feat(vela): start mocked response flow after push-to-talk commit

apps/vela-gateway/src/index.js

@@ -67,21 +67,23 @@ function scheduleMockedTurnStep(session, turnId, delay, callback) {
 	session.mockedTurnTimers.push(timer);
 }
 
-function startMockedTurn(socket, session) {
+function startMockedTurn(socket, session, { transcript = MOCKED_USER_TRANSCRIPT, includeListeningState = true } = {}) {
 	if (session.mockedTurnInFlight) {
 		sendSocketError(socket, 'mocked_turn_in_flight', 'Only one mocked turn can run per session at a time.');
 		return;
 	}
 
 	clearMockedTurn(session);
-	session.audioChunkCount = 0;
 	session.mockedTurnInFlight = true;
 	const turnId = crypto.randomUUID();
 	session.activeMockedTurnId = turnId;
-	updateSessionState(socket, session, 'listening');
+
+	if (includeListeningState) {
+		updateSessionState(socket, session, 'listening');
+	}
 
 	scheduleMockedTurnStep(session, turnId, 75, () => {
-		sendSocketMessage(socket, 'transcript.final', { text: MOCKED_USER_TRANSCRIPT });
+		sendSocketMessage(socket, 'transcript.final', { text: transcript });
 		updateSessionState(socket, session, 'thinking');
 	});
 
@@ -264,11 +266,12 @@ function handleClientMessage(socket, session, rawMessage) {
 				break;
 			}
 
-			sendSocketMessage(socket, 'transcript.final', {
+			const finalTranscript = createPlaceholderFinalTranscript(session.audioChunkCount);
-				text: createPlaceholderFinalTranscript(session.audioChunkCount)
-			});
 			session.audioChunkCount = 0;
-			updateSessionState(socket, session, 'idle');
+			startMockedTurn(socket, session, {
+				transcript: finalTranscript,
+				includeListeningState: false
+			});
 			break;
 		case 'response.cancel':
 			clearMockedTurn(session);

apps/vela-gateway/test/websocket-session.test.js

@@ -303,6 +303,26 @@ test('websocket handles valid and invalid client messages safely', async () => {
 			type: 'transcript.final',
 			payload: { text: '[mocked final] Placeholder push-to-talk transcript completed from 1 appended chunk.' }
 		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'session.state',
+			payload: { value: 'thinking' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'session.state',
+			payload: { value: 'speaking' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'response.text.delta',
+			payload: { text: '[mocked assistant] ' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'response.text.delta',
+			payload: { text: 'This is a deterministic mocked response from the gateway vertical slice.' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'response.completed',
+			payload: {}
+		});
 		assert.deepEqual(await client.nextMessage(), {
 			type: 'session.state',
 			payload: { value: 'idle' }
@@ -358,6 +378,26 @@ test('websocket accepts a placeholder input cycle before a mocked turn on the sa
 			type: 'transcript.final',
 			payload: { text: '[mocked final] Placeholder push-to-talk transcript completed from 1 appended chunk.' }
 		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'session.state',
+			payload: { value: 'thinking' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'session.state',
+			payload: { value: 'speaking' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'response.text.delta',
+			payload: { text: '[mocked assistant] ' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'response.text.delta',
+			payload: { text: 'This is a deterministic mocked response from the gateway vertical slice.' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'response.completed',
+			payload: {}
+		});
 		assert.deepEqual(await client.nextMessage(), {
 			type: 'session.state',
 			payload: { value: 'idle' }
@@ -404,6 +444,37 @@ test('websocket emits deterministic partials for repeated appends and a determin
 			type: 'transcript.final',
 			payload: { text: '[mocked final] Placeholder push-to-talk transcript completed from 2 appended chunks.' }
 		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'session.state',
+			payload: { value: 'thinking' }
+		});
+
+		client.sendJson({ type: 'input_audio.commit', payload: {} });
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'error',
+			payload: {
+				code: 'mocked_turn_in_flight',
+				message: 'Wait for the mocked turn to finish before committing input.',
+				retryable: true
+			}
+		});
+
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'session.state',
+			payload: { value: 'speaking' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'response.text.delta',
+			payload: { text: '[mocked assistant] ' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'response.text.delta',
+			payload: { text: 'This is a deterministic mocked response from the gateway vertical slice.' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'response.completed',
+			payload: {}
+		});
 		assert.deepEqual(await client.nextMessage(), {
 			type: 'session.state',
 			payload: { value: 'idle' }
@@ -414,7 +485,30 @@ test('websocket emits deterministic partials for repeated appends and a determin
 			type: 'transcript.final',
 			payload: { text: '[mocked final] Placeholder push-to-talk transcript completed without appended audio.' }
 		});
-		await assert.rejects(() => client.nextMessage(150), /timed out waiting for websocket message/);
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'session.state',
+			payload: { value: 'thinking' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'session.state',
+			payload: { value: 'speaking' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'response.text.delta',
+			payload: { text: '[mocked assistant] ' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'response.text.delta',
+			payload: { text: 'This is a deterministic mocked response from the gateway vertical slice.' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'response.completed',
+			payload: {}
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'session.state',
+			payload: { value: 'idle' }
+		});
 
 		await client.close();
 	} finally {
@@ -542,6 +636,53 @@ test('websocket cancel stops an active mocked turn and allows a new one without
 	}
 });
 
+test('websocket cancel stops a push-to-talk commit response and allows another turn', async () => {
+	const server = await startServer();
+
+	try {
+		const client = await connectWebSocket(server.port);
+		await client.nextMessage();
+		await client.nextMessage();
+
+		client.sendJson({ type: 'input_audio.append', payload: { chunk: 'chunk-1' } });
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'session.state',
+			payload: { value: 'listening' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'transcript.partial',
+			payload: { text: '[mocked partial] Placeholder push-to-talk transcript in progress.' }
+		});
+
+		client.sendJson({ type: 'input_audio.commit', payload: {} });
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'transcript.final',
+			payload: { text: '[mocked final] Placeholder push-to-talk transcript completed from 1 appended chunk.' }
+		});
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'session.state',
+			payload: { value: 'thinking' }
+		});
+
+		client.sendJson({ type: 'response.cancel', payload: {} });
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'session.state',
+			payload: { value: 'idle' }
+		});
+		await assert.rejects(() => client.nextMessage(150), /timed out waiting for websocket message/);
+
+		client.sendJson({ type: 'mocked.turn.trigger', payload: {} });
+		assert.deepEqual(await client.nextMessage(), {
+			type: 'session.state',
+			payload: { value: 'listening' }
+		});
+
+		await client.close();
+	} finally {
+		await server.close();
+	}
+});
+
 test('websocket safely accepts cancel when no turn is active', async () => {
 	const server = await startServer();
 

apps/vela-ui/e2e/voice-session.spec.js

@@ -105,6 +105,7 @@ test('voice session shell supports a placeholder mic-control cycle before anothe
 	await expect(page.getByTestId('user-transcript')).toHaveText(
 		'[mocked final] Placeholder push-to-talk transcript completed from 1 appended chunk.'
 	);
+	await expect(page.getByTestId('assistant-response')).toHaveText(MOCKED_ASSISTANT_RESPONSE);
 	await expect(page.getByTestId('partial-transcript')).toHaveText('none');
 	await expect(page.getByTestId('gateway-session-state')).toHaveText('idle');
 	await expect(page.getByTestId('mocked-turn-button')).toBeEnabled();
@@ -114,3 +115,34 @@ test('voice session shell supports a placeholder mic-control cycle before anothe
 	await expect(page.getByTestId('assistant-response')).toHaveText(MOCKED_ASSISTANT_RESPONSE);
 	await expect(page.getByTestId('mocked-turn-status')).toHaveText('idle');
 });
+
+test('voice session shell can cancel a push-to-talk mocked response and start another turn', async ({ page }) => {
+	await page.goto('/');
+	await expect(page.getByTestId('hydration-status')).toHaveText('ready');
+
+	await page.getByTestId('connect-button').click();
+	await expect(page.getByTestId('connection-state')).toHaveText('connected');
+
+	await page.getByTestId('mic-control-button').dispatchEvent('mousedown');
+	await expect(page.getByTestId('gateway-session-state')).toHaveText('listening');
+	await page.getByTestId('mic-control-button').dispatchEvent('mouseup');
+
+	await expect(page.getByTestId('user-transcript')).toHaveText(
+		'[mocked final] Placeholder push-to-talk transcript completed from 1 appended chunk.'
+	);
+	await expect(page.getByTestId('cancel-turn-button')).toBeEnabled();
+	await expect(page.getByTestId('assistant-response')).toContainText('[mocked assistant]');
+
+	await page.getByTestId('cancel-turn-button').click();
+	await expect(page.getByTestId('gateway-session-state')).toHaveText('idle');
+	await expect(page.getByTestId('mocked-turn-status')).toHaveText('idle');
+	await expect(page.getByTestId('user-transcript')).toHaveText(
+		'[mocked final] Placeholder push-to-talk transcript completed from 1 appended chunk.'
+	);
+	await expect(page.getByTestId('assistant-response')).toContainText('[mocked assistant]');
+	await expect(page.getByTestId('mocked-turn-button')).toBeEnabled();
+
+	await page.getByTestId('mocked-turn-button').click();
+	await expect(page.getByTestId('assistant-response')).toHaveText(MOCKED_ASSISTANT_RESPONSE);
+	await expect(page.getByTestId('mocked-turn-status')).toHaveText('idle');
+});

apps/vela-ui/src/lib/VoiceSessionShell.svelte

@@ -53,6 +53,7 @@
 	let inProgressPartialTranscript = 'none';
 	let mockedAssistantResponse = 'none';
 	let mockedTurnInFlight = false;
+	let assistantResponseExpected = false;
 	let mockedConversationRenderOrder = [];
 	let micControlActive = false;
 	let hydrationStatus = 'mounting';
@@ -78,6 +79,7 @@
 		connectionState === 'connected' &&
 		socket?.readyState === WebSocket.OPEN &&
 		sessionReadyReceived &&
+		gatewaySessionState === 'idle' &&
 		!mockedTurnInFlight;
 
 	function clearSocketHandlers(targetSocket) {
@@ -96,6 +98,7 @@
 		inProgressPartialTranscript = 'none';
 		mockedAssistantResponse = 'none';
 		mockedTurnInFlight = false;
+		assistantResponseExpected = false;
 		mockedConversationRenderOrder = [];
 		micControlActive = false;
 	}
@@ -125,6 +128,7 @@
 		}
 
 		connectionDetail = 'Mic control shell released. Sending placeholder input_audio.commit.';
+		assistantResponseExpected = true;
 		socket.send(JSON.stringify(createMessageEnvelope('input_audio.commit', {})));
 	}
 
@@ -150,6 +154,7 @@
 		inProgressPartialTranscript = 'none';
 		mockedAssistantResponse = 'waiting for mocked response…';
 		mockedTurnInFlight = true;
+		assistantResponseExpected = true;
 		lastError = 'none';
 		socket.send(JSON.stringify(createMessageEnvelope('mocked.turn.trigger', {})));
 	}
@@ -235,6 +240,7 @@
 				gatewaySessionState = message.payload.value;
 				if (message.payload.value === 'idle') {
 					mockedTurnInFlight = false;
+					assistantResponseExpected = false;
 				}
 				return;
 			}
@@ -242,7 +248,8 @@
 			if (message.type === 'transcript.final') {
 				inProgressPartialTranscript = 'none';
 				mockedUserTranscript = message.payload.text;
-				if (mockedTurnInFlight) {
+				if (assistantResponseExpected) {
+					mockedTurnInFlight = true;
 					mockedAssistantResponse = '…';
 				}
 				if (!mockedConversationRenderOrder.includes('transcript')) {
@@ -276,12 +283,14 @@
 
 			if (message.type === 'response.completed') {
 				mockedTurnInFlight = false;
+				assistantResponseExpected = false;
 				return;
 			}
 
 			if (message.type === 'error') {
 				if (message.payload.retryable === false) {
 					mockedTurnInFlight = false;
+					assistantResponseExpected = false;
 					connectionState = 'error';
 					connectionDetail = 'Gateway reported a protocol error.';
 				} else {

apps/vela-ui/tests/voice-session.test.js

@@ -93,7 +93,7 @@ describe('voice session shell', () => {
 		});
 	});
 
-	it('runs a placeholder mic-control cycle and keeps mocked turn usable on the same socket', async () => {
+	it('runs a placeholder mic-control cycle, streams mocked assistant text, and keeps mocked turn usable on the same socket', async () => {
 		render(VoiceSessionShell);
 
 		await fireEvent.click(getByTestId('connect-button'));
@@ -154,6 +154,15 @@ describe('voice session shell', () => {
 				text: '[mocked final] Placeholder push-to-talk transcript completed from 1 appended chunk.'
 			})
 		);
+		socket.message(createMessageEnvelope('session.state', { value: 'thinking' }));
+		socket.message(createMessageEnvelope('session.state', { value: 'speaking' }));
+		socket.message(createMessageEnvelope('response.text.delta', { text: '[mocked assistant] ' }));
+		socket.message(
+			createMessageEnvelope('response.text.delta', {
+				text: 'This is a deterministic mocked response from the gateway vertical slice.'
+			})
+		);
+		socket.message(createMessageEnvelope('response.completed', {}));
 
 		socket.message(createMessageEnvelope('session.state', { value: 'idle' }));
 
@@ -163,6 +172,9 @@ describe('voice session shell', () => {
 			expect(getByTestId('user-transcript').textContent).toBe(
 				'[mocked final] Placeholder push-to-talk transcript completed from 1 appended chunk.'
 			);
+			expect(getByTestId('assistant-response').textContent).toBe(
+				'[mocked assistant] This is a deterministic mocked response from the gateway vertical slice.'
+			);
 			expect(getByTestId('partial-transcript').textContent).toBe('none');
 		});
 
@@ -172,6 +184,61 @@ describe('voice session shell', () => {
 		expect(JSON.parse(socket.sent[2]).type).toBe('mocked.turn.trigger');
 	});
 
+	it('keeps rendered push-to-talk transcript and assistant text visible after cancel', async () => {
+		render(VoiceSessionShell);
+
+		await fireEvent.click(getByTestId('connect-button'));
+		const socket = MockWebSocket.latest();
+		socket.open();
+		socket.message(createMessageEnvelope('session.ready', { sessionId: 'session-ptt-cancel' }));
+		socket.message(createMessageEnvelope('session.state', { value: 'idle' }));
+
+		await waitFor(() => {
+			expect(getByTestId('mic-control-button').hasAttribute('disabled')).toBe(false);
+		});
+
+		await fireEvent.mouseDown(getByTestId('mic-control-button'));
+		socket.message(createMessageEnvelope('session.state', { value: 'listening' }));
+		socket.message(
+			createMessageEnvelope('transcript.partial', {
+				text: '[mocked partial] Placeholder push-to-talk transcript in progress.'
+			})
+		);
+
+		await fireEvent.mouseUp(getByTestId('mic-control-button'));
+		expect(JSON.parse(socket.sent[1]).type).toBe('input_audio.commit');
+
+		socket.message(
+			createMessageEnvelope('transcript.final', {
+				text: '[mocked final] Placeholder push-to-talk transcript completed from 1 appended chunk.'
+			})
+		);
+		socket.message(createMessageEnvelope('session.state', { value: 'thinking' }));
+		socket.message(createMessageEnvelope('session.state', { value: 'speaking' }));
+		socket.message(createMessageEnvelope('response.text.delta', { text: '[mocked assistant] ' }));
+
+		await waitFor(() => {
+			expect(getByTestId('cancel-turn-button').hasAttribute('disabled')).toBe(false);
+			expect(getByTestId('assistant-response').textContent).toBe('[mocked assistant] ');
+		});
+
+		await fireEvent.click(getByTestId('cancel-turn-button'));
+		expect(JSON.parse(socket.sent[2]).type).toBe('response.cancel');
+
+		socket.message(createMessageEnvelope('session.state', { value: 'idle' }));
+
+		await waitFor(() => {
+			expect(getByTestId('gateway-session-state').textContent).toBe('idle');
+			expect(getByTestId('mocked-turn-status').textContent).toBe('idle');
+			expect(getByTestId('cancel-turn-button').hasAttribute('disabled')).toBe(true);
+			expect(getByTestId('user-transcript').textContent).toBe(
+				'[mocked final] Placeholder push-to-talk transcript completed from 1 appended chunk.'
+			);
+			expect(getByTestId('assistant-response').textContent).toBe('[mocked assistant] ');
+			expect(getByTestId('mic-control-button').hasAttribute('disabled')).toBe(false);
+		});
+	});
+
 	it('renders mocked transcript before assistant response for a connected session', async () => {
 		render(VoiceSessionShell);
 

docs/architecture.md

@@ -36,7 +36,7 @@ The repository now includes separate runnable workspaces for the UI and gateway
 - PWA enabled
 - WebSocket client
 
-The current implementation is a minimal SvelteKit app with a single voice-session shell page. The shipped UI can open and close a browser WebSocket connection to the gateway `/ws` endpoint, show explicit connection status (`not connected`, `connecting`, `connected`, `disconnected`, `error`), expose mic control shell interactions that emit placeholder `input_audio.append` / `input_audio.commit` events, trigger one deterministic mocked turn while connected, render deterministic placeholder partial/final transcripts for the push-to-talk shell, and render the mocked user transcript plus mocked assistant response for the existing mocked-turn path. This remains a shell only: there is no real microphone capture, real provider integration, or audio playback yet.
+The current implementation is a minimal SvelteKit app with a single voice-session shell page. The shipped UI can open and close a browser WebSocket connection to the gateway `/ws` endpoint, show explicit connection status (`not connected`, `connecting`, `connected`, `disconnected`, `error`), expose mic control shell interactions that emit placeholder `input_audio.append` / `input_audio.commit` events, trigger one deterministic mocked turn while connected, render deterministic placeholder partial/final transcripts for the push-to-talk shell, and stream the mocked assistant response both for `mocked.turn.trigger` and for push-to-talk commits. This remains a shell only: there is no real microphone capture, real provider integration, or audio playback yet.
 
 #### Responsibilities
 
@@ -104,8 +104,8 @@ The current implementation is a minimal Fastify service with `/`, `/health`, and
 - `GET /ws` documents the route for plain HTTP clients and returns `426 Upgrade Required`
 - WebSocket upgrades on `/ws` create an ephemeral session immediately
 - the gateway sends `session.ready` followed by `session.state` (`idle`) when the socket is established
-- valid minimal client events, including placeholder `input_audio.append` / `input_audio.commit`, can move the session between `idle` and `listening`
+- valid minimal client events, including placeholder `input_audio.append` / `input_audio.commit`, can move the session through the mocked turn states on one socket
-- placeholder `input_audio.append` emits deterministic mocked `transcript.partial` events and `input_audio.commit` emits one deterministic mocked `transcript.final`
+- placeholder `input_audio.append` emits deterministic mocked `transcript.partial` events and `input_audio.commit` emits one deterministic mocked `transcript.final` before starting the existing mocked assistant response stream
 - `mocked.turn.trigger` drives a fixed transcript/response event sequence over the existing shared protocol
 - only one mocked turn is allowed in flight per session at a time
 - invalid JSON, invalid envelopes, and malformed frames are handled defensively so the process stays up
@@ -116,13 +116,13 @@ The current implementation is a minimal Fastify service with `/`, `/health`, and
 - exposes connect, disconnect, mic-control shell interactions, and mocked-turn controls
 - does not request microphone permission or capture real microphone audio
 - only emits placeholder `input_audio.append` / `input_audio.commit` events; it does not send real audio data or play back audio
-- renders the latest placeholder partial transcript during a push-to-talk shell turn and replaces it with the final deterministic transcript on commit
+- renders the latest placeholder partial transcript during a push-to-talk shell turn, replaces it with the final deterministic transcript on commit, and appends streamed mocked assistant text for that same push-to-talk turn
 - reads mocked transcript and mocked response events from the shared protocol contract
 
 ## Voice Pipeline
 
 ```text
-Mic control shell / mocked turn button → Placeholder `input_audio.append` / `input_audio.commit` or mocked session flow → Deterministic transcript events → Mocked response text events when using mocked.turn.trigger → UI
+Mic control shell / mocked turn button → Placeholder `input_audio.append` / `input_audio.commit` or mocked session flow → Deterministic transcript events → Shared mocked response engine → Mocked response text events → UI
 ```
 
 This mocked vertical slice intentionally stands in for the future real pipeline:

docs/backlog.md

@@ -190,9 +190,9 @@ Polish the system after the core voice loop is reliable.
 - `apps/vela-gateway` now exposes a minimal `/ws` WebSocket session skeleton with ephemeral in-memory sessions and defensive message handling
 - `apps/vela-gateway` now accepts `mocked.turn.trigger` and emits protocol-valid mocked transcript/response events with one in-flight mocked turn per session
 - `apps/vela-gateway` now supports placeholder input-audio append/commit cycles before running another mocked turn on the same socket
-- `apps/vela-gateway` now emits deterministic `transcript.partial` events for placeholder `input_audio.append` messages and exactly one deterministic `transcript.final` for each placeholder `input_audio.commit`
+- `apps/vela-gateway` now emits deterministic `transcript.partial` events for placeholder `input_audio.append` messages and, after each accepted `input_audio.commit`, reuses the mocked response engine to stream a deterministic assistant reply for that push-to-talk turn
-- `apps/vela-ui` now renders the latest placeholder partial transcript during the push-to-talk shell turn and replaces it with the deterministic final transcript on commit
+- `apps/vela-ui` now renders the latest placeholder partial transcript during the push-to-talk shell turn, replaces it with the deterministic final transcript on commit, and shows streamed assistant text for the same push-to-talk flow
-- `apps/vela-ui` now exposes a cancel control for active mocked turns and keeps already-rendered transcript/response text visible after cancellation
+- `apps/vela-ui` now exposes a cancel control for active mocked turns and mocked push-to-talk responses, and keeps already-rendered transcript/response text visible after cancellation
-- `apps/vela-gateway` now honors `response.cancel` during mocked turns by stopping pending mocked response events, returning the session to `idle`, and allowing a new mocked turn on the same socket
+- `apps/vela-gateway` now honors `response.cancel` during mocked turns and push-to-talk-triggered mocked responses by stopping pending mocked response events, returning the session to `idle`, and allowing a new turn on the same socket
 - `apps/vela-protocol` now provides the shared WebSocket event contract for the UI and gateway
 - backend framework choice is now concrete: Fastify

docs/protocol.md

@@ -63,7 +63,7 @@ type ClientEvent =
 - a mocked turn emits deterministic `transcript.final`, `response.text.delta`, `response.completed`, and `session.state` events in protocol-valid order
 - `input_audio.append` updates the ephemeral session record and moves the session to `listening`
 - each accepted `input_audio.append` emits one deterministic `transcript.partial` for the current placeholder turn
-- `input_audio.commit` emits exactly one deterministic `transcript.final`, resets the minimal buffered state, and returns the session to `idle`
+- `input_audio.commit` emits exactly one deterministic `transcript.final` and then starts the same deterministic mocked assistant response stream used by `mocked.turn.trigger`
 - after a completed placeholder input cycle, the same socket can still send `mocked.turn.trigger`
 - `response.cancel` is safe to send even when no mocked turn is active
 - `response.cancel` stops any still-pending mocked turn events for the active turn and resets the minimal session state back to `idle`
@@ -92,7 +92,7 @@ Notes:
 - the UI disables the mic control while disconnected, before `session.ready`, or while a mocked turn is already in flight
 - pressing the mic control sends one placeholder `input_audio.append` chunk and releasing it sends `input_audio.commit`
 - while a placeholder push-to-talk turn is in progress, the UI renders the latest `transcript.partial`
-- after placeholder commit, the UI renders the `transcript.final` and clears the partial-only display
+- after placeholder commit, the UI renders the `transcript.final`, clears the partial-only display, and streams the mocked assistant text from the downstream response events
 - the UI copy explicitly labels the mic button as a control shell and not real microphone capture
 - the UI shows a cancel control and enables it only while a mocked turn is active
 - after cancel returns the gateway to `idle`, the UI clears the active-turn indicator but keeps any transcript or response text that was already rendered
@@ -147,9 +147,9 @@ Notes:
 - no audio, STT, LLM, TTS, or external providers participate in this flow
 - `response.cancel` can stop the mocked turn early, suppress any later mocked response events for that turn, and return the session to `idle`
 
-### Deterministic placeholder push-to-talk transcript sequence
+### Deterministic placeholder push-to-talk transcript and mocked response sequence
 
-For this increment, the existing mic-control shell still sends placeholder `input_audio.append` on press and `input_audio.commit` on release. The gateway now translates that shell flow into deterministic mocked transcript events only:
+For this increment, the existing mic-control shell still sends placeholder `input_audio.append` on press and `input_audio.commit` on release. The gateway now translates that shell flow into deterministic mocked transcript events and then reuses the existing mocked response stream:
 
 ```text
 input_audio.append #1
@@ -161,6 +161,11 @@ input_audio.append #N (N > 1)
 
 input_audio.commit after N appends
 → transcript.final("[mocked final] Placeholder push-to-talk transcript completed from N appended chunk(s).")
+→ session.state(thinking)
+→ session.state(speaking)
+→ response.text.delta("[mocked assistant] ")
+→ response.text.delta("This is a deterministic mocked response from the gateway vertical slice.")
+→ response.completed
 → session.state(idle)
 ```
 
@@ -168,7 +173,8 @@ Safe deterministic edge cases for this mocked placeholder flow:
 
 - commit without any prior append is accepted and emits `transcript.final("[mocked final] Placeholder push-to-talk transcript completed without appended audio.")`
 - repeated appends during one placeholder turn are accepted and each append replaces the latest partial transcript with a chunk-count-based deterministic value
-- placeholder commit does not automatically start assistant thinking, response streaming, or audio playback
+- after the final transcript, placeholder commit follows the same mocked `thinking → speaking → response.text.delta* → response.completed → idle` path as `mocked.turn.trigger`
+- `response.cancel` can interrupt this mocked post-commit response path the same way it interrupts `mocked.turn.trigger`; already-rendered transcript or assistant text is not retracted
 
 ## Contract Scope for This Increment
 
@@ -200,6 +206,7 @@ Current mocked-pipeline behavior:
 
 - during an active mocked turn, `response.cancel` returns the session to `idle` immediately
 - any mocked turn timers that have not fired yet are dropped, so no later `response.text.delta` or `response.completed` events are emitted for the cancelled turn
+- the same cancellation behavior applies when a mocked turn was started by `input_audio.commit`
 - once `idle` is restored, the same WebSocket session can start another mocked turn without reconnecting
 
 More general future-state expectations: