feat(search): /ask/react endpoint with Qwen native tool calling ReAct loop

PR-DocSrv-Ask-ToolCalling-ReAct-1 — Qwen3.6-27B-8bit 의 native tool calling
으로 ReAct loop 도입. 기존 /api/search/ask 무수정. 트랙 B (frontend /ask SSE)
와 파일 단위 충돌 0 (search.py 의 ask() 함수 line diff = 0, 순수 추가).

핵심 invariant:
- 별 endpoint /api/search/ask/react (qwen-macbook only, implicit opt-in)
- MacBook unavailable 시 HTTP 503 + error_reason=macbook_unavailable.
  Gemma 자동 fallback X (정정 4 의 연장)

G0 (구현 전 hard gate, plan b-velvety-hare.md):
- G0-1 fixture (tests/fixtures/qwen_tool_call_response.json): 실제 mlx-vlm
  응답 박제. shape = OpenAI 표준 호환 (choices[0].message.tool_calls +
  function.arguments JSON string). generate_with_tools() 가 본 shape 기준 구현.
- G0-2 counter semantics: max_tool_rounds=2 + max_llm_calls=3 + search_exec_max=2.
  마지막 LLM 호출은 tool_choice="none" + system instruction 으로 final 강제.
- G0-3 trace exposure: default response 의 debug_trace=null. debug=true 시만
  채움. server log 에는 항상 round 기록.

backends.py (193 → 261줄):
- QwenMacBookBackend.generate_with_tools(messages, tools, tool_choice)
  신규 method. 기존 generate() 무수정. BackendUnavailable 처리 동일.

react_loop.py 신규 (275줄):
- agentic_ask_loop(session, query, *, backend, max_tool_rounds, debug)
- tool round 안에서 run_search 호출, results dedup by id, final round 강제,
  partial=True 조건 (final content 빈 경우)

search.py (+82줄):
- POST /api/search/ask/react + AskReactRequest/Response schema
- BackendUnavailable → JSONResponse(503, error_reason=macbook_unavailable)

config.yaml + config.py:
- search.ask.react: { enabled, max_tool_rounds=2, search_tool_limit=5,
  search_tool_mode=hybrid }

tests (566줄, 18 신규 + 23 회귀 모두 PASS):
- test_react_loop.py 13건: G0-1 fixture shape / G0-2 counter cap / G0-3 trace
  exposure / BackendUnavailable propagation / sources dedup
- test_search_ask_react_endpoint.py 5건: 503 + run_search 호출 0 / 정상 200 /
  debug=true trace 노출 / max rounds partial
- 회귀 (test_ask_eval_auth 9 + test_search_ask_macbook_503 5 +
  test_backend_dispatcher 9) 모두 PASS

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/api/test_search_ask_react_endpoint.py

@@ -0,0 +1,218 @@
+"""PR-DocSrv-Ask-ToolCalling-ReAct-1: /api/search/ask/react endpoint integration.
+
+검증 항목 (G0-3 trace exposure + 정정 4 invariant):
+- backend unavailable → HTTP 503 + error_reason=macbook_unavailable
+  + ★ `run_search` mock 호출 횟수 == 0 (search 단계 진입 자체 차단)
+- 정상 응답 → 200 + final_answer + sources + debug_trace=null (default)
+- debug=true → debug_trace 채워짐
+- max rounds 도달 → iterations=2 + partial=false (final content 정상)
+
+endpoint 함수 (`api.search.ask_react`) 를 직접 호출하는 lightweight 패턴.
+TestClient 없이 FastAPI deps 를 MagicMock 으로 우회. (priority_gate / backend_dispatcher
+test 와 동일 service-layer 패턴.)
+"""
+
+from __future__ import annotations
+
+import asyncio
+import json
+import os
+import sys
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "app"))
+
+
+# ── helpers ────────────────────────────────────────────────────────────────
+
+
+def _msg_with_tool_call(q: str, tc_id: str = "tc-1") -> dict:
+    return {
+        "role": "assistant",
+        "content": None,
+        "tool_calls": [
+            {
+                "id": tc_id,
+                "type": "function",
+                "function": {
+                    "name": "search",
+                    "arguments": json.dumps({"q": q}, ensure_ascii=False),
+                },
+            }
+        ],
+    }
+
+
+def _msg_with_content(text: str) -> dict:
+    return {"role": "assistant", "content": text, "tool_calls": None}
+
+
+def _fake_chunk(chunk_id: int, doc_id: int = 100):
+    m = MagicMock()
+    m.id = chunk_id
+    m.chunk_id = chunk_id
+    m.doc_id = doc_id
+    m.title = f"doc {doc_id}"
+    m.score = 0.9
+    m.snippet = f"snippet {chunk_id}"
+    m.text = None
+    return m
+
+
+def _fake_pr(chunks: list):
+    pr = MagicMock()
+    pr.results = chunks
+    return pr
+
+
+@pytest.fixture
+def patched_backend_and_search(monkeypatch):
+    """get_backend + run_search 둘 다 mock. backend 의 generate_with_tools 는
+    각 테스트가 side_effect 설정.
+
+    Returns: (backend_mock, run_search_mock, set_backend_unavailable_fn).
+    """
+    from services.llm.backends import BackendUnavailable, QwenMacBookBackend
+    from services.llm import backends as backends_mod
+    from services.search import react_loop
+
+    backend = MagicMock(spec=QwenMacBookBackend)
+    backend.name = "qwen-macbook"
+    backend.generate_with_tools = AsyncMock()
+
+    def _fake_get_backend(name):
+        # endpoint 가 qwen-macbook 만 호출하므로 단일 backend 반환
+        return backend
+
+    monkeypatch.setattr(backends_mod, "get_backend", _fake_get_backend)
+    # search.py 의 ask_react 안에서 `from services.llm.backends import ... get_backend`
+    # 로 import 하므로 module-level patch 만으로 충분 (지연 import 라 매번 fresh).
+
+    run_search_mock = AsyncMock(return_value=_fake_pr([_fake_chunk(1)]))
+    monkeypatch.setattr(react_loop, "run_search", run_search_mock)
+
+    def _make_unavailable():
+        backend.generate_with_tools.side_effect = BackendUnavailable(
+            "qwen-macbook", "ConnectError"
+        )
+
+    return backend, run_search_mock, _make_unavailable
+
+
+def _call_endpoint(payload):
+    """ask_react 를 직접 호출. user/session 은 MagicMock 으로 우회."""
+    from api.search import ask_react
+
+    user = MagicMock()
+    session = MagicMock()
+    return asyncio.run(ask_react(payload, user=user, session=session))
+
+
+# ── ★ 정정 4 invariant: backend unavailable → 503 + run_search 호출 0 ──────
+
+
+def test_qwen_unavailable_returns_503(patched_backend_and_search):
+    """backend BackendUnavailable → HTTP 503 + error_reason=macbook_unavailable."""
+    from api.search import AskReactRequest
+
+    backend, run_search_mock, make_unavailable = patched_backend_and_search
+    make_unavailable()
+
+    response = _call_endpoint(AskReactRequest(query="Q"))
+
+    # JSONResponse instance
+    assert response.status_code == 503
+    body = json.loads(response.body)
+    assert body["error_reason"] == "macbook_unavailable"
+    assert body["backend_used"] is None
+    assert body["backend_requested"] == "qwen-macbook"
+
+    # ★ run_search 호출 0 (search 진입 자체 차단)
+    assert run_search_mock.call_count == 0
+
+
+# ── 정상 200 + G0-3 default debug_trace=null ──────────────────────────────
+
+
+def test_successful_response_default_no_debug_trace(patched_backend_and_search):
+    """debug 미지정 (default false) → 200 + debug_trace == null."""
+    from api.search import AskReactRequest, AskReactResponse
+
+    backend, run_search_mock, _ = patched_backend_and_search
+    backend.generate_with_tools.side_effect = [
+        _msg_with_tool_call("q1"),
+        _msg_with_content("최종 답입니다"),
+    ]
+
+    response = _call_endpoint(AskReactRequest(query="Q"))
+
+    # Pydantic instance (FastAPI response_model 적용 전 raw return)
+    assert isinstance(response, AskReactResponse)
+    assert response.final_answer == "최종 답입니다"
+    assert response.iterations == 2
+    assert response.partial is False
+    assert response.debug_trace is None  # ★ G0-3
+    assert len(response.sources) == 1
+
+
+# ── G0-3: debug=true → debug_trace 채워짐 ──────────────────────────────────
+
+
+def test_debug_true_populates_trace(patched_backend_and_search):
+    from api.search import AskReactRequest
+
+    backend, run_search_mock, _ = patched_backend_and_search
+    backend.generate_with_tools.side_effect = [
+        _msg_with_content("바로 답"),
+    ]
+
+    response = _call_endpoint(AskReactRequest(query="Q", debug=True))
+
+    assert response.debug_trace is not None
+    assert isinstance(response.debug_trace, list)
+    assert len(response.debug_trace) >= 1
+
+
+# ── max rounds → final content 정상 → partial=false ──────────────────────
+
+
+def test_max_rounds_with_final_content(patched_backend_and_search):
+    from api.search import AskReactRequest
+
+    backend, run_search_mock, _ = patched_backend_and_search
+    backend.generate_with_tools.side_effect = [
+        _msg_with_tool_call("q1"),
+        _msg_with_tool_call("q2", tc_id="tc-2"),
+        _msg_with_content("정리된 최종 답"),
+    ]
+
+    response = _call_endpoint(AskReactRequest(query="Q"))
+
+    assert response.iterations == 2
+    assert response.partial is False
+    assert response.final_answer == "정리된 최종 답"
+    # LLM 호출 3회, search 2회 (G0-2 cap)
+    assert backend.generate_with_tools.call_count == 3
+    assert run_search_mock.call_count == 2
+
+
+# ── max rounds + final content 빈 string → partial=true ──────────────────
+
+
+def test_max_rounds_with_empty_final_partial(patched_backend_and_search):
+    from api.search import AskReactRequest
+
+    backend, run_search_mock, _ = patched_backend_and_search
+    backend.generate_with_tools.side_effect = [
+        _msg_with_tool_call("q1"),
+        _msg_with_tool_call("q2", tc_id="tc-2"),
+        _msg_with_content(""),
+    ]
+
+    response = _call_endpoint(AskReactRequest(query="Q"))
+
+    assert response.iterations == 2
+    assert response.partial is True
+    assert response.final_answer == ""

tests/fixtures/qwen_tool_call_response.json

@@ -0,0 +1 @@
+{"id":"chatcmpl-d72e8a01-83d8-4b63-8fe2-e83df37b730b","object":"chat.completion","created":1779456681,"model":"/Users/hyungi/mlx-models/Qwen3.6-27B-8bit","choices":[{"index":0,"finish_reason":"tool_calls","message":{"role":"assistant","content":null,"reasoning":null,"tool_calls":[{"type":"function","index":0,"id":"6f30c959-c730-4901-82d2-28ff9b5967de","function":{"name":"search","arguments":"{\"q\": \"가스기사 14회 1번 문제\"}"}}],"tool_call_id":null,"name":null},"logprobs":null}],"usage":{"prompt_tokens":285,"completion_tokens":33,"total_tokens":318,"prompt_tokens_details":{"cached_tokens":0},"prompt_tps":0.0,"generation_tps":0.0,"peak_memory":30.390329063}}

tests/services/test_react_loop.py

@@ -0,0 +1,348 @@
+"""PR-DocSrv-Ask-ToolCalling-ReAct-1: agentic_ask_loop unit tests.
+
+검증 invariant:
+- G0-1: tests/fixtures/qwen_tool_call_response.json 의 shape 이 parsing 가정과 일치.
+- G0-2 counter semantics:
+    * LLM 호출 횟수 ≤ max_llm_calls (= max_tool_rounds + 1)
+    * search 실행 횟수 ≤ search_exec_max (= max_tool_rounds)
+    * 마지막 LLM 호출의 tool_choice == "none"
+    * partial=true 조건: max rounds 후 final content 비어 있을 때
+- G0-3 trace exposure: debug=False → debug_trace=None, debug=True → list[dict].
+- BackendUnavailable 은 호출자에게 그대로 전파 (정정 4 의 연장).
+"""
+
+from __future__ import annotations
+
+import asyncio
+import json
+import os
+import sys
+from pathlib import Path
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "app"))
+
+
+FIXTURE_PATH = (
+    Path(__file__).resolve().parents[1] / "fixtures" / "qwen_tool_call_response.json"
+)
+
+
+# ── helpers ────────────────────────────────────────────────────────────────
+
+
+def _msg_with_tool_call(q: str, tc_id: str = "tc-1") -> dict:
+    """G0-1 fixture shape 그대로 — assistant message with one tool_call."""
+    return {
+        "role": "assistant",
+        "content": None,
+        "tool_calls": [
+            {
+                "id": tc_id,
+                "type": "function",
+                "function": {
+                    "name": "search",
+                    "arguments": json.dumps({"q": q}, ensure_ascii=False),
+                },
+            }
+        ],
+    }
+
+
+def _msg_with_content(text: str) -> dict:
+    return {"role": "assistant", "content": text, "tool_calls": None}
+
+
+def _fake_chunk(chunk_id: int, doc_id: int = 100, score: float = 0.9):
+    m = MagicMock()
+    m.id = chunk_id
+    m.chunk_id = chunk_id
+    m.doc_id = doc_id
+    m.title = f"doc {doc_id}"
+    m.score = score
+    m.snippet = f"snippet for chunk {chunk_id}"
+    m.text = None
+    return m
+
+
+def _fake_pr(chunks: list):
+    pr = MagicMock()
+    pr.results = chunks
+    return pr
+
+
+@pytest.fixture
+def mock_backend():
+    """services.llm.backends.QwenMacBookBackend instance mock (generate_with_tools)."""
+    from services.llm.backends import QwenMacBookBackend
+
+    b = MagicMock(spec=QwenMacBookBackend)
+    b.name = "qwen-macbook"
+    b.generate_with_tools = AsyncMock()
+    return b
+
+
+@pytest.fixture
+def mock_run_search(monkeypatch):
+    """services.search.react_loop.run_search 를 monkeypatch — chunk 1건 반환 default."""
+    from services.search import react_loop
+
+    mock = AsyncMock(return_value=_fake_pr([_fake_chunk(1)]))
+    monkeypatch.setattr(react_loop, "run_search", mock)
+    return mock
+
+
+# ── G0-1: fixture shape 검증 ───────────────────────────────────────────────
+
+
+def test_fixture_shape_matches_parser_assumptions():
+    """G0-1: fixture 의 shape 이 react_loop 의 parsing 가정과 일치."""
+    assert FIXTURE_PATH.exists(), f"fixture missing at {FIXTURE_PATH}"
+    fixture = json.loads(FIXTURE_PATH.read_text(encoding="utf-8"))
+    assert "choices" in fixture and len(fixture["choices"]) >= 1
+    msg = fixture["choices"][0]["message"]
+    assert msg["role"] == "assistant"
+    assert "tool_calls" in msg
+    tcs = msg["tool_calls"]
+    assert isinstance(tcs, list) and len(tcs) >= 1
+    tc = tcs[0]
+    assert tc["type"] == "function"
+    assert tc["function"]["name"]  # non-empty str
+    # arguments 가 JSON string 임 (G0-1 핵심 발견)
+    args_str = tc["function"]["arguments"]
+    assert isinstance(args_str, str)
+    args = json.loads(args_str)
+    assert isinstance(args, dict)
+
+
+# ── early exit (LLM call #1 에 tool_calls 없음) ─────────────────────────────
+
+
+def test_no_tool_calls_early_exit(mock_backend, mock_run_search):
+    """첫 LLM 호출이 tool_calls 없이 content 반환 → iterations=1, partial=false."""
+    from services.search.react_loop import agentic_ask_loop
+
+    mock_backend.generate_with_tools.side_effect = [
+        _msg_with_content("바로 답입니다"),
+    ]
+    session = MagicMock()
+
+    result = asyncio.run(agentic_ask_loop(session, "Q", backend=mock_backend))
+
+    assert result.iterations == 1
+    assert result.partial is False
+    assert result.final_answer == "바로 답입니다"
+    assert result.sources == []
+    assert mock_backend.generate_with_tools.call_count == 1
+    assert mock_run_search.call_count == 0
+
+
+# ── 1 round + early exit ───────────────────────────────────────────────────
+
+
+def test_one_round_then_final_content(mock_backend, mock_run_search):
+    """round 1 tool_call → search → round 2 content (early exit)."""
+    from services.search.react_loop import agentic_ask_loop
+
+    mock_backend.generate_with_tools.side_effect = [
+        _msg_with_tool_call("query A"),
+        _msg_with_content("두 번째 호출 종합문"),
+    ]
+    session = MagicMock()
+
+    result = asyncio.run(agentic_ask_loop(session, "Q", backend=mock_backend))
+
+    assert result.iterations == 2
+    assert result.partial is False
+    assert result.final_answer == "두 번째 호출 종합문"
+    assert len(result.sources) == 1
+    assert mock_backend.generate_with_tools.call_count == 2
+    assert mock_run_search.call_count == 1
+
+
+# ── max rounds 도달 + final call ────────────────────────────────────────────
+
+
+def test_max_rounds_reached_final_with_content(mock_backend, mock_run_search):
+    """round 1, 2 둘 다 tool_call → final call → content 정상 → partial=false."""
+    from services.search.react_loop import agentic_ask_loop
+
+    mock_backend.generate_with_tools.side_effect = [
+        _msg_with_tool_call("q1"),
+        _msg_with_tool_call("q2", tc_id="tc-2"),
+        _msg_with_content("최종 답입니다"),
+    ]
+    session = MagicMock()
+
+    result = asyncio.run(agentic_ask_loop(session, "Q", backend=mock_backend))
+
+    assert result.iterations == 2  # = max_tool_rounds
+    assert result.partial is False
+    assert result.final_answer == "최종 답입니다"
+    assert mock_backend.generate_with_tools.call_count == 3
+    assert mock_run_search.call_count == 2
+
+
+# ── G0-2: 마지막 호출 tool_choice="none" ─────────────────────────────────────
+
+
+def test_final_call_uses_tool_choice_none(mock_backend, mock_run_search):
+    """G0-2 invariant: max_tool_rounds 도달 시 final call 의 tool_choice == 'none'."""
+    from services.search.react_loop import agentic_ask_loop
+
+    mock_backend.generate_with_tools.side_effect = [
+        _msg_with_tool_call("q1"),
+        _msg_with_tool_call("q2", tc_id="tc-2"),
+        _msg_with_content("종합"),
+    ]
+    session = MagicMock()
+
+    asyncio.run(agentic_ask_loop(session, "Q", backend=mock_backend))
+
+    last_call = mock_backend.generate_with_tools.call_args_list[-1]
+    assert last_call.kwargs.get("tool_choice") == "none"
+    # final call 은 tools=[] 를 keyword 로 넘김 (positional 아님)
+    assert last_call.kwargs.get("tools") == []
+
+
+# ── G0-2: max LLM calls + search exec cap ──────────────────────────────────
+
+
+def test_max_llm_calls_capped_at_three(mock_backend, mock_run_search):
+    """LLM 호출 횟수 ≤ 3 (= max_tool_rounds + 1)."""
+    from services.search.react_loop import agentic_ask_loop
+
+    mock_backend.generate_with_tools.side_effect = [
+        _msg_with_tool_call("q1"),
+        _msg_with_tool_call("q2", tc_id="tc-2"),
+        _msg_with_content("종합"),
+    ]
+    asyncio.run(agentic_ask_loop(MagicMock(), "Q", backend=mock_backend))
+    assert mock_backend.generate_with_tools.call_count <= 3
+
+
+def test_search_exec_capped_at_two(mock_backend, mock_run_search):
+    """search 실제 실행 횟수 ≤ max_tool_rounds (=2)."""
+    from services.search.react_loop import agentic_ask_loop
+
+    mock_backend.generate_with_tools.side_effect = [
+        _msg_with_tool_call("q1"),
+        _msg_with_tool_call("q2", tc_id="tc-2"),
+        _msg_with_content("종합"),
+    ]
+    asyncio.run(agentic_ask_loop(MagicMock(), "Q", backend=mock_backend))
+    assert mock_run_search.call_count <= 2
+
+
+# ── G0-2: partial=true (final content 비어 있음) ───────────────────────────
+
+
+def test_partial_when_final_content_empty(mock_backend, mock_run_search):
+    """max rounds 도달 + final call content 비어 있음 → partial=true."""
+    from services.search.react_loop import agentic_ask_loop
+
+    mock_backend.generate_with_tools.side_effect = [
+        _msg_with_tool_call("q1"),
+        _msg_with_tool_call("q2", tc_id="tc-2"),
+        _msg_with_content(""),  # 빈 content
+    ]
+    result = asyncio.run(agentic_ask_loop(MagicMock(), "Q", backend=mock_backend))
+
+    assert result.iterations == 2
+    assert result.partial is True
+    assert result.final_answer == ""
+
+
+# ── sources dedup ──────────────────────────────────────────────────────────
+
+
+def test_sources_dedup_by_id(mock_backend, monkeypatch):
+    """같은 chunk id 가 두 round 에 나오면 sources 에서 dedup."""
+    from services.search import react_loop
+    from services.search.react_loop import agentic_ask_loop
+
+    # round 1 → chunk id=1, round 2 → chunk id=1 + id=2
+    run_search_mock = AsyncMock(side_effect=[
+        _fake_pr([_fake_chunk(1)]),
+        _fake_pr([_fake_chunk(1), _fake_chunk(2)]),
+    ])
+    monkeypatch.setattr(react_loop, "run_search", run_search_mock)
+
+    mock_backend.generate_with_tools.side_effect = [
+        _msg_with_tool_call("q1"),
+        _msg_with_tool_call("q2", tc_id="tc-2"),
+        _msg_with_content("종합"),
+    ]
+    result = asyncio.run(agentic_ask_loop(MagicMock(), "Q", backend=mock_backend))
+
+    src_ids = [s["id"] for s in result.sources]
+    assert src_ids == [1, 2]  # id=1 중복 없음
+    assert len(result.sources) == 2
+
+
+# ── G0-3: trace exposure ───────────────────────────────────────────────────
+
+
+def test_debug_trace_none_when_debug_false(mock_backend, mock_run_search):
+    """G0-3: debug=False (default) → debug_trace=None."""
+    from services.search.react_loop import agentic_ask_loop
+
+    mock_backend.generate_with_tools.side_effect = [
+        _msg_with_content("바로 답"),
+    ]
+    result = asyncio.run(
+        agentic_ask_loop(MagicMock(), "Q", backend=mock_backend, debug=False)
+    )
+    assert result.debug_trace is None
+
+
+def test_debug_trace_populated_when_debug_true(mock_backend, mock_run_search):
+    """G0-3: debug=True → debug_trace 가 list[dict]."""
+    from services.search.react_loop import agentic_ask_loop
+
+    mock_backend.generate_with_tools.side_effect = [
+        _msg_with_tool_call("q1"),
+        _msg_with_content("종합"),
+    ]
+    result = asyncio.run(
+        agentic_ask_loop(MagicMock(), "Q", backend=mock_backend, debug=True)
+    )
+    assert isinstance(result.debug_trace, list)
+    assert len(result.debug_trace) >= 1
+    # 첫 entry 는 tool_round
+    assert result.debug_trace[0].get("phase") == "tool_round"
+
+
+# ── BackendUnavailable propagation ─────────────────────────────────────────
+
+
+def test_backend_unavailable_propagates(mock_backend, mock_run_search):
+    """BackendUnavailable 은 그대로 raise — 호출자 (search.py) 가 503 매핑."""
+    from services.llm.backends import BackendUnavailable
+    from services.search.react_loop import agentic_ask_loop
+
+    mock_backend.generate_with_tools.side_effect = BackendUnavailable(
+        "qwen-macbook", "ConnectError"
+    )
+
+    with pytest.raises(BackendUnavailable):
+        asyncio.run(agentic_ask_loop(MagicMock(), "Q", backend=mock_backend))
+
+    # ★ run_search 가 한 번도 호출되지 않음 (search 시도 0)
+    assert mock_run_search.call_count == 0
+
+
+def test_backend_unavailable_in_final_call_propagates(mock_backend, mock_run_search):
+    """final call 에서 unavailable 발생도 그대로 raise."""
+    from services.llm.backends import BackendUnavailable
+    from services.search.react_loop import agentic_ask_loop
+
+    mock_backend.generate_with_tools.side_effect = [
+        _msg_with_tool_call("q1"),
+        _msg_with_tool_call("q2", tc_id="tc-2"),
+        BackendUnavailable("qwen-macbook", "ReadTimeout"),
+    ]
+    with pytest.raises(BackendUnavailable):
+        asyncio.run(agentic_ask_loop(MagicMock(), "Q", backend=mock_backend))