Take the time off together, in long stretches

A season preset in a two-parent household now means both parents, not
either one — being off together is the point of the period.

Within such a period the planner used to pick whoever had the most
parental-benefit days left, date by date, so spending kept moving the
lead and the funding flipped almost daily. Six weeks came out as a
patchwork that costs nothing to hold but a great deal to request. The
payer is now chosen once per half of the window, reusing the same split
the swap strategy already uses: one parent on parental benefit while the
other takes vacation, then they change over. Both are still off every
day, and if one runs out mid-stretch the roles swap for the rest rather
than day by day.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ei7gCZzdd751uZtPzb1Pep

backend/app/services/allocation_service.py

@@ -968,6 +968,29 @@ class _Ledger:
         else:
             pool.lagsta_left -= 1
 
+    def _try_co_leave_pair(
+        self,
+        fp_parent: ParentFacts,
+        fp_pool: _MutablePool,
+        vacation_parent: ParentFacts,
+        d: date,
+    ) -> tuple[ParentFacts, str, ParentFacts] | None:
+        """§fix (long-chunk co-leave, 2026-09-19) — peek-then-commit a single
+
+        co-leave pairing (one parent on FP, the other on vacation) for `d`.
+        Returns `None` (spending NEITHER side) unless both can actually pay
+        — used by `_fund_both` to try its chunk-designated pairing first
+        and the swapped pairing as a same-date fallback.
+        """
+        fp_tier = self._peek_fp_tier(fp_pool)
+        vacation_budget = self._vacation.get((vacation_parent.uuid, d.year))
+        vacation_can_pay = vacation_budget is not None and vacation_budget.available > 0
+        if fp_tier is None or not vacation_can_pay:
+            return None
+        self._commit_fp_tier(fp_pool, fp_tier)
+        self._spend_vacation(vacation_parent.uuid, d)
+        return fp_parent, fp_tier, vacation_parent
+
     def _maybe_clamp_age4(self, pool_key: tuple[UUID, UUID] | None, window_start: date) -> None:
         """§9.1: from the 4th birthday onward, the FP pool is hard-capped at
         min(remaining, `self._input.age4_savable_cap`). Applied once per
@@ -1219,13 +1242,22 @@ class _Ledger:
              both parents currently hold FP (peeked, then committed
              together — never a lone half-spend).
           2. Co-leave (PL + vacation) — the default for every other date:
-             the parent with the MOST remaining FP (`sjukpenning_left +
-             lagsta_left`, at the live, mutating balance — ties go to
-             `responsible.parents[0]`, deterministic since that list order
-             is itself deterministic) takes an FP day (sjukpenning
-             preferred); the other parent takes a vacation day, same date.
-             Peeked-then-committed together too: if either side can't pay,
-             NEITHER is spent for that date.
+             which parent PAYS with FP is decided per CHUNK, not per day —
+             §fix (long-chunk co-leave, 2026-09-19). The window's dates are
+             split into two contiguous halves with `_split_dates_by_
+             spendable_cost` (the same balanced-by-cost split `_fund_swap`
+             uses); `responsible.parents[0]` is the designated FP-payer for
+             the first half, `responsible.parents[1]` for the second — so a
+             long window reads as "parent A on PL while parent B is on
+             vacation for ~half the window, then they switch", not a
+             daily-alternating patchwork. Both parents are still off on
+             EVERY date regardless of chunk. If the chunk's designated
+             FP-payer has no FP left for a date (or their partner has no
+             vacation left), the roles are tried swapped for just that date
+             — since the exhausted resource stays exhausted, this sticks
+             for the rest of the chunk rather than flip-flopping day to
+             day. Each attempted pairing is peeked-then-committed together:
+             if either side can't pay, NEITHER is spent for that date.
           3. Swap (last resort) — a date co-leave genuinely cannot fund
              (the higher-FP parent has none left, or the other parent has
              no vacation left) is handed to `_fund_swap` together with the
@@ -1271,6 +1303,16 @@ class _Ledger:
         )
         want_dubbeldag = wish.wants_dubbeldag
 
+        # §fix (long-chunk co-leave, 2026-09-19) — decide the FP-payer per
+        # CHUNK, not per day, so a long window reads as one switch instead
+        # of a daily-alternating patchwork (see the tier-2 docstring above).
+        # Reuses `_fund_swap`'s own balanced-by-spendable-cost split rather
+        # than a second splitter.
+        first_half, _second_half = _split_dates_by_spendable_cost(
+            dates, self._input.holidays, self._input.day_overrides
+        )
+        first_half_dates: frozenset[date] = frozenset(first_half)
+
         days: list[ProposedDay] = []
         cost = _new_cost()
         dubbeldag_dates: set[date] = set()
@@ -1354,23 +1396,34 @@ class _Ledger:
                     funded = True
 
             if not funded and pool_a is not None and pool_b is not None:
-                total_a = pool_a.sjukpenning_left + pool_a.lagsta_left
-                total_b = pool_b.sjukpenning_left + pool_b.lagsta_left
-                # Ties go to parent_a (`responsible.parents[0]`) — that list
-                # is itself deterministically ordered, so this stays stable
-                # across repeated `propose()` runs (§5 test #11).
-                if total_a >= total_b:
-                    fp_parent, fp_pool, other_parent = parent_a, pool_a, parent_b
+                # §fix (long-chunk co-leave, 2026-09-19) — the FP-payer is
+                # decided once per half of the window (`first_half_dates`),
+                # not by comparing live remaining balances on every date —
+                # that per-day comparison is exactly what made the payer
+                # flip almost daily as spending shifted the lead back and
+                # forth. `responsible.parents[0]` pays with FP across the
+                # first half, `responsible.parents[1]` across the second.
+                if d in first_half_dates:
+                    primary, primary_pool = parent_a, pool_a
+                    secondary, secondary_pool = parent_b, pool_b
                 else:
-                    fp_parent, fp_pool, other_parent = parent_b, pool_b, parent_a
+                    primary, primary_pool = parent_b, pool_b
+                    secondary, secondary_pool = parent_a, pool_a
 
-                fp_tier = self._peek_fp_tier(fp_pool)
-                other_budget = self._vacation.get((other_parent.uuid, d.year))
-                other_can_pay_vacation = other_budget is not None and other_budget.available > 0
+                funded_pair = self._try_co_leave_pair(primary, primary_pool, secondary, d)
+                if funded_pair is None:
+                    # Graceful fallback: the chunk's designated FP-payer
+                    # can't afford THIS date (or their partner can't pay
+                    # vacation) — try the pairing swapped for just this
+                    # date rather than losing "both off" for it entirely.
+                    # Because the exhausted resource (FP or vacation) stays
+                    # exhausted, once this triggers it keeps triggering for
+                    # the rest of the chunk instead of flip-flopping day to
+                    # day.
+                    funded_pair = self._try_co_leave_pair(secondary, secondary_pool, primary, d)
 
-                if fp_tier is not None and other_can_pay_vacation:
-                    self._commit_fp_tier(fp_pool, fp_tier)
-                    self._spend_vacation(other_parent.uuid, d)
+                if funded_pair is not None:
+                    fp_parent, fp_tier, vacation_parent = funded_pair
                     cost[fp_tier] += 1
                     cost["vacation"] += 1
                     fp_reason = (
@@ -1382,7 +1435,7 @@ class _Ledger:
                     days.append(
                         ProposedDay(
                             d,
-                            other_parent.uuid,
+                            vacation_parent.uuid,
                             child.id,
                             "vacation",
                             False,

backend/tests/unit/test_services/test_allocation_service.py

@@ -2765,3 +2765,80 @@ class TestCase32TakePlOnRedDaysBothParents:
             assert len(entries) == 2
             assert all(e.day_type == "sjukpenning" for e in entries)
             assert all(e.reason == REASON_FP_RED_DAY for e in entries)
+
+
+# ---------------------------------------------------------------------------
+# §fix (long-chunk co-leave, 2026-09-19) — the FP-payer for a `parent_mode=
+# 'both'` window is decided per CHUNK (first half / second half, split by
+# `_split_dates_by_spendable_cost`), not per day. The old per-day "whichever
+# parent has more remaining FP" comparison flipped the payer almost every
+# day once spending started shifting the lead back and forth -- a real
+# household reported exactly this as an alternating PL/vacation pattern
+# that produced no benefit, just extra admin requesting leave.
+# ---------------------------------------------------------------------------
+
+
+class TestCase33LongChunkCoLeave:
+    def test_six_week_window_switches_fp_payer_at_most_once(self) -> None:
+        # Feb 1 -> Mar 14 2027: 42 calendar days, clean of red days (Easter
+        # 2027 starts Fri 3/26, per TestCase9's own verified figures) so
+        # every non-weekend date reaches the co-leave tier.
+        child = _child(birth_date=date(2024, 1, 1))  # 15mo cutoff well past this window
+        parent_a = _parent(name="A")
+        parent_b = _parent(name="B")
+        wish = _wish(
+            child_id=child.id,
+            start=(2, 1),
+            end=(3, 14),
+            weeks=6,
+            parent_mode="both",
+        )
+        alloc_input = _build_input(
+            today=date(2027, 1, 1),
+            horizon_end=date(2027, 12, 31),
+            children=[child],
+            parents=[parent_a, parent_b],
+            pools={
+                (parent_a.uuid, child.id): DayPool(100, 90, 60),
+                (parent_b.uuid, child.id): DayPool(100, 90, 60),
+            },
+            vacation={
+                (parent_a.uuid, 2027): VacationBudget(30, 0, 0),
+                (parent_b.uuid, 2027): VacationBudget(30, 0, 0),
+            },
+            wishes=[wish],
+        )
+
+        proposal = Allocator().propose(alloc_input)
+
+        outcome = proposal.outcomes[0]
+        assert outcome.strategy == "co_leave"
+        assert outcome.shortfall_days == 0
+
+        non_free = [d for d in proposal.days if d.day_type != "free"]
+        by_date: dict[date, list] = {}
+        for d in non_free:
+            by_date.setdefault(d.date, []).append(d)
+        assert by_date, "expected at least one funded date"
+
+        # Both parents present (co-leaving) on every spendable date.
+        for on_date, entries in sorted(by_date.items()):
+            parents_that_day = {e.parent_personal_data_uuid for e in entries}
+            assert parents_that_day == {parent_a.uuid, parent_b.uuid}, (on_date, parents_that_day)
+
+        # The FP-payer, walked in date order, switches at most once --
+        # never the old daily alternation.
+        fp_payer_sequence = [
+            next(e.parent_personal_data_uuid for e in entries if e.day_type != "vacation")
+            for _on_date, entries in sorted(by_date.items())
+        ]
+        switches = sum(
+            1 for prev, cur in zip(fp_payer_sequence, fp_payer_sequence[1:]) if prev != cur
+        )
+        assert switches <= 1, f"expected at most one payer switch, saw {switches}"
+
+        # Roughly equal halves -- neither parent gets only a token day.
+        count_a = fp_payer_sequence.count(parent_a.uuid)
+        count_b = fp_payer_sequence.count(parent_b.uuid)
+        assert count_a > 0 and count_b > 0
+        assert abs(count_a - count_b) <= 2

frontend/src/pages/Calendar.test.tsx

@@ -850,6 +850,43 @@ describe('Calendar', () => {
       await waitFor(() => expect(sync).toHaveBeenCalled())
     })
 
+    it('defaults a solo household preset to "either" parent', async () => {
+      const created = makeLeaveWish({ id: 'lw-preset' })
+      const add = vi.fn().mockResolvedValue(created)
+      const sync = vi.fn().mockResolvedValue(makePlanSyncResponse())
+      vi.mocked(useWorkspaceMembers).mockReturnValue(
+        makeWorkspaceMembersResult({ data: [makePersonalData({ uuid: 'pd-a', name: 'Förälder A' })] }),
+      )
+      vi.mocked(useLeaveWishes).mockReturnValue(makeLeaveWishesResult({ add, data: [] }))
+      vi.mocked(usePlanSync).mockReturnValue(makePlanSyncResult({ sync }))
+
+      render(<Calendar />)
+      fireEvent.click(screen.getByTestId('preset-click'))
+
+      await waitFor(() => expect(add).toHaveBeenCalledWith(expect.objectContaining({ parentMode: 'either' })))
+    })
+
+    it('defaults a two-parent household preset to "both" parents', async () => {
+      const created = makeLeaveWish({ id: 'lw-preset' })
+      const add = vi.fn().mockResolvedValue(created)
+      const sync = vi.fn().mockResolvedValue(makePlanSyncResponse())
+      vi.mocked(useWorkspaceMembers).mockReturnValue(
+        makeWorkspaceMembersResult({
+          data: [
+            makePersonalData({ uuid: 'pd-a', name: 'Förälder A' }),
+            makePersonalData({ uuid: 'pd-b', name: 'Förälder B' }),
+          ],
+        }),
+      )
+      vi.mocked(useLeaveWishes).mockReturnValue(makeLeaveWishesResult({ add, data: [] }))
+      vi.mocked(usePlanSync).mockReturnValue(makePlanSyncResult({ sync }))
+
+      render(<Calendar />)
+      fireEvent.click(screen.getByTestId('preset-click'))
+
+      await waitFor(() => expect(add).toHaveBeenCalledWith(expect.objectContaining({ parentMode: 'both' })))
+    })
+
     it('navigates the month view to the resolved placement and flashes the band', async () => {
       const created = makeLeaveWish({ id: 'lw-preset' })
       const add = vi.fn().mockResolvedValue(created)

frontend/src/pages/Calendar.tsx

@@ -719,7 +719,7 @@ export function Calendar(): ReactNode {
         recurrence: 'yearly',
         firstYear: new Date().getFullYear(),
         priority: wishesHook.data.length + 1,
-        parentMode: 'either',
+        parentMode: membersHook.data.length >= 2 ? 'both' : 'either',
         windowMode: 'flexible',
       })