diff --git a/packages/client/connection/src/client/fixture.ts b/packages/client/connection/src/client/fixture.ts
index 45445cc9cf..49f0aaf9dc 100644
--- a/packages/client/connection/src/client/fixture.ts
+++ b/packages/client/connection/src/client/fixture.ts
@@ -969,6 +969,9 @@ export function createFixtureApi(options: FixtureOptions = {}): ApiProxy {
     updatedAt: fixtureEpoch,
   }]
   let nextWorkspace = 1
+  // Registry-global archive set mirroring the host: archived sessions keep
+  // their workspace accounting slot and only grouping surfaces hide them.
+  const archivedSessionIds: SessionId[] = []
 
   // In-memory browse tree behind the fixture's `browse` picker capability —
   // deterministic content mirroring the design mock so assembled Web tests
@@ -1623,7 +1626,10 @@ export function createFixtureApi(options: FixtureOptions = {}): ApiProxy {
       openPath: request => ok(request, { opened: true as const }),
     },
     workspace: {
-      list: request => ok(request, { items: workspaces.map(w => ({ ...w })) }),
+      list: request => ok(request, {
+        items: workspaces.map(w => ({ ...w })),
+        archivedSessionIds: [...archivedSessionIds],
+      }),
       create: (request) => {
         const { path, name } = request.payload
         const target = path ?? `/tmp/fixture-workspaces/${name ?? ''}`
@@ -1709,6 +1715,16 @@ export function createFixtureApi(options: FixtureOptions = {}): ApiProxy {
         }
         return ok(request, { workspace: { ...workspace } })
       },
+      archiveSession: (request) => {
+        const missing = requireSession(request)
+        if (missing !== undefined) return missing
+        const { sessionId } = request.payload
+        if (!archivedSessionIds.includes(sessionId)) {
+          archivedSessionIds.push(sessionId)
+          emitHost({ type: 'host/archived-sessions-changed', archivedSessionIds: [...archivedSessionIds] })
+        }
+        return ok(request, { archivedSessionIds: [...archivedSessionIds] })
+      },
     },
     commands: {
       // The catalog mirrors one session's effective view (every fixture
@@ -2089,6 +2105,7 @@ export class FixtureApiClient extends AbstractApiClient {
       case 'workspace.rename': return this.api.workspace.rename(request)
       case 'workspace.delete': return this.api.workspace.delete(request)
       case 'workspace.insertSessionBefore': return this.api.workspace.insertSessionBefore(request)
+      case 'workspace.archiveSession': return this.api.workspace.archiveSession(request)
       case 'command.list': return this.api.commands.list(request)
       case 'command.execute': return this.api.commands.execute(request, signal)
       case 'skill.list': return this.api.skills.list(request)
diff --git a/packages/client/connection/tests/fake-api.ts b/packages/client/connection/tests/fake-api.ts
index 0d58800279..d599de1be7 100644
--- a/packages/client/connection/tests/fake-api.ts
+++ b/packages/client/connection/tests/fake-api.ts
@@ -122,7 +122,7 @@ export class FakeApiClient implements IApiClient {
   }
 
   readonly workspace: IApiClient['workspace'] = {
-    list: (payload: unknown) => this.record('workspace.list', payload, Promise.resolve(ok({ items: [] }))),
+    list: (payload: unknown) => this.record('workspace.list', payload, Promise.resolve(ok({ items: [], archivedSessionIds: [] }))),
     create: (payload: unknown) => this.record('workspace.create', payload, Promise.resolve(ok({
       workspace: { workspaceId: 'fk-ws' as never, path: '/f/ws', title: 'ws', sessionIds: [], createdAt: '0', updatedAt: '0' },
       created: true,
@@ -134,6 +134,9 @@ export class FakeApiClient implements IApiClient {
     insertSessionBefore: (payload: unknown) => this.record('workspace.insertSessionBefore', payload, Promise.resolve(ok({
       workspace: { workspaceId: 'fk-ws' as never, path: '/f/ws', title: 'ws', sessionIds: [], createdAt: '0', updatedAt: '0' },
     }))),
+    archiveSession: (payload: unknown) => this.record('workspace.archiveSession', payload, Promise.resolve(ok({
+      archivedSessionIds: [(payload as { sessionId: SessionId }).sessionId],
+    }))),
   }
 
   // Payloads stay `unknown` (lint-lane note above); response rows are the real
diff --git a/packages/client/locale/tests/language-row.spec.tsx b/packages/client/locale/tests/language-row.spec.tsx
index 2fdc5d5f45..223b9761b2 100644
--- a/packages/client/locale/tests/language-row.spec.tsx
+++ b/packages/client/locale/tests/language-row.spec.tsx
@@ -21,7 +21,7 @@ function emptySessions() {
 }
 function emptyWorkspaces() {
   const store = createSnapshotStore<WorkspaceListState>({
-    items: [], state: 'idle', phase: 'ready', error: null,
+    items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
     baselinesReady: true, recentWorkspaceId: undefined,
   })
   return bindSnapshotSelector(store)
diff --git a/packages/client/runtime/README.i18n.yaml b/packages/client/runtime/README.i18n.yaml
index f42e4f09d0..60fcf831b4 100644
--- a/packages/client/runtime/README.i18n.yaml
+++ b/packages/client/runtime/README.i18n.yaml
@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/client/runtime/README.md
-README.md: 9f2b165f1a98dcecfa3ab82386da9b094cfd2f54
-README.zh.md: 3ed047e65d3bddc14c3b6b84f327bbeebf805d4b
+README.md: 4c9122d87bfb0ea2d66478de03c69975b0577ab7
+README.zh.md: 1ee4731964ea75de29da47137933852581fc45b3
diff --git a/packages/client/runtime/README.md b/packages/client/runtime/README.md
index 9f2b165f1a..4c9122d87b 100644
--- a/packages/client/runtime/README.md
+++ b/packages/client/runtime/README.md
@@ -10,6 +10,8 @@ Workspace and Session lists have independent monotone `pending` → `ready` base
 
 `WorkspacesService.delete(workspaceId)` removes the registration from the client projection after the successful unary response; the matching `host/workspace-removed` frame is idempotent and synchronizes other tabs. Session state and the current Session selection are independent, so accounted Sessions immediately project under Ungrouped after their Workspace disappears.
 
+`WorkspaceListState.archivedSessionIds` mirrors the Host's registry-global archive set (a `ReadonlySet` replaced only when membership changes). It is full-snapshot state: the `workspace.list` baseline, the `archiveSession` unary echo, and the `host/archived-sessions-changed` frame each install the complete set. `WorkspacesService.archiveSession(sessionId)` archives over the wire and, when the archived session is the current selection, clears it into the New Session view state; grouping surfaces hide members everywhere while the session rows stay in the list store.
+
 SlotsService gives the renderer separate bare observables for `useSessions` and `useWorkspaces`; web-react creates the hooks. Workspace business state does not enter `SessionListState` or an entry store.
 
 `SessionsService.search(query, signal)` is a stateless one-shot action over the `session.search` RPC. It returns ranked session/snippet pairs without putting query, loading, or error state into the shared Session list, so each UI owner controls debounce, cancellation, stale-response suppression, and fallback presentation. `searchResultLimit` re-exposes `SESSION_SEARCH_RESULT_LIMIT` — the bound the response schema itself enforces — as injected presentation data, so client plugins do not duplicate it. It is a protocol constant rather than per-connection state, so the connection handle does not carry it.
diff --git a/packages/client/runtime/README.zh.md b/packages/client/runtime/README.zh.md
index 3ed047e65d..1ee4731964 100644
--- a/packages/client/runtime/README.zh.md
+++ b/packages/client/runtime/README.zh.md
@@ -10,6 +10,8 @@ Workspace 和 Session 列表各自具有单调的 `pending` → `ready` 基线
 
 `WorkspacesService.delete(workspaceId)` 在一元响应成功后从客户端投影中移除注册记录；对应的 `host/workspace-removed` 帧具有幂等性，并负责同步其他标签页。Session 状态与当前 Session selection 相互独立，因此 Workspace 消失后，其已纳入客户端投影的 Session 会立即投影到 Ungrouped 下。
 
+`WorkspaceListState.archivedSessionIds` 镜像 Host 的注册表级全局归档集合（一个 `ReadonlySet`，仅在成员变化时才替换）。它是全快照状态：`workspace.list` 基线、`archiveSession` 一元回声和 `host/archived-sessions-changed` 帧各自安装完整集合。`WorkspacesService.archiveSession(sessionId)` 通过 wire 归档；当被归档的会话正是当前 selection 时，将其清空为 New Session 视图状态。各分组视图在所有位置隐藏集合成员，而会话行本身仍留在列表 store 中。
+
 SlotsService 分别为 renderer 提供 `useSessions` 与 `useWorkspaces` 的裸 observable；web-react 创建钩子。Workspace 业务状态不会进入 `SessionListState` 或配置项 store。
 
 `SessionsService.search(query, signal)` 是基于 `session.search` RPC 的无状态单次操作。它返回经过排序的会话／snippet 对，但不会将查询条件、加载状态或错误状态写入共享 Session 列表，因此每个 UI 所有者都自行负责防抖、取消、抑制陈旧响应和回退呈现。`searchResultLimit` 将 `SESSION_SEARCH_RESULT_LIMIT`——即响应 schema 自身强制执行的上限——作为注入的呈现数据重新公开，使客户端插件无需复制该值。它是协议常量而非逐连接状态，因此连接 handle 不携带它。
diff --git a/packages/client/runtime/src/client/contract/workspaces.ts b/packages/client/runtime/src/client/contract/workspaces.ts
index 9238ea5fd0..3e64ef3717 100644
--- a/packages/client/runtime/src/client/contract/workspaces.ts
+++ b/packages/client/runtime/src/client/contract/workspaces.ts
@@ -76,4 +76,11 @@ export interface IWorkspaces {
    * @returns the updated Workspace view.
    */
   insertSessionBefore(workspaceId: WorkspaceId, sessionId: SessionId, beforeSessionId?: SessionId): Promise<WorkspaceView>
+  /**
+   * Archive a session into the registry-global set (hidden from grouping
+   * surfaces; session log and accounting slot remain). Archiving the current
+   * session clears the selection into the New Session view state.
+   * @param sessionId - session to archive.
+   */
+  archiveSession(sessionId: SessionId): Promise<void>
 }
diff --git a/packages/client/runtime/src/client/workspaces/manager.ts b/packages/client/runtime/src/client/workspaces/manager.ts
index ce4198cd01..0eddae8445 100644
--- a/packages/client/runtime/src/client/workspaces/manager.ts
+++ b/packages/client/runtime/src/client/workspaces/manager.ts
@@ -14,6 +14,8 @@ export type WorkspaceListPhase = 'pending' | 'ready'
 /** Immutable workspace-list snapshot. */
 export interface WorkspaceListSnapshot {
   items: readonly WorkspaceView[]
+  /** Registry-global archive set (hidden from grouping surfaces; accounting slots retained). */
+  archivedSessionIds: ReadonlySet<SessionId>
   state: 'idle' | 'loading' | 'error'
   phase: WorkspaceListPhase
   error: RpcError | null
@@ -28,6 +30,9 @@ export class WorkspaceManager {
   private items: Workspace[] = []
   private itemViewsSource: readonly Workspace[] | null = null
   private itemViewsCache: readonly WorkspaceView[] = []
+  // Full-snapshot state (list response / unary response / changed frame all
+  // carry the complete set), so deltas never merge — installs replace.
+  private archivedSessionIds: ReadonlySet<SessionId> = new Set()
   private state: WorkspaceListSnapshot['state'] = 'idle'
   private phase: WorkspaceListPhase = 'pending'
   private error: RpcError | null = null
@@ -77,6 +82,7 @@ export class WorkspaceManager {
           items = items.filter(workspace => !this.removedIds.has(workspace.workspaceId))
           for (const delta of frames) items = applyWorkspaceDelta(items, delta)
           this.installViews(items)
+          this.installArchived(result.value.archivedSessionIds)
           this.state = 'idle'
           this.phase = 'ready'
         } else {
@@ -158,6 +164,18 @@ export class WorkspaceManager {
     return result
   }
 
+  /**
+   * Archive one session in the registry-global set, then install the
+   * returned full set without waiting for the changed frame.
+   * @param sessionId - session to archive.
+   * @returns the wire result.
+   */
+  async archiveSession(sessionId: SessionId): Promise<RpcResult<{ archivedSessionIds: SessionId[] }>> {
+    const { result } = await this.api.workspace.archiveSession({ sessionId })
+    if (result.ok) this.installArchived(result.value.archivedSessionIds)
+    return result
+  }
+
   /**
    * Host-frame entry. Non-workspace frames are ignored so the runtime can
    * fan one host stream out to both object managers.
@@ -166,6 +184,9 @@ export class WorkspaceManager {
   handleHostEnvelope(envelope: RpcRequest<HostFrame>): void {
     if (envelope.payload.type === 'host/workspace-changed') this.upsert(envelope.payload.workspace)
     else if (envelope.payload.type === 'host/workspace-removed') this.remove(envelope.payload.workspaceId)
+    else if (envelope.payload.type === 'host/archived-sessions-changed') {
+      this.installArchived(envelope.payload.archivedSessionIds)
+    }
   }
 
   /** Re-pull the baseline after each connection generation. */
@@ -194,12 +215,21 @@ export class WorkspaceManager {
   private buildSnapshot(): WorkspaceListSnapshot {
     return {
       items: this.itemViews(),
+      archivedSessionIds: this.archivedSessionIds,
       state: this.state,
       phase: this.phase,
       error: this.error,
     }
   }
 
+  /** Replace the archive set when membership actually changed (set identity backs Object.is short-circuits). */
+  private installArchived(archivedSessionIds: readonly SessionId[]): void {
+    if (archivedSessionIds.length === this.archivedSessionIds.size
+      && archivedSessionIds.every(id => this.archivedSessionIds.has(id))) return
+    this.archivedSessionIds = new Set(archivedSessionIds)
+    this.notifier.markDirty()
+  }
+
   /** Upsert one Host view, optionally retaining the local object that materialized it. */
   private upsert(view: WorkspaceView, identity?: Workspace): void {
     if (this.removedIds.has(view.workspaceId)) return
diff --git a/packages/client/runtime/src/client/workspaces/service.ts b/packages/client/runtime/src/client/workspaces/service.ts
index 1dd3319e79..f3125e6494 100644
--- a/packages/client/runtime/src/client/workspaces/service.ts
+++ b/packages/client/runtime/src/client/workspaces/service.ts
@@ -14,6 +14,12 @@ import { WorkspaceManager, type WorkspaceListPhase } from './manager.ts'
 /** Workspace list plus the two-baseline readiness and default-target projection. */
 export interface WorkspaceListState {
   items: readonly WorkspaceView[]
+  /**
+   * Registry-global archive set: grouping surfaces hide these sessions
+   * everywhere (workspace groups and the ungrouped bucket) while their
+   * session logs and workspace accounting slots remain.
+   */
+  archivedSessionIds: ReadonlySet<SessionId>
   state: 'idle' | 'loading' | 'error'
   phase: WorkspaceListPhase
   error: RpcError | null
@@ -58,7 +64,7 @@ export class WorkspacesService implements IWorkspaces {
   constructor(ctx: Context, private readonly api: IApiClient, private readonly sessions: SessionsPort) {
     this.manager = new WorkspaceManager(api)
     this.list = createSnapshotStore<WorkspaceListState>({
-      items: [], state: 'idle', phase: 'pending', error: null,
+      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'pending', error: null,
       baselinesReady: false, recentWorkspaceId: undefined,
     })
     this.manager.subscribe(() => { this.project() })
@@ -249,6 +255,18 @@ export class WorkspacesService implements IWorkspaces {
     if (!result.ok) throw new Error(`workspace delete failed: ${result.error.code}: ${result.error.message}`)
   }
 
+  /**
+   * Archive a session into the registry-global set. When the archived
+   * session is the current one, the selection is cleared into the New
+   * Session view state — a hidden row must not stay open behind the list.
+   * @param sessionId - session to archive.
+   */
+  async archiveSession(sessionId: SessionId): Promise<void> {
+    const result = await this.manager.archiveSession(sessionId)
+    if (!result.ok) throw new Error(`session archive failed: ${result.error.code}: ${result.error.message}`)
+    if (this.sessions.list.getSnapshot().current === sessionId) this.sessions.clear()
+  }
+
   /**
    * Move a session within its Workspace's manual order (DOM-insertBefore-like).
    * @param workspaceId - owning workspace.
@@ -293,6 +311,7 @@ export class WorkspacesService implements IWorkspaces {
     const baselinesReady = workspace.phase === 'ready' && sessions.phase === 'ready'
     this.list.set({
       items: workspace.items,
+      archivedSessionIds: workspace.archivedSessionIds,
       state: workspace.state,
       phase: workspace.phase,
       error: workspace.error,
diff --git a/packages/client/runtime/tests/fake-api.ts b/packages/client/runtime/tests/fake-api.ts
index 06d948ae83..9cf7971049 100644
--- a/packages/client/runtime/tests/fake-api.ts
+++ b/packages/client/runtime/tests/fake-api.ts
@@ -140,7 +140,10 @@ export class FakeApiClient implements IApiClient {
     openPath: (payload: unknown) => this.record('host.openPath', payload, this.onOpenPath(payload)),
   }
 
-  onWorkspaceList: (payload: unknown) => Promise<RpcResponse<{ items: never[] }>> = () => Promise.resolve(ok({ items: [] }))
+  // The archive-set field defaults at the binding below so list stubs keep
+  // the pre-archive `{ items }` shape; a stub carrying the field wins.
+  onWorkspaceList: (payload: unknown) => Promise<RpcResponse<{ items: never[]; archivedSessionIds?: never[] }>> =
+    () => Promise.resolve(ok({ items: [] }))
   onWorkspaceCreate: (payload: unknown) => Promise<RpcResponse<{ workspace: WorkspaceView; created: boolean }>> =
     () => Promise.resolve(ok({ workspace: fakeWorkspace('fk-ws'), created: true }))
 
@@ -153,13 +156,22 @@ export class FakeApiClient implements IApiClient {
   onWorkspaceInsertSessionBefore: (payload: unknown) => Promise<RpcResponse<{ workspace: WorkspaceView }>> =
     () => Promise.resolve(ok({ workspace: fakeWorkspace('fk-ws') }))
 
+  onWorkspaceArchiveSession: (payload: unknown) => Promise<RpcResponse<{ archivedSessionIds: SessionId[] }>> =
+    payload => Promise.resolve(ok({ archivedSessionIds: [(payload as { sessionId: SessionId }).sessionId] }))
+
   readonly workspace: IApiClient['workspace'] = {
-    list: (payload: unknown) => this.record('workspace.list', payload, this.onWorkspaceList(payload)),
+    list: (payload: unknown) => this.record('workspace.list', payload, this.onWorkspaceList(payload).then(response => (
+      response.result.ok
+        ? { ...response, result: { ok: true as const, value: { archivedSessionIds: [] as never[], ...response.result.value } } }
+        : response
+    )) as ReturnType<IApiClient['workspace']['list']>),
     create: (payload: unknown) => this.record('workspace.create', payload, this.onWorkspaceCreate(payload)),
     rename: (payload: unknown) => this.record('workspace.rename', payload, this.onWorkspaceRename(payload)),
     delete: (payload: unknown) => this.record('workspace.delete', payload, this.onWorkspaceDelete(payload)),
     insertSessionBefore: (payload: unknown) =>
       this.record('workspace.insertSessionBefore', payload, this.onWorkspaceInsertSessionBefore(payload)),
+    archiveSession: (payload: unknown) =>
+      this.record('workspace.archiveSession', payload, this.onWorkspaceArchiveSession(payload)),
   }
 
   // Payloads stay `unknown` (lint-lane note above); response rows are the real
diff --git a/packages/client/runtime/tests/workspaces-service.spec.ts b/packages/client/runtime/tests/workspaces-service.spec.ts
index 7fd6934827..2bd462e5ee 100644
--- a/packages/client/runtime/tests/workspaces-service.spec.ts
+++ b/packages/client/runtime/tests/workspaces-service.spec.ts
@@ -285,6 +285,52 @@ describe('WorkspacesService', () => {
     }))
     await expect(workspaces.delete(wid('ghost'))).rejects.toThrow(/workspace-not-found: gone/)
   })
+
+  it('archives a session, projects the set from the response, list, and frame, and clears only the current one', async () => {
+    const ctx = new Context()
+    const api = new FakeApiClient()
+    const sessions = new SessionsService(ctx, api)
+    const workspaces = new WorkspacesService(ctx, api, sessions)
+    api.onList = () => Promise.resolve(ok({
+      items: [
+        { sessionId: sid('s-open'), updatedAt: 2, running: false, blank: false },
+        { sessionId: sid('s-idle'), updatedAt: 1, running: false, blank: false },
+      ],
+    }) as never)
+    await sessions.refresh()
+    sessions.open(sid('s-open'))
+
+    // Archiving a non-current session installs the unary echo and keeps the selection.
+    await expect(workspaces.archiveSession(sid('s-idle'))).resolves.toBeUndefined()
+    expect(api.callsOf('workspace.archiveSession')).toEqual([{ sessionId: 's-idle' }])
+    expect([...workspaces.list.getSnapshot().archivedSessionIds]).toEqual(['s-idle'])
+    expect(sessions.list.getSnapshot().current).toBe('s-open')
+
+    // Archiving the current session clears it into the New Session view state.
+    api.onWorkspaceArchiveSession = () => Promise.resolve(ok({ archivedSessionIds: [sid('s-idle'), sid('s-open')] }))
+    await workspaces.archiveSession(sid('s-open'))
+    expect([...workspaces.list.getSnapshot().archivedSessionIds]).toEqual(['s-idle', 's-open'])
+    expect(sessions.list.getSnapshot().current).toBeUndefined()
+
+    // A Host failure leaves the set and the selection untouched.
+    api.onWorkspaceArchiveSession = () => Promise.resolve(err({
+      code: 'session-not-found', message: 'no session ghost', details: { sessionId: sid('ghost') },
+    }))
+    await expect(workspaces.archiveSession(sid('ghost'))).rejects.toThrow(/session-not-found/)
+    expect([...workspaces.list.getSnapshot().archivedSessionIds]).toEqual(['s-idle', 's-open'])
+
+    // The changed frame and the list baseline both re-install the full set.
+    workspaces.handleHostEnvelope({
+      rpcId: 'frame' as never,
+      payload: { type: 'host/archived-sessions-changed', archivedSessionIds: [sid('s-idle')] },
+    } as never)
+    // Frame installs ride the notifier's microtask batch before projecting.
+    await new Promise(resolve => setTimeout(resolve, 0))
+    expect([...workspaces.list.getSnapshot().archivedSessionIds]).toEqual(['s-idle'])
+    api.onWorkspaceList = () => Promise.resolve(ok({ items: [], archivedSessionIds: [sid('s-open')] }) as never)
+    await workspaces.refresh()
+    expect([...workspaces.list.getSnapshot().archivedSessionIds]).toEqual(['s-open'])
+  })
 })
 
 describe('startInitialSelection', () => {
diff --git a/packages/client/test-runtime/src/fixtures.ts b/packages/client/test-runtime/src/fixtures.ts
index 4219d233e2..d81adf7698 100644
--- a/packages/client/test-runtime/src/fixtures.ts
+++ b/packages/client/test-runtime/src/fixtures.ts
@@ -73,6 +73,7 @@ export function conversationSnapshot(sessionId: SessionId): ConversationSnapshot
 export function workspaceListState(): WorkspaceListState {
   return {
     items: [],
+    archivedSessionIds: new Set(),
     state: 'idle',
     phase: 'ready',
     error: null,
diff --git a/packages/client/test-runtime/src/workspaces.ts b/packages/client/test-runtime/src/workspaces.ts
index 6c1a9d0aad..fab209c96c 100644
--- a/packages/client/test-runtime/src/workspaces.ts
+++ b/packages/client/test-runtime/src/workspaces.ts
@@ -186,4 +186,21 @@ export class TestWorkspaces implements IWorkspaces {
     if (stub !== undefined) return await (stub(workspaceId, sessionId, beforeSessionId) as Promise<WorkspaceView>)
     return { workspaceId, title: '', path: '', sessionIds: [sessionId] } as unknown as WorkspaceView
   }
+
+  /**
+   * Archive a session (recorded). The default mirrors the production face's
+   * observable effect: the id joins the list state's archive set.
+   * @param sessionId - session to archive.
+   */
+  async archiveSession(sessionId: SessionId): Promise<void> {
+    this.calls.push({ method: 'archiveSession', args: [sessionId] })
+    const stub = this.stubs.get('archiveSession')
+    if (stub !== undefined) {
+      await (stub(sessionId) as Promise<void>)
+      return
+    }
+    await this.update((draft) => {
+      draft.archivedSessionIds = new Set([...draft.archivedSessionIds, sessionId])
+    })
+  }
 }
diff --git a/packages/client/ui-conversation/tests/chat-code-subcalls.spec.tsx b/packages/client/ui-conversation/tests/chat-code-subcalls.spec.tsx
index 49eeec29fd..16d070b04d 100644
--- a/packages/client/ui-conversation/tests/chat-code-subcalls.spec.tsx
+++ b/packages/client/ui-conversation/tests/chat-code-subcalls.spec.tsx
@@ -128,7 +128,7 @@ async function bench(snapshot: ConversationSnapshot) {
   ctx.provide('sessions', sessionsFake)
   const workspaces = {
     list: createSnapshotStore<WorkspaceListState>({
-      items: [], state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     }),
     startSession: vi.fn(),
diff --git a/packages/client/ui-conversation/tests/chat-view.spec.tsx b/packages/client/ui-conversation/tests/chat-view.spec.tsx
index bfaab93375..b59335fa3a 100644
--- a/packages/client/ui-conversation/tests/chat-view.spec.tsx
+++ b/packages/client/ui-conversation/tests/chat-view.spec.tsx
@@ -94,7 +94,7 @@ function emptySessions() {
 
 function emptyWorkspaces() {
   const store = createSnapshotStore<WorkspaceListState>({
-    items: [], state: 'idle', phase: 'ready', error: null,
+    items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
     baselinesReady: true, recentWorkspaceId: undefined,
   })
   return bindSnapshotSelector(store)
diff --git a/packages/client/ui-conversation/tests/gate-branch-tails.spec.tsx b/packages/client/ui-conversation/tests/gate-branch-tails.spec.tsx
index 354b94ecdd..192470abfc 100644
--- a/packages/client/ui-conversation/tests/gate-branch-tails.spec.tsx
+++ b/packages/client/ui-conversation/tests/gate-branch-tails.spec.tsx
@@ -74,7 +74,7 @@ describe('render branch tails', () => {
     const emptyList = createSnapshotStore<SessionListState>(
       { ids: [], byId: {}, current: undefined, phase: 'ready' })
     const emptyWorkspaces = createSnapshotStore<WorkspaceListState>({
-      items: [], state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })
     const view = render(
@@ -111,7 +111,7 @@ describe('render branch tails', () => {
     const emptyList = createSnapshotStore<SessionListState>(
       { ids: [], byId: {}, current: undefined, phase: 'ready' })
     const emptyWorkspaces = createSnapshotStore<WorkspaceListState>({
-      items: [], state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })
     const view = render(
diff --git a/packages/client/ui-conversation/tests/input-bar.spec.tsx b/packages/client/ui-conversation/tests/input-bar.spec.tsx
index 6831b1a5a0..1974881357 100644
--- a/packages/client/ui-conversation/tests/input-bar.spec.tsx
+++ b/packages/client/ui-conversation/tests/input-bar.spec.tsx
@@ -96,7 +96,7 @@ function bench(over?: BenchOptions) {
       ids: [], byId: {}, current: undefined, phase: 'ready',
     })),
     useWorkspaces: bindSnapshotSelector(createSnapshotStore({
-      items: [], state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })),
     useProjection: ((key: string, selector?: (v: unknown) => unknown) =>
diff --git a/packages/client/ui-conversation/tests/input-matrix.spec.tsx b/packages/client/ui-conversation/tests/input-matrix.spec.tsx
index f619d380a0..4901ec2f9f 100644
--- a/packages/client/ui-conversation/tests/input-matrix.spec.tsx
+++ b/packages/client/ui-conversation/tests/input-matrix.spec.tsx
@@ -39,7 +39,7 @@ function mountBar(shell: SessionInputShell, over?: { running?: boolean; disabled
       ids: [], byId: {}, current: undefined, phase: 'ready',
     })),
     useWorkspaces: bindSnapshotSelector(createSnapshotStore({
-      items: [], state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })),
     useProjection: (() => undefined),
diff --git a/packages/client/ui-conversation/tests/input-scenarios.spec.tsx b/packages/client/ui-conversation/tests/input-scenarios.spec.tsx
index 60b02795fe..167b9c96af 100644
--- a/packages/client/ui-conversation/tests/input-scenarios.spec.tsx
+++ b/packages/client/ui-conversation/tests/input-scenarios.spec.tsx
@@ -125,7 +125,7 @@ async function scopedBench(register?: (slash: SlashService) => void) {
       ids: [], byId: {}, current: undefined, phase: 'ready',
     })),
     useWorkspaces: bindSnapshotSelector(createSnapshotStore({
-      items: [], state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })),
     useProjection: (() => undefined),
diff --git a/packages/client/ui-conversation/tests/skeleton.spec.tsx b/packages/client/ui-conversation/tests/skeleton.spec.tsx
index a37074bd07..2669cbb8ab 100644
--- a/packages/client/ui-conversation/tests/skeleton.spec.tsx
+++ b/packages/client/ui-conversation/tests/skeleton.spec.tsx
@@ -62,7 +62,7 @@ function workspace(id = 'w1'): WorkspaceView {
 }
 
 const workspaceState = (items: readonly WorkspaceView[]): WorkspaceListState => ({
-  items, state: 'idle', phase: 'ready', error: null,
+  items, archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
   baselinesReady: true, recentWorkspaceId: undefined,
 })
 
diff --git a/packages/client/ui-conversation/tests/terminal-card.spec.tsx b/packages/client/ui-conversation/tests/terminal-card.spec.tsx
index 213897e0f5..29f246d603 100644
--- a/packages/client/ui-conversation/tests/terminal-card.spec.tsx
+++ b/packages/client/ui-conversation/tests/terminal-card.spec.tsx
@@ -429,7 +429,7 @@ describe('DetailsPanel Output section', () => {
         phase: 'ready',
       })
     const workspaces = createSnapshotStore<WorkspaceListState>({
-      items: [], state: 'idle', phase: 'ready', error: null,
+      items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
       baselinesReady: true, recentWorkspaceId: undefined,
     })
     return render(
@@ -607,7 +607,7 @@ describe('DetailsPanel Output section', () => {
         useSessions={bindSnapshotSelector(createSnapshotStore<SessionListState>(
           { ids: [], byId: {}, current: undefined, phase: 'ready' }))}
         useWorkspaces={bindSnapshotSelector(createSnapshotStore<WorkspaceListState>({
-          items: [], state: 'idle', phase: 'ready', error: null,
+          items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
           baselinesReady: true, recentWorkspaceId: undefined,
         }))}
         useInput={(() => { throw new Error('unused') })}
diff --git a/packages/client/ui-layout/tests/app-frame.spec.tsx b/packages/client/ui-layout/tests/app-frame.spec.tsx
index ab8a11706d..d30cd17877 100644
--- a/packages/client/ui-layout/tests/app-frame.spec.tsx
+++ b/packages/client/ui-layout/tests/app-frame.spec.tsx
@@ -77,7 +77,7 @@ function mountFrame() {
     return sel(sessionState)
   }) as never
   const workspaceState: WorkspaceListState = {
-    items: [], state: 'idle', phase: 'ready', error: null,
+    items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
     baselinesReady: baselinesReady.current, recentWorkspaceId: undefined,
   }
   const element = () => (
diff --git a/packages/client/ui-theme/tests/appearance-row.spec.tsx b/packages/client/ui-theme/tests/appearance-row.spec.tsx
index f21fb26bdd..00cc0e3052 100644
--- a/packages/client/ui-theme/tests/appearance-row.spec.tsx
+++ b/packages/client/ui-theme/tests/appearance-row.spec.tsx
@@ -27,7 +27,7 @@ function emptySessions() {
 }
 function emptyWorkspaces() {
   const store = createSnapshotStore<WorkspaceListState>({
-    items: [], state: 'idle', phase: 'ready', error: null,
+    items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null,
     baselinesReady: true, recentWorkspaceId: undefined,
   })
   return bindSnapshotSelector(store)
diff --git a/packages/client/ui-trajectory/tests/views.spec.tsx b/packages/client/ui-trajectory/tests/views.spec.tsx
index 5c1e6c691d..d31b43d3e6 100644
--- a/packages/client/ui-trajectory/tests/views.spec.tsx
+++ b/packages/client/ui-trajectory/tests/views.spec.tsx
@@ -113,7 +113,7 @@ function emptySessions() {
 
 function emptyWorkspaces() {
   const store = createSnapshotStore<WorkspaceListState>({
-    items: [], state: 'idle', phase: 'ready', error: null, baselinesReady: true,
+    items: [], archivedSessionIds: new Set(), state: 'idle', phase: 'ready', error: null, baselinesReady: true,
     recentWorkspaceId: undefined,
   })
   return bindSnapshotSelector(store)