feat(client): ISession.rename settles the title projection cell from the response

rename is a per-session verb on the outward session face (prompt/cancel
precedent), not a list-service verb: the Session calls session.rename and
applies the response {title, seq} to its projection store under
higher-seq-wins, so the list row updates before the push frame. The fixture
api and the test-runtime double follow the same face.

packages/client/connection/src/client/fixture.ts

@@ -943,6 +943,33 @@ export function createFixtureApi(options: FixtureOptions = {}): ApiProxy {
         if (options.dropSessionCreateResponse) throw new Error('fixture: dropped session.create response after publication')
         return ok(request, { sessionId: created.sessionId })
       },
+      rename: (request) => {
+        const { sessionId, title } = request.payload
+        const source = summaryOf(sessionId)
+        if (source === undefined) {
+          return err(request, {
+            code: 'session-not-found',
+            message: `no session ${sessionId}`,
+            details: { sessionId },
+          })
+        }
+        const normalized = title.trim().replace(/\s+/g, ' ')
+        if (normalized.length === 0) {
+          return err(request, {
+            code: 'title-invalid',
+            message: `rename rejected for session ${sessionId}: empty title`,
+            details: { sessionId },
+          })
+        }
+        // The append emits the session/event and its session/projection frame
+        // (host parallel); the unary response settles the caller first.
+        append(sessionId, {
+          type: 'session/title',
+          data: { title: normalized, messageSeqs: [], source: { kind: 'user' } },
+        })
+        const log = logOf(sessionId)
+        return ok(request, { title: normalized, seq: log.length - 1 })
+      },
       history: async (request) => {
         const log = logs.get(request.payload.sessionId) ?? []
         // Snapshot at request time, deliver after the transit delay (mirrors a real host under latency).
@@ -1486,6 +1513,7 @@ export class FixtureApiClient extends AbstractApiClient {
       case 'session.history': return this.api.sessions.history(request)
       case 'session.models': return this.api.sessions.models(request)
       case 'session.selectModel': return this.api.sessions.selectModel(request)
+      case 'session.rename': return this.api.sessions.rename(request)
       case 'session.prompt': return this.api.sessions.prompt(request)
       case 'session.cancel': return this.api.sessions.cancel(request)
       case 'host.describe': return this.api.host.describe(request)

packages/client/connection/tests/fake-api.ts

@@ -45,6 +45,7 @@ export class FakeApiClient implements IApiClient {
   // Programmable slots (defaults answer OK-empty); reassign per case.
   onList: (payload: unknown) => Promise<RpcResponse<{ items: never[] }>> = () => Promise.resolve(ok({ items: [] }))
   onCreate: (payload: unknown) => Promise<RpcResponse<{ sessionId: SessionId }>> = () => Promise.resolve(ok({ sessionId: 'fk-new' as SessionId }))
+  onRename: (payload: unknown) => Promise<RpcResponse<{ title: string; seq: number }>> = () => Promise.resolve(ok({ title: 'fk-renamed', seq: 0 }))
   onHistory: (payload: { sessionId: SessionId; beforeSeq?: number; maxMessages?: number })
   => Promise<RpcResponse<{ events: never[]; hasMore: boolean; modelTarget: ModelTarget }>> =
     () => Promise.resolve(ok({
@@ -96,6 +97,7 @@ export class FakeApiClient implements IApiClient {
     models: (payload: unknown) => this.record('session.models', payload, this.onModels(payload)),
     selectModel: (payload: ModelTarget & { sessionId: SessionId }) =>
       this.record('session.selectModel', payload, this.onSelectModel(payload)),
+    rename: (payload: unknown) => this.record('session.rename', payload, this.onRename(payload)),
     prompt: (payload: unknown) => this.record('session.prompt', payload, this.onPrompt(payload)),
     cancel: (payload: unknown) => this.record('session.cancel', payload, this.onCancel(payload)),
   }

packages/client/runtime/src/client/contract/session.ts

@@ -41,6 +41,13 @@ export interface ISession {
    * @returns acceptance, or the business error.
    */
   cancel(): Promise<RpcResult<{ accepted: true }>>
+  /**
+   * Rename this session (explicit user title; pins it against automatic
+   * regeneration).
+   * @param title - raw title text (the host normalizes acceptance).
+   * @returns the normalized accepted title and its event seq, or the business error.
+   */
+  rename(title: string): Promise<RpcResult<{ title: string; seq: number }>>
   /**
    * Extend the history window backwards (older messages pagination).
    * @returns completion; failures land in snapshot.openState/loadingOlder.

packages/client/runtime/src/client/sessions/session.ts

@@ -252,6 +252,25 @@ export class Session implements SessionFace {
     return result
   }
 
+  /**
+   * Rename: contract session.rename 1:1. On success settle the 'title'
+   * projection cell from the response's `{title, seq}` under the store's
+   * higher-seq-wins rule (the push frame arriving later is a no-op replay),
+   * so the list row and any useProjection('title') reader update without
+   * waiting for the mux frame.
+   * @param title - raw title text (the host normalizes acceptance).
+   * @returns the rename result (normalized accepted title + title event seq).
+   */
+  async rename(title: string): Promise<RpcResult<{ title: string; seq: number }>> {
+    try {
+      const { result } = await this.api.sessions.rename({ sessionId: this.sessionId, title })
+      if (result.ok) this.projections.apply('title', result.value.title, result.value.seq)
+      return result
+    } catch (error) {
+      return transportError(error)
+    }
+  }
+
   /**
    * Execute one slash-command line against this session's agent — pure
    * admission semantics (the host executor durably logs the lifecycle;

packages/client/runtime/tests/fake-api.ts

@@ -63,6 +63,7 @@ export class FakeApiClient implements IApiClient {
   onList: (payload: unknown) => Promise<RpcResponse<{ items: never[] }>> = () => Promise.resolve(ok({ items: [] }))
   onCreate: (payload: unknown) => Promise<RpcResponse<{ sessionId: SessionId }>> = () => Promise.resolve(ok({ sessionId: 'fk-new' as SessionId }))
   readonly defaultModel: ModelTarget = { provider: 'deepseek', model: 'deepseek-v4-flash' }
+  onRename: (payload: unknown) => Promise<RpcResponse<{ title: string; seq: number }>> = () => Promise.resolve(ok({ title: 'fk-renamed', seq: 0 }))
   onHistory: (payload: { sessionId: SessionId; beforeSeq?: number; maxMessages?: number })
   => Promise<RpcResponse<{ events: never[]; hasMore: boolean }>> =
     () => Promise.resolve(ok({ events: [], hasMore: false }))
@@ -115,6 +116,7 @@ export class FakeApiClient implements IApiClient {
     models: (payload: unknown) => this.record('session.models', payload, this.onModels(payload)),
     selectModel: (payload: { provider: string; model: string }) =>
       this.record('session.selectModel', payload, this.onSelectModel(payload)),
+    rename: (payload: unknown) => this.record('session.rename', payload, this.onRename(payload)),
     prompt: (payload: unknown) => this.record('session.prompt', payload, this.onPrompt(payload)),
     cancel: (payload: unknown) => this.record('session.cancel', payload, this.onCancel(payload)),
   }

packages/client/runtime/tests/session.spec.ts

@@ -301,6 +301,32 @@ describe('prompt and cancel errors', () => {
   })
 })
 
+describe('rename', () => {
+  it('settles the title projection cell from the unary response (higher-seq-wins vs the push frame)', async () => {
+    const { api, session } = makeSession()
+    api.onRename = () => Promise.resolve(ok({ title: '正名', seq: 7 }))
+    const result = await session.rename('  正名  ')
+    expect(result).toMatchObject({ ok: true, value: { title: '正名', seq: 7 } })
+    expect(api.callsOf('session.rename')).toMatchObject([{ sessionId: SID, title: '  正名  ' }])
+    expect(session.projections.faceOf('title').getSnapshot()).toBe('正名')
+    // A stale lower-seq apply (the push-frame path routes into this same
+    // store) must not roll the settled value back.
+    session.projections.apply('title', '旧名', 3)
+    expect(session.projections.faceOf('title').getSnapshot()).toBe('正名')
+  })
+
+  it('returns the business error untouched and folds a transport throw to internal', async () => {
+    const { api, session } = makeSession()
+    api.onRename = () => Promise.resolve(err({ code: 'title-invalid', message: 'empty', details: { sessionId: SID } }))
+    const rejected = await session.rename('   ')
+    expect(rejected).toMatchObject({ ok: false, error: { code: 'title-invalid' } })
+    expect(session.projections.faceOf('title').getSnapshot()).toBeUndefined()
+    api.onRename = () => Promise.reject(new Error('rename transport down'))
+    const folded = await session.rename('x')
+    expect(folded).toMatchObject({ ok: false, error: { code: 'internal' } })
+  })
+})
+
 describe('pending interactions', () => {
   it('adds approval/question on requested and removes them on resolved', async () => {
     const { session } = makeSession()

packages/client/test-runtime/src/sessions.ts

@@ -107,6 +107,14 @@ export class FixtureSession implements SessionFace {
   loadOlder(): never {
     throw new Error(`test session "${this.sessionId}": loadOlder is not stubbed — supply it on the fixture's session face`)
   }
+
+  /**
+   * Fail-loud stub; supply `rename` on the fixture's session face to exercise it.
+   * @returns never — always throws.
+   */
+  rename(): never {
+    throw new Error(`test session "${this.sessionId}": rename is not stubbed — supply it on the fixture's session face`)
+  }
 }
 
 /** One live test session: fixture-derived stores plus its minted scope state. */

packages/client/test-runtime/tests/runtime.spec.tsx

@@ -470,6 +470,7 @@ describe('fixture session face', () => {
     expect(() => bare.cancel()).toThrow(/cancel is not stubbed/)
     expect(() => bare.command()).toThrow(/command is not stubbed/)
     expect(() => bare.loadOlder()).toThrow(/loadOlder is not stubbed/)
+    expect(() => bare.rename()).toThrow(/rename is not stubbed/)
     await runtime.dispose()
   })