diff --git a/.changeset/notification-action-url-from-source.md b/.changeset/notification-action-url-from-source.md
new file mode 100644
index 000000000..5e908f8c7
--- /dev/null
+++ b/.changeset/notification-action-url-from-source.md
@@ -0,0 +1,22 @@
+---
+"@objectstack/service-messaging": minor
+---
+
+Synthesize the inbox `action_url` from the event `source` (ADR-0030 L5).
+
+The Console bell reads `sys_inbox_message` (the L5 in-app materialization),
+which carries only `action_url` — not the L2 `sys_notification` event's
+`source_object`/`source_id`. Producers that pass a `source` but no explicit
+`payload.url` (collaboration `@mention`, record assignment) therefore
+materialized inbox rows with no navigable link, so the bell entry couldn't
+deep-link to the originating record.
+
+`emit()` now synthesizes an app-relative `/{object}/{id}` link from `source`
+when no explicit `payload.url`/`payload.actionUrl` is supplied — in both the
+inline fan-out and the durable-outbox enqueue paths (`actionUrlFor()`).
+Precedence: explicit url → source-derived link → `undefined`. Keeps the L5
+materialization self-sufficient for navigation (the objectui bell consumes
+`action_url`).
+
+Tests: 3 new `messaging-service.test.ts` cases (source→link, explicit-url
+precedence, neither→undefined); all 95 service-messaging tests green.
diff --git a/packages/services/service-messaging/src/messaging-service.test.ts b/packages/services/service-messaging/src/messaging-service.test.ts
index 3e14d6712..a9f4b3d26 100644
--- a/packages/services/service-messaging/src/messaging-service.test.ts
+++ b/packages/services/service-messaging/src/messaging-service.test.ts
@@ -105,6 +105,39 @@ describe('MessagingService', () => {
             expect(result.deliveries[0]).toMatchObject({ channel: 'inbox', recipient: 'user_1', ok: true, externalId: 'row_1' });
         });
 
+        it('synthesizes an action_url from source when no explicit url is given (ADR-0030 L5 deep-link)', async () => {
+            const inbox = recordingChannel('inbox');
+            service.registerChannel(inbox.channel);
+            await service.emit({
+                topic: 'collab.assignment',
+                audience: ['user_1'],
+                payload: { title: 'Assigned to you' },
+                source: { object: 'showcase_task', id: 't_42' },
+            });
+            // The materialization carries a navigable link the bell can follow,
+            // even though the producer didn't set payload.url.
+            expect(inbox.seen[0].notification.actionUrl).toBe('/showcase_task/t_42');
+        });
+
+        it('prefers an explicit payload.url over the source-derived link', async () => {
+            const inbox = recordingChannel('inbox');
+            service.registerChannel(inbox.channel);
+            await service.emit({
+                topic: 't',
+                audience: ['user_1'],
+                payload: { title: 'Hi', url: '/custom/landing' },
+                source: { object: 'showcase_task', id: 't_42' },
+            });
+            expect(inbox.seen[0].notification.actionUrl).toBe('/custom/landing');
+        });
+
+        it('leaves action_url undefined when there is neither a url nor a source', async () => {
+            const inbox = recordingChannel('inbox');
+            service.registerChannel(inbox.channel);
+            await service.emit({ topic: 't', audience: ['user_1'], payload: { title: 'Hi' } });
+            expect(inbox.seen[0].notification.actionUrl).toBeUndefined();
+        });
+
         it('accepts a single (non-array) audience entry', async () => {
             const inbox = recordingChannel('inbox');
             service.registerChannel(inbox.channel);
diff --git a/packages/services/service-messaging/src/messaging-service.ts b/packages/services/service-messaging/src/messaging-service.ts
index 4bcbbcb97..3fc78b98c 100644
--- a/packages/services/service-messaging/src/messaging-service.ts
+++ b/packages/services/service-messaging/src/messaging-service.ts
@@ -239,7 +239,7 @@ export class MessagingService {
             severity: input.severity ?? 'info',
             recipients,
             channels: input.channels,
-            actionUrl: str(payload.url) ?? str(payload.actionUrl),
+            actionUrl: actionUrlFor(input, payload),
             payload: input.payload,
         };
 
@@ -267,7 +267,7 @@ export class MessagingService {
             title: str(payload.title) ?? input.topic,
             body: str(payload.body) ?? '',
             severity: input.severity ?? 'info',
-            actionUrl: str(payload.url) ?? str(payload.actionUrl),
+            actionUrl: actionUrlFor(input, payload),
         };
         const deliveries: DeliveryOutcome[] = [];
         for (const { recipient, channels, notBefore } of targets) {
@@ -392,3 +392,19 @@ function str(v: unknown): string | undefined {
     const s = String(v);
     return s.length > 0 ? s : undefined;
 }
+
+/**
+ * The deep-link the in-app materialization should carry. An explicit
+ * `payload.url`/`payload.actionUrl` wins; otherwise, when the emit names a
+ * `source` record, synthesize an app-relative `/{object}/{id}` link so the
+ * materialization is self-sufficient for navigation (the bell no longer has the
+ * L2 event's `source_object/source_id` to fall back on — ADR-0030 L5). Returns
+ * `undefined` when there is nothing to link to.
+ */
+function actionUrlFor(input: EmitInput, payload: Record<string, unknown>): string | undefined {
+    const explicit = str(payload.url) ?? str(payload.actionUrl);
+    if (explicit) return explicit;
+    const obj = str(input.source?.object);
+    const id = str(input.source?.id);
+    return obj && id ? `/${obj}/${id}` : undefined;
+}