diff --git a/src/store.ts b/src/store.ts
index 513d287..12be84b 100644
--- a/src/store.ts
+++ b/src/store.ts
@@ -1145,10 +1145,12 @@ export const store = {
         set({ outbox: enqueueMessageFront(state.outbox, { id, text, attachmentIds, attachmentNames: [], ...(taskRef ? { taskRef } : {}) }) })
         return 'busy'
       }
-      store.toast(`发送失败：${e}`)
+      const removed = isNotProjectMember(e)
+      store.toast(removed ? '无法继续发送：你已不在该项目中或项目已关闭；历史对话仍可查看' : `发送失败：${e}`)
       set({ outbox: enqueueMessageFront(state.outbox, { id, text, attachmentIds,
         attachmentNames: attachmentIds.map(attachmentId => state.artifacts.find(artifact => artifact.id === attachmentId)?.name ?? '已添加材料'),
-        ...(taskRef ? { taskRef } : {}), deliveryError: '发送未成功，内容已保留；可修改后重试或移除' }) })
+        ...(taskRef ? { taskRef } : {}),
+        deliveryError: removed ? '无法发送：你已不在该项目中或项目已关闭，内容已保留' : '发送未成功，内容已保留；可修改后重试或移除' }) })
       await store.reconcile()
       return 'rejected'
     } finally {
@@ -1720,6 +1722,11 @@ function isConversationBusy(err: unknown): boolean {
   return String(err).includes('409:')
 }
 
+/** 已被移出项目或项目已关闭：后端以 403 `conversation: not-project-member` 拒绝继续发送。 */
+function isNotProjectMember(err: unknown): boolean {
+  return String(err).includes('not-project-member')
+}
+
 function shellPhase() {
   const liveRunId = state.live?.runId ?? null
   return deriveShellPhase({
diff --git a/test/send-removed-member.test.ts b/test/send-removed-member.test.ts
new file mode 100644
index 0000000..3a1c0d7
--- /dev/null
+++ b/test/send-removed-member.test.ts
@@ -0,0 +1,57 @@
+// 被移出项目/项目关闭后的发送拒绝：403 `conversation: not-project-member` 走专用文案，
+// 其他失败保持通用文案（见 src/store.ts sendMessage 的 catch 分支）。
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
+import { api } from '../src/api'
+import { store } from '../src/store'
+import type { Conversation, Message } from '../src/contracts'
+
+const REMOVED_TOAST = '无法继续发送：你已不在该项目中或项目已关闭；历史对话仍可查看'
+const REMOVED_DELIVERY_ERROR = '无法发送：你已不在该项目中或项目已关闭，内容已保留'
+const GENERIC_DELIVERY_ERROR = '发送未成功，内容已保留；可修改后重试或移除'
+
+function detail(id: string) {
+  return {
+    conversation: { id, projectId: 'p', title: id, createdAt: '', updatedAt: '' } as Conversation,
+    messages: [] as Message[], runs: [], activeHitl: [], lastSeq: 0, resumeSeq: 0,
+  }
+}
+class FakeEventSource {
+  onopen = null
+  onerror = null
+  onmessage = null
+  addEventListener() {}
+  close() {}
+}
+
+beforeEach(async () => {
+  vi.stubGlobal('EventSource', FakeEventSource)
+  vi.spyOn(api, 'conversationDetail').mockImplementation(async id => detail(id))
+  vi.spyOn(api, 'getPlan').mockResolvedValue(null)
+  vi.spyOn(api, 'getTodos').mockResolvedValue(null)
+  vi.spyOn(api, 'listCards').mockResolvedValue([])
+  vi.spyOn(api, 'listTasks').mockResolvedValue([])
+  vi.spyOn(api, 'listArtifacts').mockResolvedValue([])
+  vi.spyOn(api, 'surfaceList').mockResolvedValue([])
+  vi.spyOn(api, 'projectSurfaces').mockResolvedValue([])
+  vi.spyOn(api, 'listProjectCards').mockResolvedValue([])
+  await store.openProject('p')
+  await store.openConversation('a')
+})
+afterEach(() => { vi.useRealTimers(); vi.restoreAllMocks(); vi.unstubAllGlobals() })
+
+describe('sendMessage rejection copy', () => {
+  it('keeps the message in the outbox with the removed-member copy on 403 not-project-member', async () => {
+    vi.spyOn(api, 'sendMessage').mockRejectedValue(new Error('403: conversation: not-project-member'))
+    expect(await store.sendMessage('这条还要继续吗', [], 'kept-id')).toBe('rejected')
+    expect(store.getState().outbox[0]).toMatchObject({ id: 'kept-id', text: '这条还要继续吗', attachmentIds: [] })
+    expect(store.getState().outbox[0].deliveryError).toBe(REMOVED_DELIVERY_ERROR)
+    expect(store.getState().toast).toBe(REMOVED_TOAST)
+  })
+
+  it('keeps the generic copy for any other send failure', async () => {
+    vi.spyOn(api, 'sendMessage').mockRejectedValue(new Error('500: upstream down'))
+    expect(await store.sendMessage('这条还要继续吗', [], 'kept-id')).toBe('rejected')
+    expect(store.getState().outbox[0].deliveryError).toBe(GENERIC_DELIVERY_ERROR)
+    expect(store.getState().toast).toContain('发送失败：')
+  })
+})
\ No newline at end of file