fix(orchestrator): stop failed phase-transition backend

src/symphony/orchestrator.py

@@ -831,20 +831,32 @@ async def _rebuild_backend_for_phase(
                 client_tools=tools,
             )
         )
-        await new_client.start()
-        await new_client.initialize()
-        first_prompt, _ = build_first_turn_prompt(
-            prompt_template=cfg.prompt_template,
-            issue=issue,
-            attempt=attempt,
-            language=doc_language,
-            max_turns=cfg.agent.max_turns,
-            is_rewind=is_rewind,
-        )
-        await new_client.start_session(
-            initial_prompt=first_prompt,
-            issue_title=f"{issue.identifier}: {issue.title}",
-        )
+        try:
+            await new_client.start()
+            await new_client.initialize()
+            first_prompt, _ = build_first_turn_prompt(
+                prompt_template=cfg.prompt_template,
+                issue=issue,
+                attempt=attempt,
+                language=doc_language,
+                max_turns=cfg.agent.max_turns,
+                is_rewind=is_rewind,
+            )
+            await new_client.start_session(
+                initial_prompt=first_prompt,
+                issue_title=f"{issue.identifier}: {issue.title}",
+            )
+        except BaseException:
+            try:
+                await new_client.stop()
+            except Exception as stop_exc:
+                log.warning(
+                    "phase_transition_new_stop_failed",
+                    issue_id=issue.id,
+                    identifier=issue.identifier,
+                    error=str(stop_exc),
+                )
+            raise
         return new_client, first_prompt
 
     async def _refresh_issue_state(

tests/test_orchestrator_phase_transition.py

@@ -316,6 +316,30 @@ def test_phase_transition_rebuilds_backend_with_fresh_first_prompt(
     assert second_run[0][1]["prompt"] == first_prompts[1]
 
 
+def test_phase_transition_stops_new_backend_when_rebuild_initialize_fails(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    cfg = _make_config(max_turns=5)
+    issue = _make_issue(state="Todo")
+    o = _orch(tmp_path)
+    _seed_running_entry(o, issue, tmp_path)
+    instances = _install_fake_backend(monkeypatch)
+    _install_state_sequence(monkeypatch, ["In Progress", "Done"])
+
+    async def _initialize(self_inst: _FakeBackend) -> None:
+        self_inst.calls.append(("initialize", {}))
+        if self_inst.init_id == 1:
+            raise RuntimeError("second backend init failed")
+
+    monkeypatch.setattr(_FakeBackend, "initialize", _initialize)
+
+    asyncio.run(o._run_agent_attempt(issue, attempt=None, cfg=cfg))
+
+    assert len(instances) == 2
+    second_calls = [name for name, _ in instances[1].calls]
+    assert second_calls == ["start", "initialize", "stop"]
+
+
 def test_same_phase_does_not_restart_backend(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch
 ) -> None: