diff --git a/app/api/search.py b/app/api/search.py
index 1877291..4d0a37a 100644
--- a/app/api/search.py
+++ b/app/api/search.py
@@ -932,3 +932,85 @@ async def ask(
         backend_used=backend_used_val,
         debug=debug_obj,
     )
+
+
+# ─── PR-DocSrv-Ask-ToolCalling-ReAct-1 ────────────────────────────────────
+# /api/search/ask/react — Qwen native tool calling 로 ReAct loop.
+# 본 endpoint 는 qwen-macbook only (endpoint 자체가 implicit opt-in).
+# MacBook unavailable 시 503 + error_reason=macbook_unavailable. Gemma 자동 fallback X.
+# G0-2 counter semantics: max_tool_rounds=2, max LLM calls=3, search exec ≤ 2.
+# G0-3 trace exposure: default response 의 debug_trace=None, debug=True 시만 채움.
+
+
+class AskReactRequest(BaseModel):
+    query: str
+    debug: bool = False
+
+
+class AskReactResponse(BaseModel):
+    final_answer: str
+    iterations: int
+    partial: bool
+    sources: list[dict]
+    debug_trace: list[dict] | None = None
+
+
+@router.post("/ask/react", response_model=AskReactResponse)
+async def ask_react(
+    payload: AskReactRequest,
+    user: Annotated[User, Depends(get_current_user)],
+    session: Annotated[AsyncSession, Depends(get_session)],
+):
+    """ReAct loop endpoint (qwen-macbook only, no fallback).
+
+    호출자가 명시 opt-in 한 endpoint. MacBook 가 sleep / unreachable / 5xx 시
+    HTTP 503 + body `{error_reason: "macbook_unavailable", backend: "qwen-macbook"}`
+    를 반환한다. Gemma Mac mini 로 자동 fallback 하지 않는다 (정정 4 의 연장).
+
+    request body:
+      - query: str (사용자 원본 질의)
+      - debug: bool (default false; true 시 응답 `debug_trace` 채움)
+
+    response body (성공 200):
+      - final_answer: str (Qwen 종합문, partial 일 수 있음)
+      - iterations: int (실제 진행된 tool round 수)
+      - partial: bool (max_tool_rounds 도달 후 LLM content 비었을 때 true)
+      - sources: list[dict] (검색에서 모인 evidence 메타, id-기준 dedup)
+      - debug_trace: list[dict] | null (debug=true 시 round 별 trace)
+    """
+    # 지연 import — 순환 의존성 회피 (react_loop 가 api.search.SearchResult 사용 안 함)
+    from services.llm.backends import BackendUnavailable, QwenMacBookBackend, get_backend
+    from services.search.react_loop import agentic_ask_loop
+
+    backend_inst = get_backend("qwen-macbook")
+    assert isinstance(backend_inst, QwenMacBookBackend)  # mypy / runtime guard
+
+    try:
+        result = await agentic_ask_loop(
+            session,
+            payload.query,
+            backend=backend_inst,
+            debug=payload.debug,
+        )
+    except BackendUnavailable as exc:
+        logger.warning(
+            "ask_react backend unavailable backend=%s reason=%s",
+            exc.backend_name, exc.reason,
+        )
+        return JSONResponse(
+            status_code=503,
+            content={
+                "error_reason": "macbook_unavailable",
+                "backend_requested": "qwen-macbook",
+                "backend_used": None,
+                "detail": exc.reason,
+            },
+        )
+
+    return AskReactResponse(
+        final_answer=result.final_answer,
+        iterations=result.iterations,
+        partial=result.partial,
+        sources=result.sources,
+        debug_trace=result.debug_trace,
+    )
diff --git a/app/core/config.py b/app/core/config.py
index 2d9b0af..d2f765c 100644
--- a/app/core/config.py
+++ b/app/core/config.py
@@ -50,8 +50,22 @@ class SearchAskBackendConfig(BaseModel):
     timeout_read_s: int = 30
 
 
+class SearchAskReactConfig(BaseModel):
+    """PR-DocSrv-Ask-ToolCalling-ReAct-1: /api/search/ask/react ReAct loop.
+
+    qwen-macbook only (endpoint 자체가 implicit opt-in). G0-2 counter semantics:
+    max_tool_rounds=2 → LLM 호출 최대 3회 (tool round 2 + final 1), search 실행 최대 2회.
+    """
+
+    enabled: bool = True
+    max_tool_rounds: int = 2
+    search_tool_limit: int = 5
+    search_tool_mode: str = "hybrid"
+
+
 class SearchAskConfig(BaseModel):
     backend: SearchAskBackendConfig = SearchAskBackendConfig()
+    react: SearchAskReactConfig = SearchAskReactConfig()
 
 
 class SearchConfig(BaseModel):
@@ -199,9 +213,14 @@ def load_settings() -> Settings:
 
     search_cfg = SearchConfig()
     if config_path.exists() and raw and "search" in raw:
-        sb = (raw.get("search") or {}).get("ask", {}).get("backend", {}) or {}
+        ask_raw = (raw.get("search") or {}).get("ask", {}) or {}
+        sb = ask_raw.get("backend", {}) or {}
+        sr = ask_raw.get("react", {}) or {}
         search_cfg = SearchConfig(
-            ask=SearchAskConfig(backend=SearchAskBackendConfig(**sb))
+            ask=SearchAskConfig(
+                backend=SearchAskBackendConfig(**sb),
+                react=SearchAskReactConfig(**sr),
+            )
         )
 
     taxonomy = raw.get("taxonomy", {}) if config_path.exists() and raw else {}
diff --git a/app/prompts/react_ask.txt b/app/prompts/react_ask.txt
new file mode 100644
index 0000000..0197621
--- /dev/null
+++ b/app/prompts/react_ask.txt
@@ -0,0 +1,10 @@
+당신은 사내 문서 자료를 기반으로 정확한 한국어 답변을 제공하는 비서입니다.
+
+작업 원칙:
+1. 사용자 질문에 답하려면 사내 문서를 검색해야 한다면, `search` 도구를 호출하세요.
+2. 첫 검색 결과가 부족하다고 판단되면 (관련도 낮음 또는 핵심 정보 누락), 다른 키워드로 한 번 더 검색하세요.
+3. 검색 결과가 충분하면 그 evidence 만으로 한국어 최종 답을 작성하세요.
+4. 근거 없는 추측은 하지 마세요. 자료에서 확인되지 않으면 "확인된 자료가 없습니다" 라고 답하세요.
+5. 검색 도구는 최대 2회까지만 호출 가능합니다. 그 이후에는 모은 정보로 답을 마무리해야 합니다.
+
+답변 시 출처를 본문에 따로 표시할 필요는 없습니다. sources 필드로 별도 노출됩니다.
diff --git a/app/services/llm/backends.py b/app/services/llm/backends.py
index 7af868d..004a621 100644
--- a/app/services/llm/backends.py
+++ b/app/services/llm/backends.py
@@ -149,6 +149,74 @@ class QwenMacBookBackend(BackendBase):
                     ) from exc
                 raise
 
+    async def generate_with_tools(
+        self,
+        messages: list[dict],
+        tools: list[dict],
+        *,
+        tool_choice: str = "auto",
+        timeout_read_s: int,
+    ) -> dict:
+        """OpenAI 호환 chat completion with tool calling (ReAct loop 용).
+
+        Returns: `choices[0].message` dict 그대로 — `content` (Optional[str]) +
+        `tool_calls` (Optional[list]) 둘 다 포함.
+
+        Response shape = G0-1 fixture `tests/fixtures/qwen_tool_call_response.json`
+        기준 (mlx-vlm OpenAI 표준 호환). tool_calls[].function.arguments 는
+        **JSON string** 으로 옴 — 호출자가 json.loads 필요.
+
+        - `tool_choice="auto"`: 모델이 tool 호출 여부 결정
+        - `tool_choice="none"`: tool 호출 금지, content 만 반환 (final round)
+        - `tools=[]` + `tool_choice="none"`: tool 정의 없이 final answer 강제
+        """
+        gate = self._get_gate()
+        timeout = httpx.Timeout(
+            connect=float(self.timeout_connect_s),
+            read=float(timeout_read_s),
+            write=10.0,
+            pool=5.0,
+        )
+        url = f"{self.base_url}/v1/chat/completions"
+        payload: dict = {
+            "model": self.model,
+            "messages": messages,
+            "max_tokens": 4096,
+        }
+        if tools:
+            payload["tools"] = tools
+        if tool_choice in ("auto", "none"):
+            payload["tool_choice"] = tool_choice
+        async with gate:
+            try:
+                async with httpx.AsyncClient(timeout=timeout) as client:
+                    resp = await client.post(url, json=payload)
+                    resp.raise_for_status()
+                    data = resp.json()
+                    return data["choices"][0]["message"]
+            except (
+                httpx.ConnectError,
+                httpx.ConnectTimeout,
+                httpx.ReadTimeout,
+                httpx.PoolTimeout,
+                httpx.WriteTimeout,
+                httpx.RemoteProtocolError,
+            ) as exc:
+                logger.warning(
+                    "qwen-macbook(tools) unavailable url=%s exc=%s",
+                    url, type(exc).__name__,
+                )
+                raise BackendUnavailable(self.name, type(exc).__name__) from exc
+            except httpx.HTTPStatusError as exc:
+                if 500 <= exc.response.status_code < 600:
+                    logger.warning(
+                        "qwen-macbook(tools) 5xx status=%d", exc.response.status_code,
+                    )
+                    raise BackendUnavailable(
+                        self.name, f"http_{exc.response.status_code}"
+                    ) from exc
+                raise
+
 
 # ── dispatcher ─────────────────────────────────────────────────────────────
 
diff --git a/app/services/search/react_loop.py b/app/services/search/react_loop.py
new file mode 100644
index 0000000..7405574
--- /dev/null
+++ b/app/services/search/react_loop.py
@@ -0,0 +1,275 @@
+"""PR-DocSrv-Ask-ToolCalling-ReAct-1: Qwen native tool calling 로 ReAct loop.
+
+G0-2 counter semantics ([[b-velvety-hare]] § Pre-Implementation Gate):
+- max_tool_rounds = 2  (tool 호출 round cap)
+- max_llm_calls   = 3  (= max_tool_rounds + 1, final round 포함)
+- search_exec_max = max_tool_rounds (round 당 search 1회 이상 가능 — 모델 결정)
+- 마지막 LLM call 은 tool_choice="none" + system instruction 으로 final answer 강제
+
+G0-1 fixture (tests/fixtures/qwen_tool_call_response.json) 기준 parsing —
+mlx-vlm 의 OpenAI 표준 호환, `tool_calls[].function.arguments` 는 JSON string.
+
+G0-3 trace exposure:
+- `debug=True` 시만 `debug_trace` 채움. server log 에는 항상 round 기록.
+- default response = `debug_trace=None`.
+
+Invariant (정정 4 의 자연 연장):
+- backend = `QwenMacBookBackend` only. Gemma 자동 fallback 금지.
+- `BackendUnavailable` 은 호출자 (search.py) 가 503 + `error_reason=macbook_unavailable`
+  로 매핑.
+"""
+
+from __future__ import annotations
+
+import json
+from dataclasses import dataclass, field
+from pathlib import Path
+from typing import Any
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from core.config import settings
+from core.utils import setup_logger
+from services.llm.backends import QwenMacBookBackend
+from services.search.search_pipeline import run_search
+
+logger = setup_logger("react_loop")
+
+_PROMPT_PATH = Path(__file__).resolve().parents[2] / "prompts" / "react_ask.txt"
+_FINAL_INSTRUCTION = (
+    "이제는 검색 도구를 더 이상 호출하지 마시고, 위 evidence 만으로 "
+    "한국어 최종 답을 작성하세요."
+)
+_TOOLS = [
+    {
+        "type": "function",
+        "function": {
+            "name": "search",
+            "description": "사내 문서 청크 검색. q 만 넘기면 hybrid 모드로 limit 건 반환.",
+            "parameters": {
+                "type": "object",
+                "properties": {
+                    "q": {
+                        "type": "string",
+                        "description": "검색 질의문 (한국어 가능)",
+                    },
+                },
+                "required": ["q"],
+            },
+        },
+    }
+]
+
+
+@dataclass
+class ReactResult:
+    final_answer: str
+    iterations: int
+    partial: bool
+    sources: list[dict[str, Any]] = field(default_factory=list)
+    debug_trace: list[dict[str, Any]] | None = None
+
+
+def _load_system_prompt() -> str:
+    try:
+        return _PROMPT_PATH.read_text(encoding="utf-8")
+    except OSError:
+        logger.warning("react_ask.txt missing path=%s — fallback prompt", _PROMPT_PATH)
+        return (
+            "당신은 사내 문서 자료를 기반으로 정확한 한국어 답변을 제공하는 비서입니다. "
+            "필요하면 `search` 도구를 호출해 evidence 를 모으고, 충분하다 판단되면 "
+            "최종 답을 작성하세요. 근거 없는 추측은 피하세요."
+        )
+
+
+def _result_payload(pr, *, limit: int) -> tuple[str, list[dict[str, Any]]]:
+    """run_search() PipelineResult → (LLM-side JSON string, sources-side dict list).
+
+    LLM-side: snippet 600자 컷, score / title / doc_id 포함.
+    Sources-side: snippet 제외, id / doc_id / title / score 만.
+    """
+    items_llm: list[dict[str, Any]] = []
+    items_src: list[dict[str, Any]] = []
+    for r in (pr.results or [])[:limit]:
+        rid = getattr(r, "id", None) or getattr(r, "chunk_id", None)
+        doc_id = getattr(r, "doc_id", None)
+        title = getattr(r, "title", "") or ""
+        score = getattr(r, "score", None)
+        snippet = (getattr(r, "snippet", "") or getattr(r, "text", "") or "")[:600]
+        items_llm.append(
+            {
+                "id": rid,
+                "doc_id": doc_id,
+                "title": title,
+                "snippet": snippet,
+                "score": score,
+            }
+        )
+        items_src.append(
+            {"id": rid, "doc_id": doc_id, "title": title, "score": score}
+        )
+    return (
+        json.dumps({"results": items_llm, "count": len(items_llm)}, ensure_ascii=False),
+        items_src,
+    )
+
+
+async def agentic_ask_loop(
+    session: AsyncSession,
+    query: str,
+    *,
+    backend: QwenMacBookBackend,
+    max_tool_rounds: int | None = None,
+    debug: bool = False,
+) -> ReactResult:
+    """ReAct loop entry point.
+
+    Args:
+        session: AsyncSession (caller-managed)
+        query: 사용자 원본 질의
+        backend: QwenMacBookBackend instance (qwen-macbook only — Gemma 미지원)
+        max_tool_rounds: None 시 config.search.ask.react.max_tool_rounds
+        debug: True 시 `debug_trace` 채움
+    """
+    cfg = settings.search.ask.react
+    if max_tool_rounds is None:
+        max_tool_rounds = cfg.max_tool_rounds
+    timeout_read_s = settings.search.ask.backend.timeout_read_s
+    limit = cfg.search_tool_limit
+    mode = cfg.search_tool_mode
+
+    messages: list[dict] = [
+        {"role": "system", "content": _load_system_prompt()},
+        {"role": "user", "content": query},
+    ]
+    sources: list[dict[str, Any]] = []
+    seen_ids: set[Any] = set()
+    trace: list[dict[str, Any]] = []
+
+    # Tool rounds — 최대 max_tool_rounds 회 (LLM call #1 .. #max_tool_rounds)
+    for round_idx in range(max_tool_rounds):
+        msg = await backend.generate_with_tools(
+            messages,
+            _TOOLS,
+            tool_choice="auto",
+            timeout_read_s=timeout_read_s,
+        )
+        tool_calls = msg.get("tool_calls") or []
+        trace.append(
+            {
+                "phase": "tool_round",
+                "round": round_idx,
+                "tool_call_count": len(tool_calls),
+                "content_present": bool(msg.get("content")),
+            }
+        )
+        logger.info(
+            "react_loop round=%d tool_calls=%d content=%s",
+            round_idx,
+            len(tool_calls),
+            "yes" if msg.get("content") else "no",
+        )
+
+        if not tool_calls:
+            # LLM 이 tool 호출 안 함 → 종합문 직접 반환 (early exit)
+            content = msg.get("content") or ""
+            return ReactResult(
+                final_answer=content,
+                iterations=round_idx + 1,
+                partial=not bool(content),
+                sources=sources,
+                debug_trace=trace if debug else None,
+            )
+
+        # assistant message (tool_calls 포함) 추가
+        messages.append(
+            {
+                "role": "assistant",
+                "content": msg.get("content"),
+                "tool_calls": tool_calls,
+            }
+        )
+
+        # 각 tool call 실행
+        for tc in tool_calls:
+            fn = tc.get("function") or {}
+            tc_id = tc.get("id") or ""
+            fn_name = fn.get("name")
+            if fn_name != "search":
+                messages.append(
+                    {
+                        "role": "tool",
+                        "tool_call_id": tc_id,
+                        "content": json.dumps(
+                            {"error": f"unknown tool {fn_name!r}"},
+                            ensure_ascii=False,
+                        ),
+                    }
+                )
+                trace.append({"phase": "tool_unknown", "name": fn_name})
+                continue
+            try:
+                args = json.loads(fn.get("arguments") or "{}")
+            except json.JSONDecodeError:
+                args = {}
+            q_arg = (args.get("q") or "").strip() or query
+            pr = await run_search(
+                session,
+                q_arg,
+                mode=mode,
+                limit=limit,
+                rerank=True,
+                analyze=False,
+            )
+            tool_content, round_sources = _result_payload(pr, limit=limit)
+            for s in round_sources:
+                sid = s.get("id")
+                if sid is not None and sid in seen_ids:
+                    continue
+                if sid is not None:
+                    seen_ids.add(sid)
+                sources.append(s)
+            messages.append(
+                {
+                    "role": "tool",
+                    "tool_call_id": tc_id,
+                    "content": tool_content,
+                }
+            )
+            trace.append(
+                {
+                    "phase": "search",
+                    "q": q_arg,
+                    "result_count": len(pr.results or []),
+                }
+            )
+
+    # Final round — LLM call #(max_tool_rounds + 1). tool_choice="none" 강제
+    messages.append({"role": "system", "content": _FINAL_INSTRUCTION})
+    final_msg = await backend.generate_with_tools(
+        messages,
+        tools=[],
+        tool_choice="none",
+        timeout_read_s=timeout_read_s,
+    )
+    final_content = final_msg.get("content") or ""
+    trace.append(
+        {
+            "phase": "final",
+            "content_present": bool(final_content),
+            "tool_calls_ignored": len(final_msg.get("tool_calls") or []),
+        }
+    )
+    logger.info(
+        "react_loop final content=%s tool_calls_ignored=%d",
+        "yes" if final_content else "no",
+        len(final_msg.get("tool_calls") or []),
+    )
+
+    return ReactResult(
+        final_answer=final_content,
+        iterations=max_tool_rounds,
+        partial=not bool(final_content),
+        sources=sources,
+        debug_trace=trace if debug else None,
+    )
diff --git a/config.yaml b/config.yaml
index f970bf9..690dd88 100644
--- a/config.yaml
+++ b/config.yaml
@@ -82,6 +82,12 @@ search:
       macbook_model: "mlx-community/Qwen3.6-27B-8bit"
       timeout_connect_s: 1   # MacBook sleep/wake 빠른 감지 (자동 fallback 부재 → 빠른 503)
       timeout_read_s: 30     # synthesis_service.LLM_TIMEOUT_MS=30000 와 align
+    # PR-DocSrv-Ask-ToolCalling-ReAct-1: /api/search/ask/react ReAct loop (qwen-macbook only)
+    react:
+      enabled: true
+      max_tool_rounds: 2     # G0-2: LLM 호출 최대 3회 (tool round 2 + final 1), search 실행 최대 2회
+      search_tool_limit: 5
+      search_tool_mode: "hybrid"
 
 nas:
   mount_path: "/documents"
diff --git a/tests/api/test_search_ask_react_endpoint.py b/tests/api/test_search_ask_react_endpoint.py
new file mode 100644
index 0000000..380d70f
--- /dev/null
+++ b/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 == ""
diff --git a/tests/fixtures/qwen_tool_call_response.json b/tests/fixtures/qwen_tool_call_response.json
new file mode 100644
index 0000000..c6069cc
--- /dev/null
+++ b/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}}
\ No newline at end of file
diff --git a/tests/services/test_react_loop.py b/tests/services/test_react_loop.py
new file mode 100644
index 0000000..2f31e7f
--- /dev/null
+++ b/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))