fix(backend): 多章链设显式 recursion_limit 防御硬化（CR-C2）

apps/api/tests/test_chain.py

@@ -572,6 +572,62 @@ async def test_run_chain_job_no_conflict_completes_done(_patched_runner: None) -
     assert [a["chapter_no"] for a in fakes["accepted"]] == [1, 2]
 
 
+async def test_run_chain_job_sets_recursion_limit_scaled_to_count(
+    _patched_runner: None, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """CR-C2：run_chain_job 给 graph.ainvoke 传显式 top-level recursion_limit（防御硬化）。
+
+    langgraph 默认上界已足够（无实时崩溃），但每链上界应显式 = count*NODES_PER_CHAPTER +
+    HEADROOM。断言 config 键值 + 仍正常跑完（行为不变）。用 PROXY 包裹真 graph 记录 config
+    （不 setattr 到可能冻结的 CompiledStateGraph 上）。
+    """
+    from ww_core.orchestrator.chain import build_chain_graph as real_build
+
+    captured: dict[str, Any] = {}
+
+    class _SpyGraph:
+        def __init__(self, inner: Any) -> None:
+            self._inner = inner
+
+        async def ainvoke(self, inp: Any, *, config: Any = None, **kw: Any) -> Any:
+            captured["config"] = config
+            return await self._inner.ainvoke(inp, config=config, **kw)
+
+    def _spy_build(*args: Any, **kwargs: Any) -> Any:
+        return _SpyGraph(real_build(*args, **kwargs))
+
+    monkeypatch.setattr("ww_api.services.chain_runner.build_chain_graph", _spy_build)
+
+    fakes = _chain_runner_fakes()
+    gateway = _FakeChainGateway(conflicts=[])
+    job_id = uuid.uuid4()
+    saver = MemorySaver()
+
+    await run_chain_job(
+        _session_factory,
+        job_id,
+        project_id=uuid.uuid4(),
+        user_id=USER,
+        chain_key="draft_volume",
+        start_chapter_no=1,
+        count=7,
+        chain_gateway_builder=_async_returning(gateway),
+        checkpointer_ctx=lambda: _memsaver_ctx(saver),
+        accept_op=fakes["accept_op"],
+        chapter_repo_factory=fakes["chapter_repo_factory"],
+        review_repo_factory=fakes["review_repo_factory"],
+    )
+
+    # (1) 显式上界 = count*NODES_PER_CHAPTER + HEADROOM = 7*4+10 = 38。
+    assert captured["config"]["recursion_limit"] == 7 * 4 + 10 == 38
+    # (2) recursion_limit 是 top-level 键（不在 configurable 下——langgraph 只认 top-level）。
+    assert "recursion_limit" not in captured["config"]["configurable"]
+    # (3) 仍正常跑完（行为不变）。
+    rec = _RecordingJobRepo.state[job_id]
+    assert rec["status"] == STATUS_DONE
+    assert rec["result"]["written"] == list(range(1, 8))
+
+
 async def test_run_chain_job_conflict_sets_awaiting_then_resume_done(
     _patched_runner: None,
 ) -> None:

apps/api/ww_api/services/chain_runner.py

@@ -50,6 +50,11 @@ log = structlog.get_logger(__name__)
 # 链 job 的 kind（jobs.kind 自由 Text 列，零迁移复用；§7）。
 JOB_KIND_CHAIN = "chain"
 
+# 每章在链图里的超级步数（write→review→decide→accept），用于算 recursion_limit 上界。
+NODES_PER_CHAPTER = 4  # 每章 4 个超级步：write→review→decide→accept
+# START 边 + interrupt/resume 重入的余量（防御性冗余，非精确计数）。
+RECURSION_LIMIT_HEADROOM = 10  # START 边 + interrupt/resume 重入余量
+
 
 # 「按 session 建 digest 网关」缝：accept 节点在自建短事务里需 light 档网关跑终稿提炼。
 DigestGatewayBuilder = Callable[[Any], Awaitable[Gateway]]
@@ -207,7 +212,14 @@ async def run_chain_job(
     try:
         async with checkpointer_ctx() as checkpointer:
             await _set_running(session_factory, job_id)
-            config: RunnableConfig = {"configurable": {"thread_id": str(job_id)}}
+            # 显式每链 recursion_limit 上界（防御硬化，CR-C2）：count 已被端点上限 MAX_CHAIN_COUNT
+            # 夹住，故 count*每章步数+余量 是安全且贴合的上界。**必须放 config 顶层**——放到
+            # `configurable` 下 langgraph 会静默忽略。单份 config 同时喂初始 run 与 resume 分支。
+            recursion_limit = count * NODES_PER_CHAPTER + RECURSION_LIMIT_HEADROOM
+            config: RunnableConfig = {
+                "configurable": {"thread_id": str(job_id)},
+                "recursion_limit": recursion_limit,
+            }
             graph = build_chain_graph(
                 chain_gateway_builder,
                 session_factory=session_factory,