diff --git a/docs/developers/qwen-serve-protocol.md b/docs/developers/qwen-serve-protocol.md
index 74224ab1cfc..5673bb4c036 100644
--- a/docs/developers/qwen-serve-protocol.md
+++ b/docs/developers/qwen-serve-protocol.md
@@ -69,7 +69,9 @@ Attaches to existing sessions are NOT counted toward the cap, so an idle daemon'
 
 ## Capabilities
 
-Every Stage 1 daemon advertises 9 feature tags. Clients **must** gate UI off `features`, not off `mode` (per design §10).
+The daemon advertises its supported feature tags from the serve capability
+registry. Clients **must** gate UI off `features`, not off `mode` (per design
+§10).
 
 ```
 ['health', 'capabilities', 'session_create', 'session_list',
@@ -104,6 +106,10 @@ Pass `?deep=1` (also accepts `?deep=true` or bare `?deep`) for a probe that expo
 ```json
 {
   "v": 1,
+  "protocolVersions": {
+    "current": "v1",
+    "supported": ["v1"]
+  },
   "mode": "http-bridge",
   "features": ["health", "capabilities", "..."],
   "modelServices": [],
@@ -113,6 +119,8 @@ Pass `?deep=1` (also accepts `?deep=true` or bare `?deep`) for a probe that expo
 
 Stable contract: when `v` increments the frame layout has changed in a backwards-incompatible way.
 
+> **`protocolVersions`** describes the serve protocol versions the daemon can speak. `current` is the daemon's preferred protocol version and `supported` is the compatible set. Additive to v=1: older v=1 daemons omit this field, so SDK clients that target older builds should treat it as optional.
+
 > **`modelServices` is always `[]` in Stage 1.** The agent uses its single default model service and doesn't enumerate it over the wire. Stage 2 will populate this from registered model adapters so SDK clients can build service-pickers; until then, do NOT rely on this field being non-empty.
 
 > **`workspaceCwd`** is the canonical absolute path this daemon binds to (#3803 §02 — 1 daemon = 1 workspace). Use it to (a) detect mismatch before posting `/session` and (b) omit `cwd` on `POST /session` (the route falls back to this path). Multi-workspace deployments expose multiple daemons on different ports, each with its own `workspaceCwd`. Additive to v=1: pre-§02 v=1 daemons omit the field — clients that target older builds should null-check before consuming it.
diff --git a/packages/cli/src/serve/capabilities.ts b/packages/cli/src/serve/capabilities.ts
new file mode 100644
index 00000000000..0f96352d315
--- /dev/null
+++ b/packages/cli/src/serve/capabilities.ts
@@ -0,0 +1,52 @@
+/**
+ * @license
+ * Copyright 2025 Qwen Team
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+export const SERVE_PROTOCOL_VERSION = 'v1' as const;
+
+export const SUPPORTED_SERVE_PROTOCOL_VERSIONS = [
+  SERVE_PROTOCOL_VERSION,
+] as const;
+
+export type ServeProtocolVersion =
+  (typeof SUPPORTED_SERVE_PROTOCOL_VERSIONS)[number];
+
+export interface ServeProtocolVersions {
+  current: ServeProtocolVersion;
+  supported: ServeProtocolVersion[];
+}
+
+export interface ServeCapabilityDescriptor {
+  since: ServeProtocolVersion;
+}
+
+export const SERVE_CAPABILITY_REGISTRY = {
+  health: { since: SERVE_PROTOCOL_VERSION },
+  capabilities: { since: SERVE_PROTOCOL_VERSION },
+  session_create: { since: SERVE_PROTOCOL_VERSION },
+  session_list: { since: SERVE_PROTOCOL_VERSION },
+  session_prompt: { since: SERVE_PROTOCOL_VERSION },
+  session_cancel: { since: SERVE_PROTOCOL_VERSION },
+  session_events: { since: SERVE_PROTOCOL_VERSION },
+  session_set_model: { since: SERVE_PROTOCOL_VERSION },
+  permission_vote: { since: SERVE_PROTOCOL_VERSION },
+} as const satisfies Record<string, ServeCapabilityDescriptor>;
+
+export type ServeFeature = keyof typeof SERVE_CAPABILITY_REGISTRY;
+
+export const SERVE_FEATURES = Object.freeze(
+  Object.keys(SERVE_CAPABILITY_REGISTRY) as ServeFeature[],
+);
+
+export function getServeFeatures(): ServeFeature[] {
+  return [...SERVE_FEATURES];
+}
+
+export function getServeProtocolVersions(): ServeProtocolVersions {
+  return {
+    current: SERVE_PROTOCOL_VERSION,
+    supported: [...SUPPORTED_SERVE_PROTOCOL_VERSIONS],
+  };
+}
diff --git a/packages/cli/src/serve/index.ts b/packages/cli/src/serve/index.ts
index 52288bbe2ed..85a5d12db6a 100644
--- a/packages/cli/src/serve/index.ts
+++ b/packages/cli/src/serve/index.ts
@@ -12,10 +12,20 @@ export {
 } from './runQwenServe.js';
 export {
   CAPABILITIES_SCHEMA_VERSION,
+  SERVE_CAPABILITY_REGISTRY,
+  SERVE_FEATURES,
+  SERVE_PROTOCOL_VERSION,
   STAGE1_FEATURES,
+  SUPPORTED_SERVE_PROTOCOL_VERSIONS,
+  getServeFeatures,
+  getServeProtocolVersions,
   type CapabilitiesEnvelope,
+  type ServeCapabilityDescriptor,
+  type ServeFeature,
   type ServeMode,
   type ServeOptions,
+  type ServeProtocolVersion,
+  type ServeProtocolVersions,
   type Stage1Feature,
 } from './types.js';
 export {
diff --git a/packages/cli/src/serve/server.test.ts b/packages/cli/src/serve/server.test.ts
index 1c916d271b9..7e7ef756c9d 100644
--- a/packages/cli/src/serve/server.test.ts
+++ b/packages/cli/src/serve/server.test.ts
@@ -11,6 +11,12 @@ import { describe, it, expect, afterEach, vi } from 'vitest';
 import request from 'supertest';
 import { createServeApp } from './server.js';
 import { runQwenServe, type RunHandle } from './runQwenServe.js';
+import {
+  getServeFeatures,
+  getServeProtocolVersions,
+  SERVE_CAPABILITY_REGISTRY,
+  SERVE_PROTOCOL_VERSION,
+} from './capabilities.js';
 import type {
   CancelNotification,
   PromptRequest,
@@ -30,11 +36,7 @@ import {
   type HttpAcpBridge,
 } from './httpAcpBridge.js';
 import type { BridgeEvent, SubscribeOptions } from './eventBus.js';
-import {
-  CAPABILITIES_SCHEMA_VERSION,
-  STAGE1_FEATURES,
-  type ServeOptions,
-} from './types.js';
+import { CAPABILITIES_SCHEMA_VERSION, type ServeOptions } from './types.js';
 
 const baseOpts: ServeOptions = {
   hostname: '127.0.0.1',
@@ -51,6 +53,17 @@ const baseOpts: ServeOptions = {
 // WS_B).
 const WS_BOUND = path.resolve(path.sep, 'work', 'bound');
 const WS_DIFFERENT = path.resolve(path.sep, 'work', 'different');
+const EXPECTED_STAGE1_FEATURES = [
+  'health',
+  'capabilities',
+  'session_create',
+  'session_list',
+  'session_prompt',
+  'session_cancel',
+  'session_events',
+  'session_set_model',
+  'permission_vote',
+] as const;
 
 interface FakeBridgeOpts {
   spawnImpl?: (req: BridgeSpawnRequest) => Promise<BridgeSession>;
@@ -190,6 +203,25 @@ function fakeBridge(opts: FakeBridgeOpts = {}): FakeBridge {
 }
 
 describe('createServeApp', () => {
+  describe('serve capability registry', () => {
+    it('returns a fresh ordered feature list', () => {
+      const features = getServeFeatures();
+      expect(features).toEqual([...EXPECTED_STAGE1_FEATURES]);
+
+      features.pop();
+      expect(getServeFeatures()).toEqual([...EXPECTED_STAGE1_FEATURES]);
+    });
+
+    it('marks every current feature as v1', () => {
+      expect(Object.keys(SERVE_CAPABILITY_REGISTRY)).toEqual([
+        ...EXPECTED_STAGE1_FEATURES,
+      ]);
+      expect(
+        Object.values(SERVE_CAPABILITY_REGISTRY).map(({ since }) => since),
+      ).toEqual(EXPECTED_STAGE1_FEATURES.map(() => SERVE_PROTOCOL_VERSION));
+    });
+  });
+
   describe('GET /health', () => {
     it('returns 200 ok', async () => {
       const app = createServeApp(baseOpts);
@@ -209,8 +241,9 @@ describe('createServeApp', () => {
         .set('Host', `127.0.0.1:${baseOpts.port}`);
       expect(res.status).toBe(200);
       expect(res.body.v).toBe(CAPABILITIES_SCHEMA_VERSION);
+      expect(res.body.protocolVersions).toEqual(getServeProtocolVersions());
       expect(res.body.mode).toBe('http-bridge');
-      expect(res.body.features).toEqual([...STAGE1_FEATURES]);
+      expect(res.body.features).toEqual(getServeFeatures());
       expect(res.body.modelServices).toEqual([]);
     });
 
diff --git a/packages/cli/src/serve/server.ts b/packages/cli/src/serve/server.ts
index 39cb996bde3..2144ee072a1 100644
--- a/packages/cli/src/serve/server.ts
+++ b/packages/cli/src/serve/server.ts
@@ -20,10 +20,10 @@ import {
   WorkspaceMismatchError,
   type HttpAcpBridge,
 } from './httpAcpBridge.js';
+import { getServeFeatures, getServeProtocolVersions } from './capabilities.js';
 import { SubscriberLimitExceededError, type BridgeEvent } from './eventBus.js';
 import {
   CAPABILITIES_SCHEMA_VERSION,
-  STAGE1_FEATURES,
   type CapabilitiesEnvelope,
   type ServeOptions,
 } from './types.js';
@@ -197,8 +197,9 @@ export function createServeApp(
   app.get('/capabilities', (_req, res) => {
     const envelope: CapabilitiesEnvelope = {
       v: CAPABILITIES_SCHEMA_VERSION,
+      protocolVersions: getServeProtocolVersions(),
       mode: opts.mode,
-      features: [...STAGE1_FEATURES],
+      features: getServeFeatures(),
       modelServices: [],
       // #3803 §02: surface the bound workspace so clients can detect
       // mismatch pre-flight and omit `cwd` on `POST /session`.
diff --git a/packages/cli/src/serve/types.ts b/packages/cli/src/serve/types.ts
index 19ccdf4c3e9..395ca48f752 100644
--- a/packages/cli/src/serve/types.ts
+++ b/packages/cli/src/serve/types.ts
@@ -4,6 +4,12 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+import {
+  SERVE_FEATURES,
+  type ServeFeature,
+  type ServeProtocolVersions,
+} from './capabilities.js';
+
 /**
  * Stage 1 daemon mode shape.
  *
@@ -87,6 +93,11 @@ export interface ServeOptions {
  */
 export interface CapabilitiesEnvelope {
   v: 1;
+  /**
+   * Serve protocol versions supported by this daemon. Optional because this is
+   * additive to v=1; older v=1 daemons omit it.
+   */
+  protocolVersions?: ServeProtocolVersions;
   mode: ServeMode;
   features: string[];
   /**
@@ -119,34 +130,21 @@ export interface CapabilitiesEnvelope {
 
 export const CAPABILITIES_SCHEMA_VERSION = 1 as const;
 
-/**
- * Stage 1 ships only the routes wired in `server.ts`. As routes land in
- * follow-up PRs, append the corresponding feature tag here so clients can
- * progressively enable UI affordances.
- *
- * The annotation is intentionally absent: `as const` widens to
- * `readonly ['health', 'capabilities', ...]` and the derived
- * `Stage1Feature` union catches typos at compile time. Annotating as
- * `readonly string[]` would erase the literal information.
- */
-// FIXME(stage-1.5, chiga0 finding 5):
-// `STAGE1_FEATURES` is a hard-coded constant — `extMethod` plugins
-// can't contribute to the capability set without editing the daemon.
-// Stage 1.5 should convert this to a registry that bridges and
-// plugins push into, alongside an `ext_*` event family + a
-// `POST /ext/:method` route. Tracked under #3803.
-// Reference: https://github.com/QwenLM/qwen-code/pull/3889#issuecomment-4427773706
-export const STAGE1_FEATURES = [
-  'health',
-  'capabilities',
-  'session_create',
-  'session_list',
-  'session_prompt',
-  'session_cancel',
-  'session_events',
-  'session_set_model',
-  'permission_vote',
-] as const;
+/** @deprecated Use SERVE_FEATURES from the capability registry. */
+export const STAGE1_FEATURES = SERVE_FEATURES;
+
+/** @deprecated Use ServeFeature from the capability registry. */
+export type Stage1Feature = ServeFeature;
 
-/** Compile-time-checked feature identifier — element of STAGE1_FEATURES. */
-export type Stage1Feature = (typeof STAGE1_FEATURES)[number];
+export {
+  getServeFeatures,
+  getServeProtocolVersions,
+  SERVE_CAPABILITY_REGISTRY,
+  SERVE_FEATURES,
+  SERVE_PROTOCOL_VERSION,
+  SUPPORTED_SERVE_PROTOCOL_VERSIONS,
+  type ServeCapabilityDescriptor,
+  type ServeFeature,
+  type ServeProtocolVersion,
+  type ServeProtocolVersions,
+} from './capabilities.js';
diff --git a/packages/sdk-typescript/src/daemon/index.ts b/packages/sdk-typescript/src/daemon/index.ts
index 106f6e4f42e..4939f97d3b2 100644
--- a/packages/sdk-typescript/src/daemon/index.ts
+++ b/packages/sdk-typescript/src/daemon/index.ts
@@ -18,6 +18,7 @@ export type {
   DaemonCapabilities,
   DaemonEvent,
   DaemonMode,
+  DaemonProtocolVersions,
   DaemonSession,
   DaemonSessionSummary,
   PermissionOutcome,
diff --git a/packages/sdk-typescript/src/daemon/types.ts b/packages/sdk-typescript/src/daemon/types.ts
index 7114b0f6670..9c880ff55a6 100644
--- a/packages/sdk-typescript/src/daemon/types.ts
+++ b/packages/sdk-typescript/src/daemon/types.ts
@@ -15,9 +15,19 @@
 
 export type DaemonMode = 'http-bridge' | 'native';
 
+export interface DaemonProtocolVersions {
+  current: string;
+  supported: string[];
+}
+
 /** Capabilities envelope returned from `GET /capabilities`. */
 export interface DaemonCapabilities {
   v: 1;
+  /**
+   * Serve protocol versions supported by the daemon. Optional because this is
+   * additive to v=1; older v=1 daemons omit it.
+   */
+  protocolVersions?: DaemonProtocolVersions;
   mode: DaemonMode;
   /**
    * Feature tags the client should gate UI off (e.g. `permission_vote`,
diff --git a/packages/sdk-typescript/src/index.ts b/packages/sdk-typescript/src/index.ts
index 2dc5a2e8fea..6d4a0d48fd2 100644
--- a/packages/sdk-typescript/src/index.ts
+++ b/packages/sdk-typescript/src/index.ts
@@ -16,6 +16,7 @@ export {
   type DaemonClientOptions,
   type DaemonEvent,
   type DaemonMode,
+  type DaemonProtocolVersions,
   type DaemonSession,
   type DaemonSessionSummary,
   type PermissionOutcome,
diff --git a/packages/sdk-typescript/test/unit/DaemonClient.test.ts b/packages/sdk-typescript/test/unit/DaemonClient.test.ts
index e39c93d1ba3..18164d0acde 100644
--- a/packages/sdk-typescript/test/unit/DaemonClient.test.ts
+++ b/packages/sdk-typescript/test/unit/DaemonClient.test.ts
@@ -98,6 +98,10 @@ describe('DaemonClient', () => {
     it('GETs /capabilities and returns the v1 envelope', async () => {
       const envelope = {
         v: 1 as const,
+        protocolVersions: {
+          current: 'v1',
+          supported: ['v1'],
+        },
         mode: 'http-bridge' as const,
         features: ['health', 'capabilities'],
         modelServices: [],
@@ -111,6 +115,19 @@ describe('DaemonClient', () => {
       // omit `cwd` from `POST /session` (route falls back).
       expect(caps.workspaceCwd).toBe('/work/bound');
     });
+
+    it('accepts old v1 envelopes without protocolVersions', async () => {
+      const envelope: DaemonCapabilities = {
+        v: 1,
+        mode: 'http-bridge',
+        features: ['health', 'capabilities'],
+        modelServices: [],
+        workspaceCwd: '/work/bound',
+      };
+      const { fetch } = recordingFetch(() => jsonResponse(200, envelope));
+      const client = new DaemonClient({ baseUrl: 'http://daemon', fetch });
+      await expect(client.capabilities()).resolves.toEqual(envelope);
+    });
   });
 
   describe('bearer auth', () => {