fix(agent): normalize fenced and flat-args LLM outputs in parser

核心算法 #10（Agent Loop）：仅加固 _parse_response 解析路径——
剥除 ```json 围栏 + 收拢 action 平铺参数（deepseek 稳定输出变体，
案例 637-3/615-3 三连拒 0 步阵亡）。Thinking+JSON 协议、json_repair
兜底链、hook 时序与步数语义均未改动。

core/agent/loop.py

@@ -18,6 +18,7 @@ TRM4 → TRM5 有意变更（非简化）:
 from __future__ import annotations
 
 import json
+import re
 from typing import TYPE_CHECKING, Any
 
 import pluggy
@@ -31,6 +32,9 @@ if TYPE_CHECKING:
     from core.protocols import LLMProvider
     from core.types import LLMResponse
 
+# deepseek 等模型稳定输出变体：```json 围栏包裹 JSON 体
+_CODE_FENCE_RE = re.compile(r"^\s*```(?:json)?\s*\n?|\n?\s*```\s*$")
+
 
 async def _call_hook(hook: Any, **kwargs: Any) -> list[Any]:
     """调用 pluggy hook 并 await 异步返回值。
@@ -267,7 +271,8 @@ class AgentLoop:
     ) -> tuple[str, dict, dict, str, dict, str] | None:
         """从 LLMResponse 中提取结构化决策数据。
 
-        解析流程: content → repair_json → json.loads → 校验 action/tool/args。
+        解析流程: content → 剥除 ```json 围栏 → repair_json → json.loads
+        → 收拢 action 平铺参数 → 校验 action/tool/args。
 
         参数:
             response: LLMResponse 实例。
@@ -282,7 +287,7 @@ class AgentLoop:
         if not content.strip():
             return None
 
-        repaired = repair_json(content)
+        repaired = repair_json(_CODE_FENCE_RE.sub("", content).strip())
         try:
             data = json.loads(repaired)
         except (json.JSONDecodeError, ValueError):
@@ -291,7 +296,7 @@ class AgentLoop:
         if not isinstance(data, dict) or "action" not in data:
             return None
 
-        action = data["action"]
+        action = self._normalize_action(data["action"])
         if not isinstance(action, dict) or "tool" not in action or "args" not in action:
             return None
 
@@ -299,6 +304,25 @@ class AgentLoop:
         plan = data.get("plan", {})
         return thought, reflect, plan, content, action, response.call_id
 
+    @staticmethod
+    def _normalize_action(action: Any) -> Any:
+        """收拢 deepseek 变体的 action 平铺参数。
+
+        deepseek 等模型稳定输出变体: 工具参数平铺在 action 下（缺 args
+        嵌套），确定性收拢为标准 {"tool": ..., "args": {...}} 结构。
+        标准嵌套结构与非法结构均原样返回，由调用方校验。
+
+        参数:
+            action: 从 LLM 输出解析出的 action 字段（任意类型）。
+
+        返回:
+            归一化后的 action（仅平铺变体被改写，其余原样返回）。
+        """
+        if isinstance(action, dict) and "tool" in action and "args" not in action:
+            flat_args = {k: v for k, v in action.items() if k != "tool"}
+            return {"tool": action["tool"], "args": flat_args}
+        return action
+
     async def _execute_tool(
         self,
         dispatcher: ToolDispatcher,

tests/unit/test_agent_loop.py

@@ -239,3 +239,60 @@ class TestAgentLoop:
         assert "after_tool:0" in tracker.events
         assert "after_step:0" in tracker.events
         assert "on_finish:finished" in tracker.events
+
+
+# ── A1 解析容错测试（Spec-1）──────────────────────────────────
+
+# 生产真实样本结构：尾部围栏残留 + action.args 平铺（开头围栏场景由
+# test_leading_json_fence 单独覆盖）
+_REAL_FLAT_FENCED = """{
+  "plan": {
+    "goal": "从三个L1根节点开始建立全局认知",
+    "tool": "view_node",
+    "reason": "三个L1节点覆盖整个视频"
+  },
+  "action": {
+    "tool": "view_node",
+    "node_id": "J5Npf2xJpag_L1_000",
+    "question": "What is the overall topic of this video?"
+  }
+}
+```"""
+
+
+class TestParseNormalization:
+    """deepseek 输出变体（args 平铺 + ```json 围栏）归一化。"""
+
+    def _parse(self, content: str):
+        loop = AgentLoop(llm=AsyncMock(), max_steps=10)
+        return loop._parse_response(_make_response(content))
+
+    def test_flat_args_with_trailing_fence(self) -> None:
+        """生产样本：action 平铺 node_id/question + 尾部围栏。"""
+        parsed = self._parse(_REAL_FLAT_FENCED)
+        assert parsed is not None
+        action = parsed[4]
+        assert action["tool"] == "view_node"
+        assert action["args"] == {
+            "node_id": "J5Npf2xJpag_L1_000",
+            "question": "What is the overall topic of this video?",
+        }
+
+    def test_leading_json_fence(self) -> None:
+        content = '```json\n{"reflect": {}, "plan": {}, "action": {"tool": "submit_answer", "args": {"answer": "A"}}}\n```'
+        parsed = self._parse(content)
+        assert parsed is not None
+        assert parsed[4]["args"] == {"answer": "A"}
+
+    def test_nested_args_unchanged(self) -> None:
+        """标准嵌套结构不受归一化影响。"""
+        parsed = self._parse(_submit_json("B"))
+        assert parsed is not None
+        assert parsed[4] == {"tool": "submit_answer", "args": {"answer": "B"}}
+
+    def test_action_missing_tool_still_rejected(self) -> None:
+        content = json.dumps({"reflect": {}, "plan": {}, "action": {"node_id": "x"}})
+        assert self._parse(content) is None
+
+    def test_empty_content_still_rejected(self) -> None:
+        assert self._parse("") is None