diff --git a/apps/api/tests/broker/outboxWorker.test.ts b/apps/api/tests/broker/outboxWorker.test.ts
index 6d39ef5..e8babf1 100644
--- a/apps/api/tests/broker/outboxWorker.test.ts
+++ b/apps/api/tests/broker/outboxWorker.test.ts
@@ -18,7 +18,7 @@ import { describe, it, expect, vi, beforeEach } from 'vitest'
 // This import fails (RED) — broker/outboxWorker.ts does not exist yet.
 // eslint-disable-next-line @typescript-eslint/ban-ts-comment
 // @ts-ignore intentional RED import
-import { runOutboxDrain } from '../../src/broker/outboxWorker.js'
+import { runOutboxDrain, assembleRruleString } from '../../src/broker/outboxWorker.js'
 
 // ── Drizzle DB mock ────────────────────────────────────────────────────────
 // Follows the pattern from PATTERNS.md §Drizzle DB mock in tests.
@@ -647,3 +647,69 @@ describe('runOutboxDrain — fail closed on bad credentials (CR-03) + backoff in
     expect(setArg?.nextAttemptAt?.getTime()).toBeLessThanOrEqual(expectedMaxMs)
   })
 })
+
+// ─── D-06: assembleRruleString unit tests ─────────────────────────────────────
+// These tests import the NOT-YET-EXPORTED `assembleRruleString` helper.
+// RED: will fail because assembleRruleString is not exported yet.
+
+describe('assembleRruleString (D-06)', () => {
+  it('returns base preset unchanged when no bound given', () => {
+    expect(assembleRruleString('FREQ=DAILY')).toBe('FREQ=DAILY')
+  })
+
+  it('appends COUNT when count is given (count wins over until)', () => {
+    expect(assembleRruleString('FREQ=DAILY', undefined, 5, false)).toBe('FREQ=DAILY;COUNT=5')
+  })
+
+  it('COUNT wins when both until and count are provided (mutual exclusion, RFC 5545 §3.3.10)', () => {
+    expect(assembleRruleString('FREQ=WEEKLY', '2026-06-30', 5, false)).toBe('FREQ=WEEKLY;COUNT=5')
+  })
+
+  it('appends UNTIL as DATE form (YYYYMMDD) for all-day events', () => {
+    expect(assembleRruleString('FREQ=WEEKLY', '2026-06-30', undefined, true)).toBe('FREQ=WEEKLY;UNTIL=20260630')
+  })
+
+  it('appends UNTIL as DATETIME UTC form (YYYYMMDDTHHMMSSZ) for timed events', () => {
+    expect(assembleRruleString('FREQ=WEEKLY', '2026-06-30', undefined, false)).toBe('FREQ=WEEKLY;UNTIL=20260630T235959Z')
+  })
+
+  it('COUNT=5 appended to FREQ=DAILY (matches plan behavior assertion)', () => {
+    expect(assembleRruleString('FREQ=DAILY', undefined, 5, false)).toBe('FREQ=DAILY;COUNT=5')
+  })
+})
+
+// ─── D-07: FREQ-persistence regression lock ────────────────────────────────────
+// RED: will fail because the outbox worker does not yet wire recurrenceUntil/recurrenceCount
+// and the FREQ-persistence assertion catches the D-07 regression scenario.
+
+describe('FREQ persistence (D-07 regression)', () => {
+  beforeEach(() => {
+    vi.resetAllMocks()
+    mockPendingRows = []
+    wireMockChain()
+  })
+
+  it('D-07: daily-recurrence payload assembles to FREQ=DAILY (not weekly or none) in emitted ICS', async () => {
+    const { createCalendarEvent } = await import('../../src/broker/write.js')
+    let capturedIcsString: unknown = null
+    vi.mocked(createCalendarEvent).mockImplementation(async (_client, _cal, _uid, icsString) => {
+      capturedIcsString = icsString
+      return makeResponse(201)
+    })
+    const dailyPayload = JSON.stringify({
+      title: 'Daily standup',
+      allDay: false,
+      start: '2026-06-10T09:00:00',
+      end: '2026-06-10T09:30:00',
+      recurrence: 'daily',
+    })
+    mockPendingRows = [makeRow({ payload: dailyPayload })]
+
+    await runOutboxDrain()
+
+    expect(typeof capturedIcsString).toBe('string')
+    // D-07: FREQ must be DAILY — not WEEKLY or absent
+    expect(capturedIcsString as string).toContain('RRULE:FREQ=DAILY')
+    expect(capturedIcsString as string).not.toContain('FREQ=WEEKLY')
+  })
+})
diff --git a/apps/api/tests/broker/vevent.test.ts b/apps/api/tests/broker/vevent.test.ts
index 52d608b..407e1fa 100644
--- a/apps/api/tests/broker/vevent.test.ts
+++ b/apps/api/tests/broker/vevent.test.ts
@@ -90,6 +90,43 @@ describe('buildVeventString', () => {
     expect(result.icsString).not.toContain('RRULE:')
   })
 
+  // D-06: RRULE COUNT — verified ical.js 2.2.1 output
+  it('serializes COUNT in RRULE for a timed event (D-06)', () => {
+    const result = buildVeventString({
+      summary: 'Weekly',
+      allDay: false,
+      dtstart: new Date('2026-06-10T09:00:00Z'),
+      dtend: new Date('2026-06-10T10:00:00Z'),
+      rruleString: 'FREQ=WEEKLY;COUNT=5',
+    })
+    expect(result.icsString).toContain('RRULE:FREQ=WEEKLY;COUNT=5')
+  })
+
+  // D-06: RRULE UNTIL DATE form — all-day event must NOT contain T235959Z
+  it('serializes UNTIL as DATE form for all-day events (D-06)', () => {
+    const result = buildVeventString({
+      summary: 'Daily standup',
+      allDay: true,
+      dtstart: '2026-06-10',
+      dtend: '2026-06-11',
+      rruleString: 'FREQ=DAILY;UNTIL=20260630',
+    })
+    expect(result.icsString).toContain('RRULE:FREQ=DAILY;UNTIL=20260630')
+    expect(result.icsString).not.toContain('T235959Z')
+  })
+
+  // D-06: RRULE UNTIL DATETIME UTC form — timed event
+  it('serializes UNTIL as DATETIME UTC form for timed events (D-06)', () => {
+    const result = buildVeventString({
+      summary: 'Weekly',
+      allDay: false,
+      dtstart: new Date('2026-06-10T09:00:00Z'),
+      dtend: new Date('2026-06-10T10:00:00Z'),
+      rruleString: 'FREQ=WEEKLY;UNTIL=20260630T235959Z',
+    })
+    expect(result.icsString).toContain('RRULE:FREQ=WEEKLY;UNTIL=20260630T235959Z')
+  })
+
   it('uses the provided uid when given', () => {
     const uid = 'custom-uid-001@familysync'
     const result = buildVeventString({