diff --git a/.changeset/fix-bb-agent-stream-result-tojson.md b/.changeset/fix-bb-agent-stream-result-tojson.md
new file mode 100644
index 00000000..4a4cfc1f
--- /dev/null
+++ b/.changeset/fix-bb-agent-stream-result-tojson.md
@@ -0,0 +1,7 @@
+---
+"@aws-blocks/bb-agent": patch
+---
+
+fix(bb-agent): add toJSON() to AgentStreamResult
+
+`AgentStreamResult` now serializes to `{ channelId, channel: null }` when returned from API methods. Previously `channel` serialized to an empty object `{}`; it is now explicitly `null` to signal it is server-side only.
diff --git a/packages/bb-agent/API.md b/packages/bb-agent/API.md
index 8b90feb1..85464c55 100644
--- a/packages/bb-agent/API.md
+++ b/packages/bb-agent/API.md
@@ -93,6 +93,10 @@ export interface AgentStreamResult {
     channel: Promise<RealtimeChannel<AgentStreamChunk>>;
     channelId: string;
     complete: () => Promise<AgentStreamChunk>;
+    toJSON(): {
+        channelId: string;
+        channel: null;
+    };
 }
 
 // @public
diff --git a/packages/bb-agent/src/agent.ts b/packages/bb-agent/src/agent.ts
index 211c81d6..b281118e 100644
--- a/packages/bb-agent/src/agent.ts
+++ b/packages/bb-agent/src/agent.ts
@@ -428,6 +428,7 @@ export class AgentBase<TContext = DefaultToolContext> extends Scope {
 					}
 				});
 			}),
+			toJSON() { return { channelId, channel: null }; },
 		};
 	}
 
diff --git a/packages/bb-agent/src/index.test.ts b/packages/bb-agent/src/index.test.ts
index 4c053a58..436f4ebc 100644
--- a/packages/bb-agent/src/index.test.ts
+++ b/packages/bb-agent/src/index.test.ts
@@ -115,6 +115,19 @@ describe('needsApproval and interrupt mutual exclusivity', () => {
 	});
 });
 
+// ── AgentStreamResult.toJSON() ───────────────────────────────────────────────
+
+describe('AgentStreamResult.toJSON()', () => {
+	test('serializes to { channelId, channel: null }', async () => {
+		const scope = new Scope('test-tojson');
+		const agent = new Agent(scope, 'tj', { systemPrompt: 'test', model: { deployed: { provider: 'canned' }, local: { provider: 'canned' } } });
+		const result = await agent.stream('hello', { userId: 'test-user' });
+		const serialized = JSON.parse(JSON.stringify(result));
+		assert.deepStrictEqual(serialized, { channelId: result.channelId, channel: null });
+		assert.strictEqual('complete' in serialized, false);
+	});
+});
+
 // ── tool factory enforcement (compile-time) ──────────────────────────────────
 
 describe('tool factory enforcement', () => {
diff --git a/packages/bb-agent/src/types.ts b/packages/bb-agent/src/types.ts
index 39f632cd..33ff49b7 100644
--- a/packages/bb-agent/src/types.ts
+++ b/packages/bb-agent/src/types.ts
@@ -264,14 +264,30 @@ export interface StreamOptions<TContext = DefaultToolContext> {
 	context?: TContext;
 }
 
-/** Returned by stream(). Provides the channelId and server-side convenience methods. */
+/**
+ * Returned by stream(). Provides the channelId and server-side convenience methods.
+ *
+ * Safe to return directly from API methods — `toJSON()` serializes to
+ * `{ channelId, channel: null }`. Only `channelId` is meaningful client-side;
+ * `channel` is explicitly `null` to signal the live handle is server-side only,
+ * and the `complete()` helper is dropped (functions don't serialize).
+ */
 export interface AgentStreamResult {
 	/** Realtime channel ID where chunks are published. */
 	channelId: string;
-	/** Realtime channel handle — subscribe to streaming chunks or return to client as Transferable. */
+	/**
+	 * Realtime channel handle (server-side only). Nulled by `toJSON()` — clients subscribe from `channelId` instead.
+	 *
+	 * @remarks
+	 * Unlike `RealtimeChannel.toJSON()` which produces a hydratable descriptor, this is nulled
+	 * because it's a `Promise` that can't round-trip. Clients reconstruct a subscribe-only
+	 * channel from `channelId` via the `useChat` `subscribe` callback.
+	 */
 	channel: Promise<RealtimeChannel<AgentStreamChunk>>;
 	/** Wait for the complete response (server-side). Resolves when the done chunk arrives. */
 	complete: () => Promise<AgentStreamChunk>;
+	/** Only `{ channelId, channel: null }` is serialized when this object crosses the RPC boundary. */
+	toJSON(): { channelId: string; channel: null };
 }
 
 export interface AgentStreamChunk {