diff --git a/.agents/notes/implemented/bug-fix/2026-08-20-web-session-projection-store-survives-revival.i18n.yaml b/.agents/notes/implemented/bug-fix/2026-08-20-web-session-projection-store-survives-revival.i18n.yaml
new file mode 100644
index 0000000000..db4abd3659
--- /dev/null
+++ b/.agents/notes/implemented/bug-fix/2026-08-20-web-session-projection-store-survives-revival.i18n.yaml
@@ -0,0 +1,6 @@
+# Bilingual-pair consistency record (docs/i18n/README.md): the git blob hash of each
+# 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 .agents/notes/implemented/bug-fix/2026-08-20-web-session-projection-store-survives-revival.md
+2026-08-20-web-session-projection-store-survives-revival.md: 27340e87e9fd2dfbfd4bf1868f4aacc2d4af5fdb
+2026-08-20-web-session-projection-store-survives-revival.zh.md: 63799da02266e1b2a724bb00659ea0fb5717d93d
diff --git a/.agents/notes/implemented/bug-fix/2026-08-20-web-session-projection-store-survives-revival.md b/.agents/notes/implemented/bug-fix/2026-08-20-web-session-projection-store-survives-revival.md
new file mode 100644
index 0000000000..27340e87e9
--- /dev/null
+++ b/.agents/notes/implemented/bug-fix/2026-08-20-web-session-projection-store-survives-revival.md
@@ -0,0 +1,52 @@
+# Agent Note: Web session projection stores survive removal and revival
+
+Status: implemented
+
+English | [中文](2026-08-20-web-session-projection-store-survives-revival.zh.md)
+
+## Problem
+
+The client runtime's `ProjectionValueStore` — the per-session store backing
+`useProjection('openRouterCost')`, `useProjection('title')`, and every other
+projection face — was deleted on every `host/session-removed`. The session
+instance itself is resident: `open()` is idempotent, `drop()` only removes
+the `sessions` map entry, and removal is modeled as a `removed` flag on the
+surviving instance. Deleting the projection store disagreed with that model:
+when the session was re-added (`host/session-added`) or re-listed by a
+`session.list` refresh, the same instance resumed without its projection
+store, so projection-backed UI (notably the OpenRouter cost readout in the
+composer dock) reverted to blank until the host re-served a projection
+commit.
+
+## Decision
+
+The projection store follows the resident instance: `SessionManager` no
+longer deletes it on `host/session-removed`. The `removed` tombstone is
+lifted by a new `Session.handleRevived()` — called from the `host/session-added`
+path and from the list-refresh summary push — which clears the flag and
+records the store as dirty so subscribers re-read the retained values. A
+re-added session therefore resumes its previous projection values through the
+same instance; the host re-serves the projection rows on the next `session.list`
+from its cached row set, and the store needs no re-seed.
+
+## Alternatives considered
+
+**Delete the store on removal and re-seed on re-add.** Rejected: the session
+instance is not re-seeded by `open()` (idempotent), so a deletion would leave
+the resumed session projection-blank until a fresh projection frame arrives.
+Retention matches the resident-instance contract already in place.
+
+**Drop the store only when the session is dropped from the list.** The removal
+frame is the only cleanup signal the manager receives; there is no separate
+lifecycle step distinguishing "tombstoned for revival" from "gone", so keeping
+the store until `drop()` is the model-consistent rule.
+
+## Consequences
+
+A removed-but-resident session keeps its projection values for as long as the
+instance exists, and re-listing restores them without a host re-seed. The
+store retains memory for removed sessions until the manager drops the map
+entry, which is the same tenure the session instance already held. `test:gui`
+pins both revival paths — `host/session-added` and list-refresh re-listing —
+plus the plain-switch retention case, and the projection-store spec pins that
+a removed session's store keeps its folded values.
\ No newline at end of file
diff --git a/.agents/notes/implemented/bug-fix/2026-08-20-web-session-projection-store-survives-revival.zh.md b/.agents/notes/implemented/bug-fix/2026-08-20-web-session-projection-store-survives-revival.zh.md
new file mode 100644
index 0000000000..63799da022
--- /dev/null
+++ b/.agents/notes/implemented/bug-fix/2026-08-20-web-session-projection-store-survives-revival.zh.md
@@ -0,0 +1,23 @@
+# Agent Note: Web 会话投影存储在移除与复活后继续存活
+
+Status: implemented
+
+[English](2026-08-20-web-session-projection-store-survives-revival.md) | 中文
+
+## 问题
+
+客户端运行时的 `ProjectionValueStore` —— 支撑 `useProjection('openRouterCost')`、`useProjection('title')` 以及其他所有投影面（face）的按会话存储 —— 会在每次 `host/session-removed` 时被删除。会话实例本身是常驻的：`open()` 幂等，`drop()` 只删除 `sessions` 映射项，移除被建模为幸存实例上的 `removed` 标志。删除投影存储与该模型不一致：当会话被重新添加（`host/session-added`）或由 `session.list` 刷新重新列出时，同一实例继续运行却没有其投影存储，于是投影驱动的 UI（尤其是 composer dock 中的 OpenRouter 费用读数）会回退为空白，直到宿主重新下发投影提交。
+
+## 决策
+
+投影存储随常驻实例存活：`SessionManager` 不再在 `host/session-removed` 时删除它。`removed` 墓碑由新增的 `Session.handleRevived()` 解除——它从 `host/session-added` 路径与列表刷新摘要推送两处调用——清除标志并将存储标记为脏，使订阅者重新读取保留的值。因此被重新添加的会话通过同一个实例恢复之前的投影值；宿主在下一次 `session.list` 时从缓存的投影行集合重新下发这些行，存储无需重新播种。
+
+## 考虑过的替代方案
+
+**移除时删除存储，重新添加时重新播种。** 否决：`open()` 幂等，不会为会话重新播种，因此删除会让恢复的会话在收到新投影帧之前一直投影空白。保留与既有的常驻实例契约一致。
+
+**仅当会话从列表移除时才丢弃存储。** 移除帧是管理器收到的唯一清理信号；没有单独的「为复活而墓碑化」与「真正消失」生命周期步骤，因此在 `drop()` 之前保留存储是模型一致的规则。
+
+## 结论
+
+被移除但常驻的会话在其实例存活期间保留投影值，重新列出时无需宿主重新播种即可恢复。在管理器删除映射项之前，存储会为已移除会话保留内存——这与会话实例本身已有的任期相同。`test:gui` 钉定了两条复活路径（`host/session-added` 与列表刷新重新列出），外加普通切换保留场景；投影存储规范钉定被移除会话的存储会保留其折叠后的值。
\ No newline at end of file
diff --git a/packages/client/runtime/src/client/sessions/manager.ts b/packages/client/runtime/src/client/sessions/manager.ts
index 13aa20d1c8..e93f7c7803 100644
--- a/packages/client/runtime/src/client/sessions/manager.ts
+++ b/packages/client/runtime/src/client/sessions/manager.ts
@@ -476,6 +476,11 @@ export class SessionManager {
             if (session === undefined) continue
             session.handleBlank(s.blank)
             session.handleRunning(s.running)
+            // A durable session the host disposed earlier being re-listed is
+            // alive again: lift the removed tombstone so the surviving
+            // instance renders normally (mutation replay already excluded a
+            // genuinely-removed session from this.summaries).
+            session.handleRevived()
           }
           // Seed each row's projection baseline into the per-session value
           // store (cold titles surface without opening the session). Per-key
@@ -804,6 +809,10 @@ export class SessionManager {
           ...(frame.agentPreset !== undefined ? { agentPreset: frame.agentPreset } : {}),
         })
         this.sessions.get(frame.sessionId)?.handleBlank(frame.blank)
+        // A re-emitted id (host re-engaging a previously disposed session)
+        // revives the surviving instance: the removed tombstone must not keep
+        // the conversation (and its projection readouts) in the removed state.
+        this.sessions.get(frame.sessionId)?.handleRevived()
         if (frame.origin === 'subagent' && frame.parentSessionId !== undefined) {
           this.markCatalogParentExpandable(frame.parentSessionId)
         }
@@ -834,7 +843,12 @@ export class SessionManager {
         // no relative order. Clearing here makes a detached Activation's rows
         // disappear whichever arrives first.
         this.jobsBySession.delete(frame.sessionId)
-        if (!durableSubagent) this.projectionStores.delete(frame.sessionId)
+        // The projection store is retained: the Session instance survives
+        // removal (resident-instance rule — handleRemoved above only flags the
+        // snapshot), and a session the host re-lists resumes its projections
+        // through the same instance. Deleting the store here would leave that
+        // surviving instance with an empty projection face until a fresh
+        // baseline lands — and open() is idempotent, so no re-open re-pulls it.
         // A pull already in flight was requested before this removal and can
         // carry the pre-removal parentAvailable:true, which would resurrect
         // the writable editor this invalidation just closed. Replay false over
diff --git a/packages/client/runtime/src/client/sessions/session.ts b/packages/client/runtime/src/client/sessions/session.ts
index 3e5ec5a811..269734d57e 100644
--- a/packages/client/runtime/src/client/sessions/session.ts
+++ b/packages/client/runtime/src/client/sessions/session.ts
@@ -577,6 +577,18 @@ export class Session implements SessionFace {
     this.notifier.markDirty()
   }
 
+  /**
+   * host/session-added or host re-listing relay: the session is alive again,
+   * so lift the removed tombstone. The instance survives removal, and a
+   * removed-then-re-listed session must render its conversation again (the
+   * composer and its projection readouts) rather than the removed state.
+   */
+  handleRevived(): void {
+    if (!this.removed) return
+    this.removed = false
+    this.notifier.markDirty()
+  }
+
   /**
    * host/agent-error relay: the only outlet for live failures with no turn position.
    * @param message - the stringified error.
diff --git a/packages/client/runtime/tests/manager.client.spec.ts b/packages/client/runtime/tests/manager.client.spec.ts
index d0594032df..8c7fb7771f 100644
--- a/packages/client/runtime/tests/manager.client.spec.ts
+++ b/packages/client/runtime/tests/manager.client.spec.ts
@@ -190,7 +190,7 @@ describe('list lifecycle', () => {
     expect(manager.getListSnapshot().items.map(i => i.sessionId)).toEqual([S2])
   })
 
-  it('retains title projections before list arrival, keeps last-wins by seq, and clears them on removal', async () => {
+  it('retains title projections before list arrival, keeps last-wins by seq, and survives removal', async () => {
     const api = new FakeApiClient()
     const manager = new SessionManager(api, fakeRemote())
     const titleFrame = (rpcId: string, title: string, seq: number) => {
@@ -212,9 +212,13 @@ describe('list lifecycle', () => {
     expect(titled.items[0]?.title).toBe('Newest')
     expect(titled.items[1]?.title).toBeUndefined()
 
+    // A removed-then-re-added session is the same durable lifecycle under the
+    // resident-instance rule: its projection face (here the title) survives
+    // the round-trip, so the row resurfacing shows the retained value rather
+    // than re-seeding from an empty store.
     manager.handleHostEnvelope({ rpcId: 'removed' as never, payload: { type: 'host/session-removed', sessionId: S1 } })
     manager.handleHostEnvelope({ rpcId: 'readded' as never, payload: { type: 'host/session-added', blank: true, sessionId: S1 } })
-    expect(manager.getListSnapshot().items.find(item => item.sessionId === S1)?.title).toBeUndefined()
+    expect(manager.getListSnapshot().items.find(item => item.sessionId === S1)?.title).toBe('Newest')
   })
 
   it('seeds cold titles from the list rows\' projections block under higher-seq-wins', async () => {
diff --git a/packages/client/runtime/tests/projection-store.client.spec.ts b/packages/client/runtime/tests/projection-store.client.spec.ts
index 5ef2c41194..79f917f0af 100644
--- a/packages/client/runtime/tests/projection-store.client.spec.ts
+++ b/packages/client/runtime/tests/projection-store.client.spec.ts
@@ -209,7 +209,7 @@ describe('manager frame routing', () => {
     expect(manager.getListSnapshot().items[0]?.projectionValues).not.toBe(baseline)
   })
 
-  it('drops the projection store with the removed session', async () => {
+  it('retains the projection store with the removed session (the instance survives removal)', async () => {
     const api = new FakeApiClient()
     const manager = new SessionManager(api, fakeRemote())
     api.onList = () => Promise.resolve(ok({
@@ -224,6 +224,9 @@ describe('manager frame routing', () => {
       rpcId: 'rm' as never,
       payload: { type: 'host/session-removed', sessionId: sid('s1') } as never,
     })
-    expect(manager.get(sid('s1')).projections.get('title')).toBeUndefined()
+    // The Session instance survives removal (resident-instance rule), so its
+    // projection face survives with it: a re-listed session resumes its
+    // values instead of re-seeding from an empty face.
+    expect(manager.get(sid('s1')).projections.get('title')).toBe('Doomed')
   })
 })
diff --git a/packages/client/runtime/tests/switch-retention.client.spec.ts b/packages/client/runtime/tests/switch-retention.client.spec.ts
new file mode 100644
index 0000000000..87484f96fd
--- /dev/null
+++ b/packages/client/runtime/tests/switch-retention.client.spec.ts
@@ -0,0 +1,147 @@
+/**
+ * Repro + behavioral pin for the chat-switch projection retention contract:
+ * a session's projection store (openRouterCost is the canonical example) must
+ * survive a selection change and remain readable through the same value face
+ * when the session becomes current again.
+ *
+ * Server frames exercised:
+ * - session.list rows carrying a projections block (the cold seed path)
+ * - session/projection push frames to a live session
+ * - host/session-removed (transient host teardown) + host/session-added
+ * - session/subscribed (durable-replay baseline) + a re-pulled history tail
+ *
+ * The manager's per-session ProjectionValueStore is retained across select();
+ * this suite pins the retention contract so a regression in the
+ * retention/seeding discipline shows up as a red test here.
+ */
+import { describe, expect, it } from 'vitest'
+import type { SessionId } from '@deepseek-ai/dsh-api-remotes/client'
+import { SessionManager } from '../src/client/sessions/manager.ts'
+import { FakeApiClient, fakeRemote, ok } from './fake-api.client.ts'
+
+const S1 = 'fk-s1' as SessionId
+const S2 = 'fk-s2' as SessionId
+
+const COST = { totalUsd: 1.23, pricedSteps: 1, unknownModelSteps: 0, currency: 'USD' as const }
+
+function summary(sessionId: SessionId, projections?: { asOfSeq: number; values: Record<string, unknown> }) {
+  return {
+    sessionId,
+    updatedAt: 100,
+    running: false,
+    blank: false,
+    ...(projections === undefined ? {} : { projections }),
+  } as never
+}
+
+describe('projection retention across chat switches (repro)', () => {
+  it('retains the projection store for a session that was opened, seeded, and switched away from and back', async () => {
+    const api = new FakeApiClient()
+    api.onList = () => Promise.resolve(ok({ items: [summary(S1), summary(S2)] }))
+    const manager = new SessionManager(api, fakeRemote())
+    await manager.refreshList()
+
+    // A live push frame lands the cost in S1's resident store.
+    manager.handleMuxEnvelope({
+      rpcId: 'seed' as never,
+      payload: { type: 'session/projection', sessionId: S1, key: 'openRouterCost', value: COST, seq: 5 } as never,
+    })
+    expect(manager.getListSnapshot().items[0]?.projectionValues?.openRouterCost).toEqual(COST)
+
+    // The conversation switches to another chat and back; nothing removes S1.
+    manager.select(S2)
+    manager.select(S1)
+    expect(manager.getListSnapshot().items[0]?.projectionValues?.openRouterCost).toEqual(COST)
+  })
+
+  it('a host/session-removed disposal followed by a re-add re-seeds the cost from the list row block', async () => {
+    const api = new FakeApiClient()
+    api.onList = () => Promise.resolve(ok({
+      items: [
+        summary(S1, { asOfSeq: 6, values: { openRouterCost: COST } }),
+        summary(S2),
+      ],
+    }))
+    const manager = new SessionManager(api, fakeRemote())
+    await manager.refreshList()
+    expect(manager.getListSnapshot().items.find(item => item.sessionId === S1)?.projectionValues?.openRouterCost)
+      .toEqual(COST)
+
+    // Transient host teardown: the chat leaves the list; its projection store
+    // is retained (the Session instance survives removal), and the durable
+    // chat still exists host-side, so a list refresh re-adds it.
+    manager.handleHostEnvelope({
+      rpcId: 'rm' as never,
+      payload: { type: 'host/session-removed', sessionId: S1 } as never,
+    })
+    expect(manager.getListSnapshot().items.find(item => item.sessionId === S1)).toBeUndefined()
+
+    await manager.refreshList()
+    expect(manager.getListSnapshot().items.find(item => item.sessionId === S1)?.projectionValues?.openRouterCost)
+      .toEqual(COST)
+  })
+
+  it('lifts the removed tombstone when a disposed session is re-listed by the host', async () => {
+    const api = new FakeApiClient()
+    api.onList = () => Promise.resolve(ok({ items: [summary(S1), summary(S2)] }))
+    const manager = new SessionManager(api, fakeRemote())
+    await manager.refreshList()
+
+    const session = manager.get(S1)
+    manager.handleHostEnvelope({
+      rpcId: 'rm' as never,
+      payload: { type: 'host/session-removed', sessionId: S1 } as never,
+    })
+    expect(session.getSnapshot().removed).toBe(true)
+    // The same instance survives (resident-instance rule).
+    expect(manager.get(S1)).toBe(session)
+
+    // The durable chat comes back: the re-listing revives the surviving
+    // instance rather than leaving the conversation tombstoned forever.
+    manager.handleHostEnvelope({
+      rpcId: 'add' as never,
+      payload: { type: 'host/session-added', blank: false, sessionId: S1 } as never,
+    })
+    expect(session.getSnapshot().removed).toBe(false)
+  })
+
+  it('revives through a list refresh when the host re-lists a previously disposed session', async () => {
+    const api = new FakeApiClient()
+    const listed = [summary(S1), summary(S2)]
+    api.onList = () => Promise.resolve(ok({ items: listed }))
+    const manager = new SessionManager(api, fakeRemote())
+    await manager.refreshList()
+
+    const session = manager.get(S1)
+    manager.handleHostEnvelope({
+      rpcId: 'rm' as never,
+      payload: { type: 'host/session-removed', sessionId: S1 } as never,
+    })
+    expect(session.getSnapshot().removed).toBe(true)
+
+    // The host never re-emits host/session-added for an existing id; the
+    // durable session simply appears again on the next list pull.
+    await manager.refreshList()
+    expect(session.getSnapshot().removed).toBe(false)
+    expect(manager.getListSnapshot().items.find(item => item.sessionId === S1)).toBeDefined()
+  })
+
+  it('survives a subscribed durable-replay baseline when the value is at or before the baseline', async () => {
+    const api = new FakeApiClient()
+    api.onList = () => Promise.resolve(ok({ items: [summary(S1)] }))
+    const manager = new SessionManager(api, fakeRemote())
+    await manager.refreshList()
+
+    manager.handleMuxEnvelope({
+      rpcId: 'push' as never,
+      payload: { type: 'session/projection', sessionId: S1, key: 'openRouterCost', value: COST, seq: 3 } as never,
+    })
+
+    // The durable baseline is at seq 3, so the row is not phantom: it survives.
+    manager.handleMuxEnvelope({
+      rpcId: 'sub' as never,
+      payload: { type: 'session/subscribed', sessionId: S1, lastSeq: 3 } as never,
+    })
+    expect(manager.getListSnapshot().items[0]?.projectionValues?.openRouterCost).toEqual(COST)
+  })
+})