9	Settle up	Direction ("Anna pays Jacob" with a swap control); amount; allocation list: account name, account debt, editable allocated amount, over-settle warning per row (AC-11.5); remaining-to-allocate indicator; "Add account"; date; note; per-account variant with a single fixed row	Confirm; Cancel	S-10	Settlement detail	Payer, recipient, amount, date, note, allocations per account; timeline	Edit; Delete	S-11	Analytics	Range selector (presets of AC-14.2 plus custom); granularity toggle month/week; measure toggle household / my share; group-by toggle category/account; total for range; chart 1 stacked bars over time with legend; chart 2 category breakdown; chart 3 account breakdown; empty state ("No expenses in this period")	Change filters; tap a bar or a row to open S-12	S-12	Analytics drill-down	Title naming the filter ("Mat & hushåll, oktober 2026"); total; expense rows as in S-5	Open expense	S-13	Settings hub	Own avatar and name; partner's avatar and name; rows: Profile, Categories, Recurring expenses, Devices, Invite partner or Pair a device (admin), Sync status, About	Navigate	S-14	Profile	Avatar with change/remove; display name; "Needs connection" state	Save	S-15	Categories	List: icon, colour, name, archived section; editor: name (with "Reset name" for defaults), icon grid (30 icons of §9.7), colour swatches (12 tokens); delete or archive with the reason when delete is not allowed	Add; Edit; Reorder (drag); Archive; Delete	S-16	Sync status	Last successful sync time; pending changes count with a list; "Changes not synced" list from sync_rejections with what was lost and why (copy per §14.4.6 reasons); clock-skew warning; "Sync now"	Sync now; open a rejected change (read-only); dismiss it	S-17	Update available	Banner on Home with version name; dialog explaining "Install unknown apps" the first time; download progress; failure	Update; Later	S-18	Device removed	Explanation that this phone's access was removed and local data deleted	Pair again (goes to S-1)	S-19	Recurring expenses	List: description, amount, account, "Every month on day 25", next date, paused badge; editor with the expense fields of S-7 (no date) plus day of month, first month, optional last month, pause switch	Add; Edit; Pause/Resume; Delete	S-20	Devices	Rows per AC-3.1	Revoke (with confirmation)	S-21	Invites	For the admin: "Invite partner" (only before the partner joins) and "Pair another device" for either user; a created invite shows the code in groups of 4, the link, expiry, and a share action (Android share sheet); list of pending invites with cancel	Create; Share; Cancel	S-22	About	App version name and code, server URL, server version (from /health), open-source licences	Check for update	Type	Title	Body	expense_created	" added an expense"	" · · "	expense_updated	" changed an expense"	" · "	expense_deleted	" deleted an expense"	" · "	settlement_created	" recorded a payment"	" paid "	settlement_updated	" changed a payment"	" paid "	settlement_deleted	" deleted a payment"	""	recurring_expense_created	"Recurring expense added"	" · · "	sync_batch	" made changes"	"Open Kvitt to see them"	any, when the sync failed	" changed something in Kvitt" (or "Kvitt" without actor name)	"Open Kvitt to see it"	Entity class	Table	Columns (server columns unless marked local)	Primary key	Indices	UserEntity	users	id, role, display_name, picture_version?, created_at, updated_at, server_version, local local_updated_at	id	none	AccountEntity	accounts	id, name, created_by_user_id, created_at, updated_at, archived_at?, deleted_at?, server_version, local local_updated_at	id	created_at	AccountDefaultShareEntity	account_default_shares	account_id, user_id, share_bp	(account_id, user_id)	none	CategoryEntity	categories	id, default_key?, name?, icon, color, sort_order, created_at, updated_at, archived_at?, deleted_at?, server_version, local local_updated_at	id	none	RecurringTemplateEntity	recurring_templates	every server column, plus next_occurrence_date? (from the wire), local local_updated_at	id	account_id	RecurringTemplateSplitEntity	recurring_template_splits	template_id, user_id, share_bp?, share_ore?	(template_id, user_id)	none	ExpenseEntity	expenses	id, account_id, category_id, amount_ore, description, notes, expense_date, payer_user_id, split_mode, recurring_template_id?, recurring_period?, created_by_user_id? (null until the server accepted the create), updated_by_user_id?, created_at, updated_at, deleted_at?, server_version? (null until accepted); local pending, pending_mutation_id?, local_updated_at	id	account_id, expense_date, pending	ExpenseSplitEntity	expense_splits	expense_id, user_id, share_ore, share_bp?	(expense_id, user_id)	none	SettlementEntity	settlements	id, from_user_id, to_user_id, amount_ore, settlement_date, note, created_by_user_id?, updated_by_user_id?, created_at, updated_at, deleted_at?, server_version?; local pending, pending_mutation_id?, local_updated_at	id	settlement_date, pending	SettlementAllocationEntity	settlement_allocations	settlement_id, account_id, amount_ore	(settlement_id, account_id)	account_id	AuditEntryEntity	audit_log	id, entity_type, entity_id, action, source, actor_user_id, client_at, server_at, changes (JSON text as received), server_version	id	(entity_type, entity_id)	Entity class	Table	Columns	Primary key	SyncStateEntity	sync_state	id (always 1), cursor Long (last applied next_since, 0 initially), epoch String?, last_success_at String?, last_attempt_at String?, last_error_code String? (an error code of §14.3.2 or network), clock_skew Boolean	id	SyncRejectionEntity	sync_rejections	mutation_id, entity_type, entity_id, reason (§14.4.6), message, rejected_payload (JSON of the entity as pushed), server_current (JSON or null), rejected_at String, dismissed Boolean	mutation_id	code	HTTP	Meaning	invalid_request	400	Malformed JSON, missing required field, wrong type, bad query parameter, empty PATCH	invite_invalid	400	Invite code unknown, expired, used, cancelled or malformed	unauthorized	401	Missing, malformed or unknown bearer token	device_revoked	401	The token's device is revoked	forbidden	403	Role does not allow the action (§8.4), or editing another user's profile	not_found	404	Unknown or deleted id in the path; unknown APK file; no APK published; no picture	household_full	409	Partner invite or accept when two users exist	household_incomplete	409	Account, template, expense or settlement write before the partner joined	name_taken	409	Account name already used by a non-deleted account	account_not_empty	409	Account delete while referenced (AC-6.5)	category_in_use	409	Category delete while referenced (AC-7.6)	category_is_default	409	Delete of a default category, or name: null on a non-default category	cannot_revoke_current_device	409	Revoke of the calling device	no_accounts	409	Settlement proposal with no non-deleted accounts	cursor_ahead	409	GET /sync with since greater than the server's current sequence value	payload_too_large	413	Body over the limit of §8.6	unsupported_media_type	415	Picture with another content type	validation_failed	422	A value breaks §9.6 or a split/settlement rule	image_invalid	422	Picture cannot be decoded or exceeds 8000 × 8000	batch_too_large	422	More than 100 mutations in POST /sync	protocol_unsupported	426	Missing or unsupported X-Kvitt-Protocol	rate_limited	429	§8.6, with Retry-After	internal	500	Unexpected server error (logged with a request id, returned in message)	not_implemented	501	Route registered but not yet implemented (only during milestones M2a to M4)	unavailable	503	Database busy beyond busy_timeout, or server shutting down	reason (exhaustive)	When	Phone's user-facing explanation (S-16)	stale	Someone saved a newer version first	"Your change was replaced by a newer change from "	deleted	The row was deleted	"This was deleted before your change arrived"	validation_failed	The row breaks a rule (should not happen with a correct phone)	"This change was not valid and was not saved"	reference_not_found	Its account or category was deleted	"Its account or category no longer exists"	clock_skew	Phone clock more than 10 minutes ahead	"Your phone's clock is wrong. Fix it and sync again"	Entity	Field	Value	expense	account_id, category_id, payer_user_id	string (uuid)	expense	amount_ore	int	expense	description, notes	string	expense	expense_date	string (date)	expense	split_mode	string (enum)	expense	splits	array of Split sorted by user_id	settlement	from_user_id, to_user_id	string (uuid)	settlement	amount_ore	int	settlement	settlement_date	string (date)	settlement	note	string	settlement	allocations	array of Allocation sorted by account_id	both (delete only)	deleted_at	string (ts)	Item	Value	Mutations per push	100	Rows per pull page	default 500, max 1000	Debounce after a local write	2 seconds	Periodic background sync	15 minutes (WorkManager minimum), requires a connected network	FCM-triggered sync	expedited one-off work	Retry backoff	exponential from 30 seconds (WorkManager)	sync_mutations retention	90 days	Tombstone retention	forever	Token lifetime	until revoked	Notification sync wait (AC-13.5)	20 seconds	Function	Kotlin	Rust	Split	fun computeSplit(amountOre: Long, payerUserId: String, userIds: List<String>, spec: SplitSpec): SplitResult	pub fn compute_split(amount_ore: i64, payer_user_id: &str, user_ids: &[String], spec: &SplitSpec) -> Result<BTreeMap<String, i64>, SplitError>	Balances	fun computeBalances(userIds: List<String>, accountIds: List<String>, expenses: List<BalanceExpense>, settlements: List<BalanceSettlement>): Balances	pub fn compute_balances(user_ids: &[String], account_ids: &[String], expenses: &[BalanceExpense], settlements: &[BalanceSettlement]) -> Balances	Proposal	fun proposeAllocation(amountOre: Long, accounts: List<AccountPosition>): AllocationResult	pub fn propose_allocation(amount_ore: i64, accounts: &[AccountPosition]) -> Result<AllocationProposal, AllocationError>	Settlement check	fun validateSettlement(amountOre: Long, fromUserId: String, toUserId: String, userIds: List<String>, allocations: List<Allocation>): SettlementError?	pub fn validate_settlement(amount_ore: i64, from_user_id: &str, to_user_id: &str, user_ids: &[String], allocations: &[Allocation]) -> Result<(), SettlementError>	Over-settle warnings	fun overSettledAccounts(allocations: List<Allocation>, accounts: List<AccountPosition>): List<String>	pub fn over_settled_accounts(allocations: &[Allocation], accounts: &[AccountPosition]) -> Vec<String>	type	entity_type	entity_id	actor_user_id	count	Sent to	expense_created	expense	expense id	creator	1	other user's devices	expense_updated	expense	expense id	editor	1	other user's devices	expense_deleted	expense	expense id	deleter	1	other user's devices	settlement_created	settlement	settlement id	creator	1	other user's devices	settlement_updated	settlement	settlement id	editor	1	other user's devices	settlement_deleted	settlement	settlement id	deleter	1	other user's devices	recurring_expense_created	expense	expense id	template creator	1	both users' devices	sync_batch	""	""	pusher	number of notifiable changes, as a string	other user's devices	Paths	Owner	After the owner lands	SPEC.md, CLAUDE.md, README-agents.md, docs/HANDOFF.md, docs/decisions.md, PLAN-*.md, prompts/, tools/ (except tools/contracts-check.py)	docs agent briefed by the orchestrator	same	contracts/**, tools/contracts-check.py	M1	contract-change plans only (§14.8)	server/Cargo.toml, server/rust-toolchain.toml, server/Dockerfile, server/crates/kvitt-domain/**	M2a	contract-change plans only for kvitt-domain; M3, M4 and M9 may add a dependency line to a crate's Cargo.toml (second to land resolves Cargo.lock on rebase)	kvitt-server: migrations/, src/{main,lib,config,error,router,openapi,auth,time}.rs, src/db/**, src/wire/**, src/api/{mod,health,setup}.rs, src/cli/{mod,healthcheck}.rs	M2a	contract-change plans; M3/M4 may add one line per new module to mod.rs files they need	kvitt-server: src/api/{invites,landing,me,pictures,devices,accounts,categories,recurring,proposal,apk}.rs, src/ratelimit.rs, src/cli/{bootstrap,pair_device}.rs, tests/core_*.rs	M3	later server plans	kvitt-server: src/api/sync.rs, src/sync/**, src/audit/**, src/push/**, src/scheduler/**, src/cli/rotate_epoch.rs, tests/sync_*.rs, tests/scheduler_*.rs, tests/push_*.rs	M4	later server plans	server/crates/kvitt-import/**	M9	same	android/settings.gradle.kts, android/build.gradle.kts, android/gradle.properties, android/gradle/**, android/build-logic/**	M2b	later plans may add catalog entries and module includes (rebase resolves)	android/core/model/**, android/domain/**	M2b	contract-change plans only	android/core/fakes/**	M2b	M7x add files only under fakes/<feature>/	android/core/design/**	M6	M7x may not edit; requests go through the orchestrator	android/data/**	M5	later data plans	android/adapter/fcm/**, android/app/** (after the M2b shell)	M8	later integration plans	android/feature/onboarding/**	M7a		android/feature/home/**, android/feature/account/**	M7b		android/feature/expense/**	M7c		android/feature/settle/**	M7d		android/feature/analytics/**	M7e		android/feature/settings/**	M7f		infra/**	M2c	M10 updates runbooks	docs/ux/**	U1, U2		docs/validation/<file>	the milestone that names the file		Version	Date	Change	0.1	2026-10-17	First version, written from the owner interview: scope, requirements, data model, stack, API outline, security, business rules, deployment, milestones, UX sprint brief, contracts (schema v1, Room v1, OpenAPI outline, sync protocol v1, domain functions and vectors, push payload v1, ownership map).