diff --git a/chat-agent-run-gate.mjs b/chat-agent-run-gate.mjs
index 11e8dcf..2024d94 100644
--- a/chat-agent-run-gate.mjs
+++ b/chat-agent-run-gate.mjs
@@ -39,8 +39,15 @@ export function shouldPromoteSessionIdToStreaming(chatState) {
  * @param {number | undefined | null} status
  * @returns {boolean}
  */
-export function shouldKeepStreamingAfterRunError(status) {
-  return status === 0 || status === 409 || Number(status) >= 500;
+export function shouldKeepStreamingAfterRunError(status, message = '', code = '') {
+  if (status === 0 || status === 409 || Number(status) >= 500) return true;
+  // Goose can surface the session-level concurrency guard as a failed agent
+  // run (rather than an HTTP 409). The request may already be streaming and
+  // the session SSE is still the source of truth, so recover from the session
+  // instead of showing a terminal error in the chat composer.
+  const text = `${String(code ?? '')} ${String(message ?? '')}`.toLowerCase();
+  return text.includes('session already has an active request')
+    || text.includes('active request. cancel it first');
 }
 
 /**
diff --git a/chat-agent-run-gate.test.mjs b/chat-agent-run-gate.test.mjs
index 013a613..21013e9 100644
--- a/chat-agent-run-gate.test.mjs
+++ b/chat-agent-run-gate.test.mjs
@@ -54,6 +54,14 @@ test('only ambiguous transport failures keep the chat streaming', () => {
   assert.equal(shouldKeepStreamingAfterRunError(503), true);
   assert.equal(shouldKeepStreamingAfterRunError(400), false);
   assert.equal(shouldKeepStreamingAfterRunError(undefined), false);
+  assert.equal(
+    shouldKeepStreamingAfterRunError(
+      undefined,
+      'Session already has an active request. Cancel it first.',
+    ),
+    true,
+  );
+  assert.equal(shouldKeepStreamingAfterRunError(undefined, '后台任务失败'), false);
 });
 
 test('shouldPromoteSessionIdToStreaming skips re-streaming after Finish', () => {
diff --git a/chat-finish-sync.test.mjs b/chat-finish-sync.test.mjs
index 4910a8e..32ab80f 100644
--- a/chat-finish-sync.test.mjs
+++ b/chat-finish-sync.test.mjs
@@ -80,3 +80,33 @@ test('mergeConversationSnapshot keeps local messages before the first and after
     ['u0', 'u1', 'a1', 'a2'],
   );
 });
+
+test('mergeConversationSnapshot keeps local-only messages at their streamed position', () => {
+  const local = [
+    msg('u1', 'user', '生成页面'),
+    msg('a1', 'assistant', '开始分析'),
+    msg('a2', 'assistant', '调用工具'),
+    msg('a3', 'assistant', '页面完成'),
+  ];
+  const server = [
+    msg('u1', 'user', '生成页面'),
+    msg('a3', 'assistant', '页面完成（服务端）'),
+  ];
+  const merged = mergeConversationSnapshot(local, server);
+  assert.deepEqual(merged.map((message) => message.id), ['u1', 'a1', 'a2', 'a3']);
+  assert.equal(merged[3].content[0].text, '页面完成（服务端）');
+});
+
+test('mergeConversationSnapshot preserves multiple local messages before the next server anchor', () => {
+  const local = [
+    msg('u1', 'user', '任务'),
+    msg('a1', 'assistant', '步骤一'),
+    msg('a2', 'assistant', '步骤二'),
+    msg('a3', 'assistant', '完成'),
+  ];
+  const server = [msg('u1', 'user', '任务'), msg('a3', 'assistant', '完成')];
+  assert.deepEqual(
+    mergeConversationSnapshot(local, server).map((message) => message.id),
+    ['u1', 'a1', 'a2', 'a3'],
+  );
+});
diff --git a/src/hooks/useTKMindChat.ts b/src/hooks/useTKMindChat.ts
index aa3a44f..bc9ec0f 100644
--- a/src/hooks/useTKMindChat.ts
+++ b/src/hooks/useTKMindChat.ts
@@ -1618,7 +1618,7 @@ export function useTKMindChat(
         if (
           activeSessionId &&
           err instanceof ApiError &&
-          shouldKeepStreamingAfterRunError(err.status)
+          shouldKeepStreamingAfterRunError(err.status, err.message, err.code)
         ) {
           subscribeToSession(activeSessionId);
           setChatState('streaming');
@@ -1626,6 +1626,23 @@ export function useTKMindChat(
           scheduleReplyRecoverySync(activeSessionId, submitToken);
           return;
         }
+        if (
+          activeSessionId &&
+          shouldKeepStreamingAfterRunError(
+            undefined,
+            err instanceof Error ? err.message : String(err),
+            err instanceof ApiError ? err.code : '',
+          )
+        ) {
+          // Goose may report its session concurrency guard as a failed run
+          // message instead of an HTTP 409. Reattach to the session stream
+          // and reconcile the snapshot; do not strand the composer in error.
+          subscribeToSession(activeSessionId);
+          setChatState('streaming');
+          setError(null);
+          scheduleReplyRecoverySync(activeSessionId, submitToken);
+          return;
+        }
         if (session && activeSessionId) setSessions((prev) => touchSession(prev, activeSessionId!, -1));
         if (err instanceof ApiError && err.status === 402) {
           notifyInsufficientBalance();