diff --git a/apps/vela-gateway/package.json b/apps/vela-gateway/package.json
index 25cd092..1cd96b2 100644
--- a/apps/vela-gateway/package.json
+++ b/apps/vela-gateway/package.json
@@ -8,7 +8,8 @@
     "start": "node src/index.js",
     "build": "node -e \"console.log('vela-gateway: no build step required')\""
   },
-  "dependencies": {
-    "fastify": "^5.2.1"
-  }
+	"dependencies": {
+		"@vela/protocol": "0.0.0",
+		"fastify": "^5.2.1"
+	}
 }
diff --git a/apps/vela-gateway/src/index.js b/apps/vela-gateway/src/index.js
index 57b8a5b..c5b8d2d 100644
--- a/apps/vela-gateway/src/index.js
+++ b/apps/vela-gateway/src/index.js
@@ -1,4 +1,9 @@
 const Fastify = require('fastify');
+const {
+	CLIENT_EVENT_TYPES,
+	PROTOCOL_PACKAGE_NAME,
+	SERVER_EVENT_TYPES
+} = require('@vela/protocol');
 
 function buildServer() {
 	const app = Fastify({ logger: true });
@@ -7,7 +12,12 @@ function buildServer() {
 		service: 'vela-gateway',
 		status: 'ok',
 		transport: 'http',
-		next: 'websocket session skeleton'
+		next: 'websocket session skeleton',
+		protocol: {
+			package: PROTOCOL_PACKAGE_NAME,
+			clientEventCount: CLIENT_EVENT_TYPES.length,
+			serverEventCount: SERVER_EVENT_TYPES.length
+		}
 	}));
 
 	app.get('/health', async () => ({ status: 'ok' }));
diff --git a/apps/vela-protocol/package.json b/apps/vela-protocol/package.json
new file mode 100644
index 0000000..d61c8aa
--- /dev/null
+++ b/apps/vela-protocol/package.json
@@ -0,0 +1,13 @@
+{
+	"name": "@vela/protocol",
+	"private": true,
+	"version": "0.0.0",
+	"type": "module",
+	"exports": {
+		".": {
+			"types": "./src/index.d.ts",
+			"import": "./src/index.js",
+			"require": "./src/index.cjs"
+		}
+	}
+}
diff --git a/apps/vela-protocol/src/index.cjs b/apps/vela-protocol/src/index.cjs
new file mode 100644
index 0000000..3391470
--- /dev/null
+++ b/apps/vela-protocol/src/index.cjs
@@ -0,0 +1,52 @@
+const PROTOCOL_PACKAGE_NAME = '@vela/protocol';
+
+const SESSION_STATES = Object.freeze(['idle', 'listening', 'thinking', 'speaking']);
+
+const CLIENT_EVENT_TYPES = Object.freeze([
+	'session.start',
+	'input_audio.append',
+	'input_audio.commit',
+	'response.cancel'
+]);
+
+const SERVER_EVENT_TYPES = Object.freeze([
+	'session.ready',
+	'session.state',
+	'transcript.partial',
+	'transcript.final',
+	'response.text.delta',
+	'response.completed',
+	'error'
+]);
+
+function createMessageEnvelope(type, payload) {
+	return { type, payload };
+}
+
+function isMessageEnvelope(value) {
+	return Boolean(
+		value &&
+		typeof value === 'object' &&
+		typeof value.type === 'string' &&
+		'payload' in value
+	);
+}
+
+function isClientEventType(type) {
+	return CLIENT_EVENT_TYPES.includes(type);
+}
+
+function isServerEventType(type) {
+	return SERVER_EVENT_TYPES.includes(type);
+}
+
+module.exports = {
+	PROTOCOL_PACKAGE_NAME,
+	SESSION_STATES,
+	CLIENT_EVENT_TYPES,
+	SERVER_EVENT_TYPES,
+	createMessageEnvelope,
+	isMessageEnvelope,
+	isClientEventType,
+	isServerEventType
+};
diff --git a/apps/vela-protocol/src/index.d.ts b/apps/vela-protocol/src/index.d.ts
new file mode 100644
index 0000000..bfdcfdc
--- /dev/null
+++ b/apps/vela-protocol/src/index.d.ts
@@ -0,0 +1,68 @@
+export type SessionState = 'idle' | 'listening' | 'thinking' | 'speaking';
+
+export type MessageEnvelope<TType extends string, TPayload> = {
+	type: TType;
+	payload: TPayload;
+};
+
+export type ClientEventPayloads = {
+	'session.start': Record<string, never>;
+	'input_audio.append': {
+		chunk: string;
+	};
+	'input_audio.commit': Record<string, never>;
+	'response.cancel': Record<string, never>;
+};
+
+export type ServerEventPayloads = {
+	'session.ready': {
+		sessionId: string;
+	};
+	'session.state': {
+		value: SessionState;
+	};
+	'transcript.partial': {
+		text: string;
+	};
+	'transcript.final': {
+		text: string;
+	};
+	'response.text.delta': {
+		text: string;
+	};
+	'response.completed': Record<string, never>;
+	'error': {
+		code: string;
+		message: string;
+		retryable?: boolean;
+	};
+};
+
+export type ClientEventType = keyof ClientEventPayloads;
+export type ServerEventType = keyof ServerEventPayloads;
+
+export type ClientEvent = {
+	[Type in ClientEventType]: MessageEnvelope<Type, ClientEventPayloads[Type]>;
+}[ClientEventType];
+
+export type ServerEvent = {
+	[Type in ServerEventType]: MessageEnvelope<Type, ServerEventPayloads[Type]>;
+}[ServerEventType];
+
+export const PROTOCOL_PACKAGE_NAME: '@vela/protocol';
+export const SESSION_STATES: readonly SessionState[];
+export const CLIENT_EVENT_TYPES: readonly ClientEventType[];
+export const SERVER_EVENT_TYPES: readonly ServerEventType[];
+
+export function createMessageEnvelope<TType extends ClientEventType>(
+	type: TType,
+	payload: ClientEventPayloads[TType]
+): MessageEnvelope<TType, ClientEventPayloads[TType]>;
+export function createMessageEnvelope<TType extends ServerEventType>(
+	type: TType,
+	payload: ServerEventPayloads[TType]
+): MessageEnvelope<TType, ServerEventPayloads[TType]>;
+
+export function isMessageEnvelope(value: unknown): value is MessageEnvelope<string, unknown>;
+export function isClientEventType(type: string): type is ClientEventType;
+export function isServerEventType(type: string): type is ServerEventType;
diff --git a/apps/vela-protocol/src/index.js b/apps/vela-protocol/src/index.js
new file mode 100644
index 0000000..b41f78e
--- /dev/null
+++ b/apps/vela-protocol/src/index.js
@@ -0,0 +1,41 @@
+export const PROTOCOL_PACKAGE_NAME = '@vela/protocol';
+
+export const SESSION_STATES = Object.freeze(['idle', 'listening', 'thinking', 'speaking']);
+
+export const CLIENT_EVENT_TYPES = Object.freeze([
+	'session.start',
+	'input_audio.append',
+	'input_audio.commit',
+	'response.cancel'
+]);
+
+export const SERVER_EVENT_TYPES = Object.freeze([
+	'session.ready',
+	'session.state',
+	'transcript.partial',
+	'transcript.final',
+	'response.text.delta',
+	'response.completed',
+	'error'
+]);
+
+export function createMessageEnvelope(type, payload) {
+	return { type, payload };
+}
+
+export function isMessageEnvelope(value) {
+	return Boolean(
+		value &&
+		typeof value === 'object' &&
+		typeof value.type === 'string' &&
+		'payload' in value
+	);
+}
+
+export function isClientEventType(type) {
+	return CLIENT_EVENT_TYPES.includes(type);
+}
+
+export function isServerEventType(type) {
+	return SERVER_EVENT_TYPES.includes(type);
+}
diff --git a/apps/vela-ui/package.json b/apps/vela-ui/package.json
index d45b694..87e6561 100644
--- a/apps/vela-ui/package.json
+++ b/apps/vela-ui/package.json
@@ -10,10 +10,11 @@
     "preview": "vite preview",
     "check": "svelte-kit sync && svelte-check --tsconfig ./jsconfig.json"
   },
-  "dependencies": {
-    "@sveltejs/adapter-auto": "^3.3.1",
-    "@sveltejs/kit": "^2.17.1",
-    "svelte": "^5.19.5"
+	"dependencies": {
+		"@vela/protocol": "0.0.0",
+		"@sveltejs/adapter-auto": "^3.3.1",
+		"@sveltejs/kit": "^2.17.1",
+		"svelte": "^5.19.5"
   },
   "devDependencies": {
     "@sveltejs/vite-plugin-svelte": "^5.0.3",
diff --git a/apps/vela-ui/src/routes/+page.svelte b/apps/vela-ui/src/routes/+page.svelte
index 5cfb3bf..7a6f7af 100644
--- a/apps/vela-ui/src/routes/+page.svelte
+++ b/apps/vela-ui/src/routes/+page.svelte
@@ -7,8 +7,14 @@
 </svelte:head>
 
 <script>
+	import {
+		CLIENT_EVENT_TYPES,
+		PROTOCOL_PACKAGE_NAME,
+		SERVER_EVENT_TYPES
+	} from '@vela/protocol';
+
 	const appStatus = 'Bootstrapped';
-	const nextFocus = 'Wire the voice session contract to the gateway.';
+	const nextFocus = `Build the voice session shell on top of ${PROTOCOL_PACKAGE_NAME}.`;
 </script>
 
 <div class="page">
@@ -20,6 +26,11 @@
 			streaming session UI will be added in later increments.
 		</p>
 
+		<p class="contract-note">
+			Shared protocol package loaded with {CLIENT_EVENT_TYPES.length} client event types and
+			{SERVER_EVENT_TYPES.length} server event types.
+		</p>
+
 		<div class="meta">
 			<div>
 				<span>Status</span>
@@ -76,6 +87,10 @@
 		color: #c7d6e8;
 	}
 
+	.contract-note {
+		margin-top: 1rem;
+	}
+
 	.meta {
 		margin-top: 1.5rem;
 		display: grid;
diff --git a/docs/backlog.md b/docs/backlog.md
index ce968c4..2881d77 100644
--- a/docs/backlog.md
+++ b/docs/backlog.md
@@ -11,7 +11,7 @@ Establish the boundaries, protocol, and state model for the system before integr
 ### Backlog Items
 
 - [x] define repository structure for `vela-ui` and `vela-gateway`
-- define the WebSocket event contract used by the UI and gateway
+- [x] define the WebSocket event contract used by the UI and gateway via shared package
 - define the session state machine and interrupt semantics
 - define provider adapter interfaces for STT, LLM, TTS, and tools
 - document error handling and cancellation behavior
@@ -180,4 +180,5 @@ Polish the system after the core voice loop is reliable.
 
 - `apps/vela-ui` now boots as a minimal SvelteKit app with a starter page
 - `apps/vela-gateway` now boots as a minimal Fastify app with `/` and `/health` endpoints
+- `apps/vela-protocol` now provides the shared WebSocket event contract for the UI and gateway
 - backend framework choice is now concrete: Fastify
diff --git a/docs/protocol.md b/docs/protocol.md
index 47ab4f4..d6fce51 100644
--- a/docs/protocol.md
+++ b/docs/protocol.md
@@ -2,31 +2,87 @@
 
 ## Event Protocol
 
+The shared code-level contract lives in the Yarn workspace package `@vela/protocol` so both the
+gateway and UI import the same event names and envelope shape.
+
+## WebSocket Message Envelope
+
+Every WebSocket message uses one envelope format:
+
+```ts
+type MessageEnvelope<TType extends string, TPayload> = {
+  type: TType;
+  payload: TPayload;
+};
+```
+
+This increment intentionally keeps the envelope minimal:
+
+- `type` identifies the event
+- `payload` carries the event body
+- no sequence numbers, timestamps, or protocol version fields yet
+- future changes should be additive when possible
+
 ### Client → Server
 
 ```ts
 type ClientEvent =
-  | { type: "start_listening" }
-  | { type: "stop_listening" }
-  | { type: "audio_chunk"; data: string } // PCM16 base64
-  | { type: "interrupt" };
+  | { type: "session.start"; payload: {} }
+  | { type: "input_audio.append"; payload: { chunk: string } }
+  | { type: "input_audio.commit"; payload: {} }
+  | { type: "response.cancel"; payload: {} };
 ```
 
+#### Client event intent
+
+- `session.start` initializes a voice session without locking in transport or auth details yet
+- `input_audio.append` carries a chunk of captured input audio as an encoded string
+- `input_audio.commit` marks the current buffered user turn as ready for downstream processing
+- `response.cancel` interrupts the active listen/think/speak flow
+
 ### Server → Client
 
 ```ts
 type ServerEvent =
-  | { type: "state"; value: "idle" | "listening" | "thinking" | "speaking" }
-  | { type: "partial_transcript"; text: string }
-  | { type: "final_transcript"; text: string }
-  | { type: "assistant_text_delta"; text: string }
-  | { type: "tool_call_started"; tool: string }
-  | { type: "tool_call_finished"; tool: string; result: unknown }
-  | { type: "tts_audio_chunk"; data: string }
-  | { type: "assistant_done" }
-  | { type: "error"; message: string };
+  | { type: "session.ready"; payload: { sessionId: string } }
+  | {
+      type: "session.state";
+      payload: { value: "idle" | "listening" | "thinking" | "speaking" };
+    }
+  | { type: "transcript.partial"; payload: { text: string } }
+  | { type: "transcript.final"; payload: { text: string } }
+  | { type: "response.text.delta"; payload: { text: string } }
+  | { type: "response.completed"; payload: {} }
+  | {
+      type: "error";
+      payload: { code: string; message: string; retryable?: boolean };
+    };
 ```
 
+#### Server event intent
+
+- `session.ready` confirms that the gateway created a session identity
+- `session.state` exposes the coarse session phase needed by the later UI shell
+- `transcript.partial` and `transcript.final` support incremental and completed user text display
+- `response.text.delta` supports streamed assistant text without committing to audio output details yet
+- `response.completed` marks the current assistant turn as done
+- `error` is the minimal recoverable failure shape for both UI and gateway work
+
+## Contract Scope for This Increment
+
+This contract is intentionally limited to the smallest event set needed to unblock:
+
+- the later gateway WebSocket session skeleton
+- the later UI voice-session shell
+
+Explicitly deferred for later increments:
+
+- tool-calling events
+- streamed TTS/output-audio events
+- reconnect/resume semantics
+- protocol version negotiation
+- provider-specific metadata fields
+
 ## State Machine
 
 ```text
@@ -37,13 +93,13 @@ idle
  → idle
 ```
 
-Interrupt can occur at:
+`response.cancel` can occur at:
 
 - listening → restart
 - thinking → cancel
 - speaking → stop immediately
 
-## Interrupt Handling Requirements
+## `response.cancel` Handling Requirements
 
 - immediate stop of TTS playback
 - immediate stop of LLM streaming
@@ -51,12 +107,14 @@ Interrupt can occur at:
 
 ### Mechanism
 
-The `interrupt` event cancels:
+The `response.cancel` event cancels:
 
 - TTS process
 - current LLM request
 - tool execution when possible
 
+This shared contract uses `response.cancel` consistently for that cancellation signal.
+
 ## Protocol Notes for Implementation
 
 - keep the protocol backward compatible when possible