fix: stop stuck loading after session run conflict

chat-agent-run-gate.mjs

@@ -40,12 +40,13 @@ export function shouldPromoteSessionIdToStreaming(chatState) {
  * @returns {boolean}
  */
 export function shouldKeepStreamingAfterRunError(status, message = '', code = '') {
-  if (status === 0 || status === 409 || Number(status) >= 500) return true;
+  if (status === 0 || 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();
+  if (String(code ?? '').trim() === 'SESSION_RUN_CONFLICT') return false;
   return text.includes('session already has an active request')
     || text.includes('active request. cancel it first');
 }

chat-agent-run-gate.test.mjs

@@ -50,10 +50,17 @@ test('completed run wins when the direct-chat snapshot is not ready', () => {
 
 test('only ambiguous transport failures keep the chat streaming', () => {
   assert.equal(shouldKeepStreamingAfterRunError(0), true);
-  assert.equal(shouldKeepStreamingAfterRunError(409), true);
+  assert.equal(shouldKeepStreamingAfterRunError(409), false);
   assert.equal(shouldKeepStreamingAfterRunError(503), true);
   assert.equal(shouldKeepStreamingAfterRunError(400), false);
   assert.equal(shouldKeepStreamingAfterRunError(undefined), false);
+  assert.equal(
+    shouldKeepStreamingAfterRunError(
+      409,
+      'Session already has an active request. Cancel it first.',
+    ),
+    true,
+  );
   assert.equal(
     shouldKeepStreamingAfterRunError(
       undefined,