diff --git a/src/hooks/useChatSession.test.tsx b/src/hooks/useChatSession.test.tsx
index 922f0b9b5..d2e2ef2c0 100644
--- a/src/hooks/useChatSession.test.tsx
+++ b/src/hooks/useChatSession.test.tsx
@@ -1,6 +1,8 @@
 import { act, renderHook, waitFor } from '@testing-library/react'
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 import { useChatSession } from './useChatSession'
+import { messageStore } from '../store'
+import { getSessionMessages, sendMessageAsync } from '../api'
 
 const {
   createSessionMock,
@@ -501,3 +503,105 @@ describe('useChatSession busy UI signal', () => {
     expect(result.current.messageIsStreaming).toBe(true)
   })
 })
+
+describe('useChatSession sendMessageNow new-session key scoping', () => {
+  beforeEach(() => {
+    createSessionMock.mockReset()
+    summarizeSessionMock.mockReset()
+    executeCommandMock.mockReset()
+    getSelectableAgentsMock.mockReset()
+    registerSessionConsumerMock.mockReset()
+    updateConsumerSessionIdMock.mockReset()
+    sendNotificationMock.mockReset()
+    isSystemEnabledMock.mockReset()
+    errorHandlerMock.mockReset()
+    getPaneFullAutoModeMock.mockReset()
+    onFullAutoChangeMock.mockReset()
+    autoApproveSubscribeMock.mockReset()
+    shouldAutoApproveMock.mockReset()
+    claimAutoReplyMock.mockReset()
+    releaseAutoReplyMock.mockReset()
+    useSessionFamilyMock.mockReset()
+    handlePermissionReplyMock.mockReset()
+    refreshPendingRequestsMock.mockReset()
+    useSessionStateMock.mockReset()
+    pendingPermissionRequestsMock.length = 0
+    for (const key of Object.keys(activeSessionStatusMap)) {
+      delete activeSessionStatusMap[key]
+    }
+
+    registerSessionConsumerMock.mockReturnValue(vi.fn())
+    getPaneFullAutoModeMock.mockReturnValue('off')
+    onFullAutoChangeMock.mockReturnValue(vi.fn())
+    autoApproveSubscribeMock.mockReturnValue(vi.fn())
+    shouldAutoApproveMock.mockReturnValue(false)
+    claimAutoReplyMock.mockReturnValue(true)
+    useSessionFamilyMock.mockReturnValue([])
+    useSessionStateMock.mockReturnValue(null)
+    handlePermissionReplyMock.mockResolvedValue(true)
+    refreshPendingRequestsMock.mockResolvedValue(undefined)
+    autoApproveState.approvePendingOnFullAuto = false
+    getSelectableAgentsMock.mockResolvedValue([{ name: 'build', mode: 'primary', hidden: false }])
+    isSystemEnabledMock.mockImplementation((type: string) => type !== 'permission')
+
+    vi.mocked(sendMessageAsync).mockReset().mockResolvedValue(undefined)
+    vi.mocked(getSessionMessages).mockReset()
+    vi.mocked(messageStore.getSessionState).mockReset().mockReturnValue({ messages: [] })
+    vi.mocked(messageStore.createSendRollbackSnapshot).mockReset().mockReturnValue(null)
+    vi.mocked(messageStore.setStreaming).mockReset()
+    vi.mocked(messageStore.handleMessageUpdated).mockReset()
+    vi.mocked(messageStore.handlePartUpdated).mockReset()
+
+    vi.useFakeTimers()
+  })
+
+  afterEach(() => {
+    vi.useRealTimers()
+    vi.restoreAllMocks()
+  })
+
+  it('scopes the new session with paneServerId for navigation, streaming flag and the SSE fallback pull', async () => {
+    createSessionMock.mockResolvedValue({ id: 'ses_new', directory: '/workspace/demo' })
+    vi.mocked(getSessionMessages).mockResolvedValue([
+      {
+        info: { id: 'msg_u', sessionID: 'ses_new', role: 'user', time: { created: 1, completed: 1 } },
+        parts: [{ id: 'p1', sessionID: 'ses_new', messageID: 'msg_u', type: 'text', text: 'hi' }],
+      },
+    ])
+    const navigateToSessionProp = vi.fn()
+
+    const { result } = renderHook(() =>
+      useChatSession({
+        paneId: 'pane-1',
+        chatAreaRef: { current: null },
+        currentModel: { id: 'model-1', providerId: 'provider-1', variants: [] } as never,
+        refetchModels: vi.fn(async () => {}),
+        sessionId: null,
+        navigateToSession: navigateToSessionProp,
+        navigateHome: vi.fn(),
+      }),
+    )
+
+    let sendPromise!: Promise<boolean>
+    await act(async () => {
+      sendPromise = result.current.handleSend('hello', [])
+      await vi.advanceTimersByTimeAsync(0)
+    })
+    await act(async () => {
+      await vi.advanceTimersByTimeAsync(1600)
+      await sendPromise
+    })
+
+    const scoped = 'local::ses_new'
+    expect(navigateToSessionProp).toHaveBeenCalledWith(scoped, '/workspace/demo')
+    expect(messageStore.setStreaming).toHaveBeenCalledWith(scoped, true)
+    expect(vi.mocked(sendMessageAsync)).toHaveBeenCalledWith(expect.objectContaining({ sessionId: scoped }), 'local')
+    expect(vi.mocked(getSessionMessages)).toHaveBeenCalledWith(scoped, 5, '/workspace/demo', 'local')
+    expect(messageStore.handleMessageUpdated).toHaveBeenCalledWith(
+      expect.objectContaining({ sessionID: scoped, id: 'msg_u' }),
+    )
+    expect(messageStore.handlePartUpdated).toHaveBeenCalledWith(
+      expect.objectContaining({ sessionID: scoped, messageID: 'msg_u' }),
+    )
+  })
+})
diff --git a/src/hooks/useChatSession.ts b/src/hooks/useChatSession.ts
index d2c01ebe3..d0b1435a7 100644
--- a/src/hooks/useChatSession.ts
+++ b/src/hooks/useChatSession.ts
@@ -46,7 +46,7 @@ import { getMessageText, isUserMessage, type AssistantMessageInfo, type Message
 import { clipboardErrorHandler, copyTextToClipboard, createErrorHandler } from '../utils'
 import { clearSessionRuntimeState } from '../utils/sessionLifecycle'
 import { serverStorage } from '../utils/perServerStorage'
-import { sessionKeyToServerId, splitSessionKey } from '../utils/sessionKey'
+import { makeSessionKey, sessionKeyToServerId, splitSessionKey } from '../utils/sessionKey'
 import { serverStore } from '../store/serverStore'
 import { STORAGE_KEY_SELECTED_AGENT } from '../constants'
 import type { ChatAreaHandle } from '../features/chat'
@@ -55,6 +55,9 @@ import { themeStore } from '../store/themeStore'
 
 const handleError = createErrorHandler('session')
 
+/** 发送后等待 SSE 推送用户消息的兜底时延，超时未收到则主动拉取补齐 */
+const SEND_SSE_FALLBACK_DELAY_MS = 1500
+
 /**
  * Stable empty session state singleton.
  *
@@ -683,7 +686,9 @@ export function useChatSession({
         if (!sessionId) {
           if (!input.allowCreateSession) return false
           const newSession = await createSession()
-          sessionId = newSession.id
+          // store 与路由一律用复合键：SSE / loadSession / 兜底拉取都写复合键，
+          // 裸 id 会让 setStreaming 和 1500ms 兜底落到 UI 永远不读的键上
+          sessionId = makeSessionKey(paneServerId, newSession.id)
           navigateToSession(sessionId, newSession.directory)
         }
 
@@ -725,7 +730,7 @@ export function useChatSession({
           getSessionMessages(pullSessionId, 5, pullDir, paneServerId)
             .then(apiMessages => {
               for (const msg of apiMessages) {
-                messageStore.handleMessageUpdated(msg.info)
+                messageStore.handleMessageUpdated({ ...msg.info, sessionID: pullSessionId })
                 if (msg.parts) {
                   for (const part of msg.parts) {
                     messageStore.handlePartUpdated({
@@ -740,7 +745,7 @@ export function useChatSession({
             .catch(() => {
               // 拉取失败不影响主流程，SSE 重连后仍可补齐
             })
-        }, 1500)
+        }, SEND_SSE_FALLBACK_DELAY_MS)
 
         return true
       } catch (error) {
diff --git a/src/hooks/useSessionManager.test.tsx b/src/hooks/useSessionManager.test.tsx
index 1331931f9..e0373fe3b 100644
--- a/src/hooks/useSessionManager.test.tsx
+++ b/src/hooks/useSessionManager.test.tsx
@@ -80,4 +80,50 @@ describe('useSessionManager', () => {
       expect.objectContaining({ name: 'APIError' }),
     )
   })
+
+
+  it('preserves the SSE-delivered user message when the initial snapshot is stale-empty', async () => {
+    // 新建会话竞态：SSE 已把用户消息落到 loading 中的会话，
+    // 而初始快照读于发送提交之前（不含用户消息）——快照无权删掉它
+    messageStoreMock.getSessionState.mockReturnValue({
+      messages: [{ info: { id: 'msg-user', role: 'user', time: { created: 200, completed: 200 } }, parts: [] }],
+      loadState: 'loading',
+      isStale: false,
+      isStreaming: false,
+    })
+    getSessionMessagesMock.mockResolvedValue([])
+
+    const { result } = renderHook(() => useSessionManager({ sessionId: null, directory: '/workspace/demo' }))
+
+    await result.current.loadSession('session-1')
+
+    expect(messageStoreMock.setMessages).toHaveBeenCalledWith(
+      'session-1',
+      [expect.objectContaining({ info: expect.objectContaining({ id: 'msg-user' }) })],
+      expect.anything(),
+    )
+  })
+
+  it('drops local-only messages older than the snapshot newest while loading', async () => {
+    // 截断窗口外的旧历史不走 SSE 补齐通道，加载期替换照常丢弃
+    messageStoreMock.getSessionState.mockReturnValue({
+      messages: [{ info: { id: 'msg-old', role: 'user', time: { created: 100, completed: 100 } }, parts: [] }],
+      loadState: 'loading',
+      isStale: false,
+      isStreaming: false,
+    })
+    getSessionMessagesMock.mockResolvedValue([
+      { info: { id: 'msg-new', role: 'user', time: { created: 200, completed: 200 } }, parts: [] },
+    ])
+
+    const { result } = renderHook(() => useSessionManager({ sessionId: null, directory: '/workspace/demo' }))
+
+    await result.current.loadSession('session-1')
+
+    expect(messageStoreMock.setMessages).toHaveBeenCalledWith(
+      'session-1',
+      [expect.objectContaining({ info: expect.objectContaining({ id: 'msg-new' }) })],
+      expect.anything(),
+    )
+  })
 })
diff --git a/src/hooks/useSessionManager.ts b/src/hooks/useSessionManager.ts
index d0276b70f..fb9952dd2 100644
--- a/src/hooks/useSessionManager.ts
+++ b/src/hooks/useSessionManager.ts
@@ -94,8 +94,18 @@ function mergeWithLocalStreamingMessages(
     }
   })
 
-  const localOnly = localState.isStreaming
-    ? localState.messages.filter(m => !apiIds.has(m.info.id)).map(toApiMessageWithParts)
+  // 初始加载在途（loading）或仍在流式时，本地独有且不比快照旧的消息，
+  // 必然是在快照查询之后经 SSE 到达的——快照无权删除它们；
+  // 比快照最新的还旧的本地消息属于窗口外历史（截断/会话被清空），照常丢弃
+  const snapshotNewestCreated = apiMessages.reduce(
+    (max, m) => Math.max(max, m.info.time?.created ?? 0),
+    0,
+  )
+  const keepLocalOnly = localState.isStreaming || localState.loadState === 'loading'
+  const localOnly = keepLocalOnly
+    ? localState.messages
+        .filter(m => !apiIds.has(m.info.id) && (m.info.time?.created ?? 0) >= snapshotNewestCreated)
+        .map(toApiMessageWithParts)
     : []
 
   if (localOnly.length === 0) return mergedApi