diff --git a/apps/api/tests/broker/outboxWorker.test.ts b/apps/api/tests/broker/outboxWorker.test.ts
index ab1fe8c..b4fab7d 100644
--- a/apps/api/tests/broker/outboxWorker.test.ts
+++ b/apps/api/tests/broker/outboxWorker.test.ts
@@ -18,7 +18,8 @@ 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, assembleRruleString } from '../../src/broker/outboxWorker.js';
+import { runOutboxDrain, assembleRruleString, scheduleOutboxDrain } from '../../src/broker/outboxWorker.js';
+import { signalOutboxDrain } from '../../src/lib/outboxTrigger.js';
 
 // ── Drizzle DB mock ────────────────────────────────────────────────────────
 // Follows the pattern from PATTERNS.md §Drizzle DB mock in tests.
@@ -734,3 +735,111 @@ describe('FREQ persistence (D-07 regression)', () => {
     expect(capturedIcsString as string).not.toContain('FREQ=WEEKLY');
   });
 });
+
+// ─── scheduleOutboxDrain trigger-wiring tests (D-09) ─────────────────────────
+// These tests verify the trigger-wiring behavior introduced in Plan 09-01:
+//   SC-1: signalOutboxDrain() → drain fires promptly (no 15s wait)
+//   D-05: mid-drain signal collapses to exactly one trailing re-drain
+//   D-07: concurrent scheduleOutboxDrain() calls dispatch each row exactly once
+//
+// RED: these tests fail because scheduleOutboxDrain is not yet exported.
+
+describe('scheduleOutboxDrain — trigger wiring (D-09)', () => {
+  beforeEach(() => {
+    vi.resetAllMocks();
+    mockPendingRows = [];
+    wireMockChain();
+  });
+
+  // Test A (SC-1): signalOutboxDrain() triggers a drain immediately — no 15s wait.
+  // With one pending row and createCalendarEvent mocked to 201, calling signalOutboxDrain()
+  // then flushing microtasks results in createCalendarEvent called exactly once.
+  // No vi.useFakeTimers() — drain fires via the EventEmitter signal, not the interval.
+  it('SC-1: signalOutboxDrain() triggers drain promptly — createCalendarEvent called once without advancing timers', async () => {
+    const { createCalendarEvent } = await import('../../src/broker/write.js');
+    vi.mocked(createCalendarEvent).mockResolvedValue(makeResponse(201));
+    mockPendingRows = [makeRow()];
+
+    signalOutboxDrain();
+
+    // Flush microtasks so the async drain has a chance to run
+    await new Promise<void>((resolve) => setImmediate(resolve));
+    // One more flush to let the drain's internal async steps complete
+    await new Promise<void>((resolve) => setImmediate(resolve));
+
+    expect(createCalendarEvent).toHaveBeenCalledTimes(1);
+  });
+
+  // Test B (SC-4 / D-05): a signal arriving during an in-flight drain triggers exactly
+  // one trailing re-drain — not two, not zero.
+  it('D-05: two signals mid-drain collapse to exactly one trailing re-drain', async () => {
+    const { createCalendarEvent } = await import('../../src/broker/write.js');
+
+    // Capture the resolve function so we can hold the first drain in flight
+    let resolveFirst!: () => void;
+    const firstDone = new Promise<void>((resolve) => {
+      resolveFirst = resolve;
+    });
+
+    // First call: held until we release it; subsequent calls resolve immediately
+    vi.mocked(createCalendarEvent)
+      .mockImplementationOnce(async () => {
+        await firstDone;
+        return makeResponse(201);
+      })
+      .mockResolvedValue(makeResponse(201));
+
+    // Two pending rows: drain 1 picks up row 1, trailing drain picks up row 2
+    const row1 = makeRow({ id: 1, uid: 'uid-1@familysync' });
+    const row2 = makeRow({ id: 2, uid: 'uid-2@familysync' });
+    mockPendingRows = [row1];
+
+    // Start drain 1 (via signal) — it blocks on firstDone
+    signalOutboxDrain();
+    await new Promise<void>((resolve) => setImmediate(resolve));
+
+    // While drain 1 is in flight, send two more signals — both should collapse to one trailing drain
+    mockPendingRows = [row2];
+    signalOutboxDrain();
+    signalOutboxDrain();
+
+    // Release drain 1 — this lets it finish, then the trailing re-drain fires
+    resolveFirst();
+
+    // Flush: drain 1 finally-block + trailing scheduleOutboxDrain() + trailing drain steps
+    await new Promise<void>((resolve) => setImmediate(resolve));
+    await new Promise<void>((resolve) => setImmediate(resolve));
+    await new Promise<void>((resolve) => setImmediate(resolve));
+
+    // Drain 1 called createCalendarEvent once (row1); trailing drain called it once (row2).
+    // Total = 2 — never a third pass.
+    expect(createCalendarEvent).toHaveBeenCalledTimes(2);
+  });
+
+  // Test C (SC-4 / D-07): two concurrent scheduleOutboxDrain() calls dispatch each row
+  // exactly once — the second call is a no-op via the isDraining guard.
+  it('D-07: two concurrent scheduleOutboxDrain() calls invoke createCalendarEvent exactly once', async () => {
+    vi.useFakeTimers();
+    try {
+      const { createCalendarEvent } = await import('../../src/broker/write.js');
+      // Slow create so the second scheduleOutboxDrain starts while first is still running
+      vi.mocked(createCalendarEvent).mockImplementation(
+        () =>
+          new Promise((resolve) =>
+            setTimeout(() => resolve(makeResponse(201)), 20),
+          ),
+      );
+      mockPendingRows = [makeRow({ id: 1 })];
+
+      // Both calls are synchronous — second must no-op via isDraining guard
+      scheduleOutboxDrain();
+      scheduleOutboxDrain();
+
+      await vi.runAllTimersAsync();
+
+      expect(createCalendarEvent).toHaveBeenCalledTimes(1);
+    } finally {
+      vi.useRealTimers();
+    }
+  });
+});