diff --git a/frontend/src/components/auth/CredentialsForm.test.tsx b/frontend/src/components/auth/CredentialsForm.test.tsx
new file mode 100644
index 0000000..15111d6
--- /dev/null
+++ b/frontend/src/components/auth/CredentialsForm.test.tsx
@@ -0,0 +1,101 @@
+import { describe, it, expect, vi } from 'vitest'
+import { render, screen, fireEvent } from '@testing-library/react'
+import { CredentialsForm, type CredentialsFormMode } from './CredentialsForm'
+
+const labels = {
+  loginTab: 'Log in',
+  registerTab: 'Create account',
+  nameLabel: 'Name',
+  namePlaceholder: 'Your name',
+  emailLabel: 'Email',
+  emailPlaceholder: 'Email address',
+  passwordLabel: 'Password',
+  passwordPlaceholder: 'Password',
+  submitLabel: 'Submit',
+}
+
+function renderForm(mode: CredentialsFormMode, overrides: { error?: string; disabled?: boolean } = {}) {
+  const onModeChange = vi.fn()
+  const onSubmit = vi.fn()
+  render(
+    <CredentialsForm
+      mode={mode}
+      onModeChange={onModeChange}
+      onSubmit={onSubmit}
+      error={overrides.error}
+      disabled={overrides.disabled}
+      idPrefix="cf"
+      classPrefix="cf-page"
+      labels={labels}
+    />,
+  )
+  return { onModeChange, onSubmit }
+}
+
+describe('CredentialsForm', () => {
+  it('renders the login mode without a name field', () => {
+    renderForm('login')
+    expect(screen.getByTestId('cf-tab-login')).toHaveAttribute('aria-pressed', 'true')
+    expect(screen.getByTestId('cf-tab-register')).toHaveAttribute('aria-pressed', 'false')
+    expect(screen.queryByPlaceholderText('Your name')).not.toBeInTheDocument()
+    expect(screen.getByTestId('cf-email')).toBeInTheDocument()
+    expect(screen.getByTestId('cf-password')).toBeInTheDocument()
+  })
+
+  it('renders the register mode with a name field', () => {
+    renderForm('register')
+    expect(screen.getByTestId('cf-tab-register')).toHaveAttribute('aria-pressed', 'true')
+    expect(screen.getByTestId('cf-tab-login')).toHaveAttribute('aria-pressed', 'false')
+    expect(screen.getByPlaceholderText('Your name')).toBeInTheDocument()
+  })
+
+  it('calls onModeChange when a tab is clicked', () => {
+    const { onModeChange } = renderForm('login')
+    fireEvent.click(screen.getByTestId('cf-tab-register'))
+    expect(onModeChange).toHaveBeenCalledWith('register')
+
+    fireEvent.click(screen.getByTestId('cf-tab-login'))
+    expect(onModeChange).toHaveBeenCalledWith('login')
+  })
+
+  it('shows the error alert when an error is given', () => {
+    renderForm('login', { error: 'Something went wrong' })
+    expect(screen.getByRole('alert')).toHaveTextContent('Something went wrong')
+  })
+
+  it('renders no alert when there is no error', () => {
+    renderForm('login')
+    expect(screen.queryByRole('alert')).not.toBeInTheDocument()
+  })
+
+  it('submits the entered name, email and password for register mode', () => {
+    const { onSubmit } = renderForm('register')
+
+    fireEvent.change(screen.getByPlaceholderText('Your name'), { target: { value: 'Alex' } })
+    fireEvent.change(screen.getByTestId('cf-email'), { target: { value: 'a@b.com' } })
+    fireEvent.change(screen.getByTestId('cf-password'), { target: { value: 'secret' } })
+    fireEvent.click(screen.getByTestId('cf-submit'))
+
+    expect(onSubmit).toHaveBeenCalledWith({ displayName: 'Alex', email: 'a@b.com', password: 'secret' })
+  })
+
+  it('submits an empty displayName for login mode, where the field is hidden', () => {
+    const { onSubmit } = renderForm('login')
+
+    fireEvent.change(screen.getByTestId('cf-email'), { target: { value: 'a@b.com' } })
+    fireEvent.change(screen.getByTestId('cf-password'), { target: { value: 'secret' } })
+    fireEvent.click(screen.getByTestId('cf-submit'))
+
+    expect(onSubmit).toHaveBeenCalledWith({ displayName: '', email: 'a@b.com', password: 'secret' })
+  })
+
+  it('disables the submit button when disabled is true', () => {
+    renderForm('login', { disabled: true })
+    expect(screen.getByTestId('cf-submit')).toBeDisabled()
+  })
+
+  it('does not disable the submit button when disabled is not given', () => {
+    renderForm('login')
+    expect(screen.getByTestId('cf-submit')).not.toBeDisabled()
+  })
+})
diff --git a/frontend/src/components/auth/CredentialsForm.tsx b/frontend/src/components/auth/CredentialsForm.tsx
new file mode 100644
index 0000000..8955024
--- /dev/null
+++ b/frontend/src/components/auth/CredentialsForm.tsx
@@ -0,0 +1,135 @@
+import { useState, type ReactNode } from 'react'
+import { FormField } from '@/components/common/FormField'
+import { Alert } from '@/components/common/Alert'
+
+export type CredentialsFormMode = 'login' | 'register'
+
+export interface CredentialsFormValues {
+  displayName: string
+  email: string
+  password: string
+}
+
+/** All copy the form needs, so the two callers (`Auth`, `Join`) can supply
+ * their own translated strings without this component knowing their i18n
+ * keys. */
+export interface CredentialsFormLabels {
+  loginTab: ReactNode
+  registerTab: ReactNode
+  nameLabel: ReactNode
+  namePlaceholder: string
+  emailLabel: ReactNode
+  emailPlaceholder: string
+  passwordLabel: ReactNode
+  passwordPlaceholder: string
+  submitLabel: ReactNode
+}
+
+export interface CredentialsFormProps {
+  mode: CredentialsFormMode
+  onModeChange: (mode: CredentialsFormMode) => void
+  onSubmit: (values: CredentialsFormValues) => void
+  error?: ReactNode
+  disabled?: boolean | undefined
+  /** Prefixes every id/data-testid, e.g. `auth` → `auth-email`. */
+  idPrefix: string
+  /** Prefixes the BEM classes for the tabs/form, e.g. `auth-page` →
+   * `auth-page__tabs`. */
+  classPrefix: string
+  labels: CredentialsFormLabels
+}
+
+/**
+ * The login/register tab pair + credentials form shared by `Auth` (the
+ * app's front door) and `Join` (the partner-invite landing page) — until
+ * now hand-duplicated between them. Owns the field values internally;
+ * callers only see the submitted `{ displayName, email, password }` tuple
+ * and the mode toggle, and stay responsible for the error banner's content,
+ * the submit button's disabled state and label, and what happens on submit.
+ */
+export function CredentialsForm({
+  mode,
+  onModeChange,
+  onSubmit,
+  error,
+  disabled,
+  idPrefix,
+  classPrefix,
+  labels,
+}: CredentialsFormProps): ReactNode {
+  const [displayName, setDisplayName] = useState('')
+  const [email, setEmail] = useState('')
+  const [password, setPassword] = useState('')
+  const isRegister = mode === 'register'
+
+  const handleSubmit = (e: React.FormEvent) => {
+    e.preventDefault()
+    onSubmit({ displayName, email, password })
+  }
+
+  return (
+    <>
+      <div className={`segmented ${classPrefix}__tabs`} role="group">
+        <button
+          type="button"
+          className={`segmented__option ${classPrefix}__tab`}
+          aria-pressed={!isRegister}
+          onClick={() => onModeChange('login')}
+          data-testid={`${idPrefix}-tab-login`}
+        >
+          {labels.loginTab}
+        </button>
+        <button
+          type="button"
+          className={`segmented__option ${classPrefix}__tab`}
+          aria-pressed={isRegister}
+          onClick={() => onModeChange('register')}
+          data-testid={`${idPrefix}-tab-register`}
+        >
+          {labels.registerTab}
+        </button>
+      </div>
+      <form className={`${classPrefix}__form`} onSubmit={handleSubmit}>
+        {isRegister && (
+          <FormField id={`${idPrefix}-name`} label={labels.nameLabel}>
+            <input
+              className="field__control"
+              placeholder={labels.namePlaceholder}
+              value={displayName}
+              onChange={(e) => setDisplayName(e.target.value)}
+            />
+          </FormField>
+        )}
+        <FormField id={`${idPrefix}-email`} label={labels.emailLabel}>
+          <input
+            type="email"
+            data-testid={`${idPrefix}-email`}
+            className="field__control"
+            placeholder={labels.emailPlaceholder}
+            value={email}
+            onChange={(e) => setEmail(e.target.value)}
+          />
+        </FormField>
+        <FormField id={`${idPrefix}-password`} label={labels.passwordLabel}>
+          <input
+            type="password"
+            data-testid={`${idPrefix}-password`}
+            className="field__control"
+            placeholder={labels.passwordPlaceholder}
+            value={password}
+            onChange={(e) => setPassword(e.target.value)}
+          />
+        </FormField>
+        {error && <Alert variant="error">{error}</Alert>}
+        <button
+          type="submit"
+          className="btn btn--primary btn--block"
+          disabled={disabled}
+          data-testid={`${idPrefix}-submit`}
+        >
+          {labels.submitLabel}
+        </button>
+      </form>
+    </>
+  )
+}
diff --git a/frontend/src/pages/Auth.tsx b/frontend/src/pages/Auth.tsx
index 1ecdbbe..17ae3a5 100644
--- a/frontend/src/pages/Auth.tsx
+++ b/frontend/src/pages/Auth.tsx
@@ -4,8 +4,11 @@ import { useAuth } from '@/context/AuthContext'
 import { useWorkspace } from '@/context/WorkspaceContext'
 import { useTranslation } from '@/i18n'
 import { LanguageToggle } from '@/components/layout/LanguageToggle'
-import { FormField } from '@/components/common/FormField'
-import { Alert } from '@/components/common/Alert'
+import {
+  CredentialsForm,
+  type CredentialsFormMode,
+  type CredentialsFormValues,
+} from '@/components/auth/CredentialsForm'
 import './auth.css'
 
 export function Auth() {
@@ -13,11 +16,9 @@ export function Auth() {
   const { isLoading: wsLoading } = useWorkspace()
   const { t } = useTranslation()
   const navigate = useNavigate()
-  const [isRegister, setIsRegister] = useState(false)
-  const [email, setEmail] = useState('')
-  const [password, setPassword] = useState('')
-  const [displayName, setDisplayName] = useState('')
+  const [mode, setMode] = useState<CredentialsFormMode>('login')
   const [error, setError] = useState<string | null>(null)
+  const isRegister = mode === 'register'
 
   useEffect(() => {
     if (isAuthenticated && !wsLoading) {
@@ -25,17 +26,15 @@ export function Auth() {
     }
   }, [isAuthenticated, wsLoading, navigate])
 
-  const handleSubmit = async (e: React.FormEvent) => {
-    e.preventDefault()
+  const handleSubmit = async ({ displayName, email, password }: CredentialsFormValues) => {
     setError(null)
     try {
       if (isRegister) {
         await register({ email, password, displayName })
-        navigate('/', { replace: true })
       } else {
         await login({ email, password })
-        navigate('/', { replace: true })
       }
+      navigate('/', { replace: true })
     } catch (err: unknown) {
       setError(err instanceof Error ? err.message : t('auth.genericError'))
     }
@@ -44,8 +43,8 @@ export function Auth() {
   // Switching tabs clears any error from the previous tab's attempt — a
   // failed login's error text should not linger, confusingly, once the
   // person has moved on to creating an account (or back).
-  const switchTab = (register: boolean) => {
-    setIsRegister(register)
+  const switchMode = (next: CredentialsFormMode) => {
+    setMode(next)
     setError(null)
   }
 
@@ -57,62 +56,25 @@ export function Auth() {
       <div className="auth-page__column">
         <img className="auth-page__mark" src="/img/icon-64.png" alt="" width={32} height={32} />
         <h1 className="auth-page__title">{isRegister ? t('auth.createAccountTitle') : t('auth.loginTitle')}</h1>
-        <div className="segmented auth-page__tabs" role="group">
-          <button
-            type="button"
-            className="segmented__option auth-page__tab"
-            aria-pressed={!isRegister}
-            onClick={() => switchTab(false)}
-            data-testid="auth-tab-login"
-          >
-            {t('auth.loginButton')}
-          </button>
-          <button
-            type="button"
-            className="segmented__option auth-page__tab"
-            aria-pressed={isRegister}
-            onClick={() => switchTab(true)}
-            data-testid="auth-tab-register"
-          >
-            {t('auth.createAccountButton')}
-          </button>
-        </div>
-        <form className="auth-page__form" onSubmit={(e) => void handleSubmit(e)}>
-          {isRegister && (
-            <FormField id="auth-name" label={t('auth.nameLabel')}>
-              <input
-                className="field__control"
-                placeholder={t('auth.namePlaceholder')}
-                value={displayName}
-                onChange={(e) => setDisplayName(e.target.value)}
-              />
-            </FormField>
-          )}
-          <FormField id="auth-email" label={t('auth.emailLabel')}>
-            <input
-              type="email"
-              data-testid="auth-email"
-              className="field__control"
-              placeholder={t('auth.emailPlaceholder')}
-              value={email}
-              onChange={(e) => setEmail(e.target.value)}
-            />
-          </FormField>
-          <FormField id="auth-password" label={t('auth.passwordLabel')}>
-            <input
-              type="password"
-              data-testid="auth-password"
-              className="field__control"
-              placeholder={t('auth.passwordPlaceholder')}
-              value={password}
-              onChange={(e) => setPassword(e.target.value)}
-            />
-          </FormField>
-          {error && <Alert variant="error">{error}</Alert>}
-          <button type="submit" className="btn btn--primary btn--block" data-testid="auth-submit">
-            {isRegister ? t('auth.registerButton') : t('auth.loginButton')}
-          </button>
-        </form>
+        <CredentialsForm
+          mode={mode}
+          onModeChange={switchMode}
+          onSubmit={(values) => void handleSubmit(values)}
+          error={error}
+          idPrefix="auth"
+          classPrefix="auth-page"
+          labels={{
+            loginTab: t('auth.loginButton'),
+            registerTab: t('auth.createAccountButton'),
+            nameLabel: t('auth.nameLabel'),
+            namePlaceholder: t('auth.namePlaceholder'),
+            emailLabel: t('auth.emailLabel'),
+            emailPlaceholder: t('auth.emailPlaceholder'),
+            passwordLabel: t('auth.passwordLabel'),
+            passwordPlaceholder: t('auth.passwordPlaceholder'),
+            submitLabel: isRegister ? t('auth.registerButton') : t('auth.loginButton'),
+          }}
+        />
       </div>
     </main>
   )
diff --git a/frontend/src/pages/Join.tsx b/frontend/src/pages/Join.tsx
index a4f4498..08052cf 100644
--- a/frontend/src/pages/Join.tsx
+++ b/frontend/src/pages/Join.tsx
@@ -5,12 +5,15 @@ import { useWorkspace } from '@/context/WorkspaceContext'
 import { redeemInvitation } from '@/services/api'
 import { useTranslation, type TranslationKey } from '@/i18n'
 import { LanguageToggle } from '@/components/layout/LanguageToggle'
-import { FormField } from '@/components/common/FormField'
 import { Alert } from '@/components/common/Alert'
+import {
+  CredentialsForm,
+  type CredentialsFormMode,
+  type CredentialsFormValues,
+} from '@/components/auth/CredentialsForm'
 import './join.css'
 
 type Phase = 'checking' | 'form' | 'redeeming' | 'error' | 'done'
-type Mode = 'login' | 'register'
 
 /** Maps a redeem/register failure's backend message to a translated,
  * distinct reason. The backend (see `InvitationError` call sites in
@@ -43,10 +46,7 @@ export function Join() {
 
   const [phase, setPhase] = useState<Phase>('checking')
   const [errorKey, setErrorKey] = useState<TranslationKey | null>(null)
-  const [mode, setMode] = useState<Mode>('register')
-  const [email, setEmail] = useState('')
-  const [password, setPassword] = useState('')
-  const [displayName, setDisplayName] = useState('')
+  const [mode, setMode] = useState<CredentialsFormMode>('register')
   const [isSubmitting, setIsSubmitting] = useState(false)
 
   // Guards against redeeming twice: once from a submit handler and again
@@ -84,8 +84,7 @@ export function Join() {
     }
   }, [authLoading, isAuthenticated, token, redeemAndGo])
 
-  const handleSubmit = async (e: React.FormEvent) => {
-    e.preventDefault()
+  const handleSubmit = async ({ displayName, email, password }: CredentialsFormValues) => {
     if (!token) return
     setErrorKey(null)
     setIsSubmitting(true)
@@ -113,7 +112,7 @@ export function Join() {
     }
   }
 
-  const switchMode = (next: Mode) => {
+  const switchMode = (next: CredentialsFormMode) => {
     setMode(next)
     setErrorKey(null)
   }
@@ -152,75 +151,32 @@ export function Join() {
         )}
 
         {phase === 'form' && (
-          <>
-            <div className="segmented join-page__tabs" role="group">
-              <button
-                type="button"
-                className="segmented__option join-page__tab"
-                aria-pressed={mode === 'login'}
-                onClick={() => switchMode('login')}
-                data-testid="join-tab-login"
-              >
-                {t('invite.joinLoginTab')}
-              </button>
-              <button
-                type="button"
-                className="segmented__option join-page__tab"
-                aria-pressed={mode === 'register'}
-                onClick={() => switchMode('register')}
-                data-testid="join-tab-register"
-              >
-                {t('invite.joinRegisterTab')}
-              </button>
-            </div>
-            <form className="join-page__form" onSubmit={(e) => void handleSubmit(e)}>
-              {mode === 'register' && (
-                <FormField id="join-name" label={t('invite.joinNameLabel')}>
-                  <input
-                    className="field__control"
-                    placeholder={t('invite.joinNamePlaceholder')}
-                    value={displayName}
-                    onChange={(e) => setDisplayName(e.target.value)}
-                  />
-                </FormField>
-              )}
-              <FormField id="join-email" label={t('invite.joinEmailLabel')}>
-                <input
-                  type="email"
-                  data-testid="join-email"
-                  className="field__control"
-                  placeholder={t('invite.joinEmailPlaceholder')}
-                  value={email}
-                  onChange={(e) => setEmail(e.target.value)}
-                />
-              </FormField>
-              <FormField id="join-password" label={t('invite.joinPasswordLabel')}>
-                <input
-                  type="password"
-                  data-testid="join-password"
-                  className="field__control"
-                  placeholder={t('invite.joinPasswordPlaceholder')}
-                  value={password}
-                  onChange={(e) => setPassword(e.target.value)}
-                />
-              </FormField>
-              {errorKey && <Alert variant="error">{t(errorKey)}</Alert>}
-              <button
-                type="submit"
-                className="btn btn--primary btn--block"
-                disabled={isSubmitting}
-                data-testid="join-submit"
-              >
-                {isSubmitting
-                  ? mode === 'register'
-                    ? t('invite.joinSubmittingRegister')
-                    : t('invite.joinSubmittingLogin')
-                  : mode === 'register'
-                    ? t('invite.joinSubmitRegister')
-                    : t('invite.joinSubmitLogin')}
-              </button>
-            </form>
-          </>
+          <CredentialsForm
+            mode={mode}
+            onModeChange={switchMode}
+            onSubmit={(values) => void handleSubmit(values)}
+            error={errorKey ? t(errorKey) : null}
+            disabled={isSubmitting}
+            idPrefix="join"
+            classPrefix="join-page"
+            labels={{
+              loginTab: t('invite.joinLoginTab'),
+              registerTab: t('invite.joinRegisterTab'),
+              nameLabel: t('invite.joinNameLabel'),
+              namePlaceholder: t('invite.joinNamePlaceholder'),
+              emailLabel: t('invite.joinEmailLabel'),
+              emailPlaceholder: t('invite.joinEmailPlaceholder'),
+              passwordLabel: t('invite.joinPasswordLabel'),
+              passwordPlaceholder: t('invite.joinPasswordPlaceholder'),
+              submitLabel: isSubmitting
+                ? mode === 'register'
+                  ? t('invite.joinSubmittingRegister')
+                  : t('invite.joinSubmittingLogin')
+                : mode === 'register'
+                  ? t('invite.joinSubmitRegister')
+                  : t('invite.joinSubmitLogin'),
+            }}
+          />
         )}
       </div>
     </main>