diff --git a/src/queue.ts b/src/queue.ts
index 0411613..8af13f8 100644
--- a/src/queue.ts
+++ b/src/queue.ts
@@ -38,9 +38,10 @@ export function createQueues(cfg: Config): Record<Channel, Queue<SendJob>> {
 }
 
 /**
- * Job id = `${channel}:${notificationId}` so BullMQ itself dedupes enqueues:
+ * Job id = `${channel}_${notificationId}` so BullMQ itself dedupes enqueues:
  * re-polling the same pending doc will not create a second job.
+ * (BullMQ rejects custom job ids containing ':', hence the underscore.)
  */
 export function jobId(job: SendJob): string {
-  return `${job.channel}:${job.notificationId}`;
+  return `${job.channel}_${job.notificationId}`;
 }
diff --git a/test/queue.test.ts b/test/queue.test.ts
new file mode 100644
index 0000000..bf1cfdb
--- /dev/null
+++ b/test/queue.test.ts
@@ -0,0 +1,21 @@
+import { describe, expect, it } from 'vitest';
+import { jobId, QUEUE_NAMES } from '../src/queue.js';
+
+describe('jobId', () => {
+  it('is deterministic per (channel, notificationId) for dedupe', () => {
+    const a = jobId({ channel: 'fcm', notificationId: 'abc' });
+    const b = jobId({ channel: 'fcm', notificationId: 'abc' });
+    expect(a).toBe(b);
+  });
+  it('never contains ":" (BullMQ rejects custom ids with a colon)', () => {
+    expect(jobId({ channel: 'fcm', notificationId: '6a540e689f1e1fd0f9a98788' })).not.toContain(':');
+    expect(jobId({ channel: 'email', notificationId: 'x' })).not.toContain(':');
+  });
+  it('distinguishes channels for the same notification', () => {
+    expect(jobId({ channel: 'fcm', notificationId: 'x' })).not.toBe(jobId({ channel: 'email', notificationId: 'x' }));
+  });
+  it('queue names are colon-free too', () => {
+    expect(QUEUE_NAMES.fcm).not.toContain(':');
+    expect(QUEUE_NAMES.email).not.toContain(':');
+  });
+});