diff --git a/.changeset/fix-bb-agent-empty-channel-id.md b/.changeset/fix-bb-agent-empty-channel-id.md
new file mode 100644
index 00000000..995073d0
--- /dev/null
+++ b/.changeset/fix-bb-agent-empty-channel-id.md
@@ -0,0 +1,7 @@
+---
+"@aws-blocks/bb-agent": patch
+---
+
+fix(bb-agent): treat empty channelId as unset in stream()
+
+An empty `channelId` now falls back to `conversationId` or a random UUID, preventing all streams from sharing the same channel. Empty strings are treated as unset rather than used literally.
diff --git a/packages/bb-agent/README.md b/packages/bb-agent/README.md
index 4e4c974a..f46b3ec7 100644
--- a/packages/bb-agent/README.md
+++ b/packages/bb-agent/README.md
@@ -47,7 +47,7 @@ const agent = new Agent(scope, id, config)
 | `getPendingInterrupts(conversationId)` | `Promise<Array<...>>` | Get unanswered interrupts (for reload support). |
 | `getChannel(channelId)` | `Promise<RealtimeChannel>` | Get a Realtime channel for subscribing to chunks. |
 
-`stream()` submits the message to AsyncJob and returns immediately — no API Gateway timeout risk. The agent runs asynchronously and publishes chunks to Realtime.
+`stream()` submits the message to AsyncJob and returns immediately — no API Gateway timeout risk. The agent runs asynchronously and publishes chunks to Realtime. The channel ID is resolved as `options.channelId || options.conversationId || crypto.randomUUID()` — empty strings are treated as unset and fall through to the next value.
 
 **Important: Subscribe before sending.** The agent starts emitting chunks immediately after `stream()` is called. If you subscribe to the channel after calling `stream()`, early chunks may be dropped. Always subscribe first, await `established`, then send:
 
diff --git a/packages/bb-agent/src/agent.ts b/packages/bb-agent/src/agent.ts
index 211c81d6..6dfebf32 100644
--- a/packages/bb-agent/src/agent.ts
+++ b/packages/bb-agent/src/agent.ts
@@ -403,7 +403,7 @@ export class AgentBase<TContext = DefaultToolContext> extends Scope {
 	 */
 	async stream(message: string, options?: StreamOptions<TContext>): Promise<AgentStreamResult> {
 		const conversationId = options?.conversationId;
-		const channelId = options?.channelId ?? conversationId ?? crypto.randomUUID();
+		const channelId = options?.channelId || conversationId || crypto.randomUUID();
 		if (!options?.userId && !this.config.inferenceOnly) throw blocksAgentError(AgentErrors.PersistenceRequired, 'userId is required when persistence is enabled. Pass it via options.userId.');
 		const userId = options?.userId ?? 'anonymous';
 		const context = this.resolveContext(options?.context);
diff --git a/packages/bb-agent/src/index.test.ts b/packages/bb-agent/src/index.test.ts
index 4c053a58..40b23168 100644
--- a/packages/bb-agent/src/index.test.ts
+++ b/packages/bb-agent/src/index.test.ts
@@ -115,6 +115,26 @@ describe('needsApproval and interrupt mutual exclusivity', () => {
 	});
 });
 
+// ── stream() empty channelId fallback ────────────────────────────────────────
+
+describe('stream() empty channelId fallback', () => {
+	test('empty channelId is treated as unset', async () => {
+		const scope = new Scope('test-empty-ch');
+		const agent = new Agent(scope, 'ec', { systemPrompt: 'test', model: { deployed: { provider: 'canned' }, local: { provider: 'canned' } } });
+		const result = await agent.stream('hello', { userId: 'test-user', channelId: '' });
+		assert.notStrictEqual(result.channelId, '');
+		assert.ok(result.channelId.length > 0);
+	});
+
+	test('empty conversationId is treated as unset', async () => {
+		const scope = new Scope('test-empty-conv');
+		const agent = new Agent(scope, 'ev', { systemPrompt: 'test', model: { deployed: { provider: 'canned' }, local: { provider: 'canned' } } });
+		const result = await agent.stream('hello', { userId: 'test-user', conversationId: '' });
+		assert.notStrictEqual(result.channelId, '');
+		assert.ok(result.channelId.length > 0);
+	});
+});
+
 // ── 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..118a9a01 100644
--- a/packages/bb-agent/src/types.ts
+++ b/packages/bb-agent/src/types.ts
@@ -251,7 +251,7 @@ export interface ToolCallRecord {
 
 export interface StreamOptions<TContext = DefaultToolContext> {
 	conversationId?: string;
-	/** Channel ID for Realtime delivery. Defaults to conversationId or a random UUID. */
+	/** Channel ID for Realtime delivery. Defaults to conversationId or a random UUID. Empty strings are treated as unset. */
 	channelId?: string;
 	/** User ID for conversation scoping. Defaults to 'anonymous'. */
 	userId?: string;