diff --git a/app/agent/api/arguments.py b/app/agent/api/arguments.py
index b7e9599f7..bef7bc369 100644
--- a/app/agent/api/arguments.py
+++ b/app/agent/api/arguments.py
@@ -1,5 +1,6 @@
 """按已生成的 API 输入合同规范实际请求，使相同写入使用稳定参数身份。"""
 
+import json
 import re
 from copy import deepcopy
 from typing import Any
@@ -18,6 +19,24 @@ _MISSING = object()
 _CONTRACT_DESCRIPTION_MAX_CHARS = 180
 
 
+def coerce_string_body(value: Any) -> Any:
+    """把模型联合类型 schema 下常见的 JSON 字符串请求体还原为原生值。
+
+    部分模型在 body 声明为 anyOf（对象/数组/字面量）时会把整个请求体
+    编码成 JSON 字符串；此处仅在解析成功且结果为对象或数组时替换，
+    其余输入保持原值，交由 operation 合同继续校验并给出纠错回执。
+    """
+    if not isinstance(value, str):
+        return value
+    try:
+        parsed = json.loads(value)
+    except (TypeError, ValueError):
+        return value
+    if isinstance(parsed, (dict, list, type(None))):
+        return parsed
+    return value
+
+
 class ApiArgumentError(ValueError):
     """只携带字段位置和合同说明，不回显模型提交的参数值。"""
 
@@ -270,6 +289,9 @@ def canonical_api_arguments(arguments: dict[str, Any], schema: dict[str, Any]) -
         raise ValueError("API 操作没有规范参数合同")
     _validate_argument_locations(arguments, schema)
     values = deepcopy(arguments)
+    # 模型可能把联合类型的 body 编码为 JSON 字符串；先还原再按合同规范。
+    if "body" in values:
+        values["body"] = coerce_string_body(values["body"])
     if route.method == "GET" and isinstance(values.get("body"), dict):
         values["query"] = {**(values.get("query") or {}), **values.pop("body")}
     properties = branch.get("properties", {})
diff --git a/app/agent/tools/impl/api.py b/app/agent/tools/impl/api.py
index df952855f..04e1ba7b2 100644
--- a/app/agent/tools/impl/api.py
+++ b/app/agent/tools/impl/api.py
@@ -7,9 +7,13 @@ from pathlib import Path
 from typing import Any, Dict, Literal, Optional, Type, Union, cast
 
 from langchain_core.tools import ToolException
-from pydantic import BaseModel, ConfigDict, Field, PrivateAttr
+from pydantic import BaseModel, ConfigDict, Field, PrivateAttr, field_validator
 
-from app.agent.api.arguments import api_input_contract, canonical_api_arguments
+from app.agent.api.arguments import (
+    api_input_contract,
+    canonical_api_arguments,
+    coerce_string_body,
+)
 from app.agent.api.executor import ApiExecutionContext, ApiExecutionError, MoviePilotApiExecutor
 from app.agent.policy.api import resolve_api_operation
 from app.agent.policy.contracts import ExecutionOutcome, PrincipalRole
@@ -76,6 +80,12 @@ class MoviePilotApiInput(BaseModel):  # type: ignore[misc]
         ),
     )
 
+    @field_validator("body", mode="before")
+    @classmethod
+    def _restore_json_string_body(cls, value: Any) -> Any:
+        """模型在联合类型 schema 下可能把 body 编码为 JSON 字符串，入模前还原。"""
+        return coerce_string_body(value)
+
 
 class MoviePilotApiTool(MoviePilotTool):
     """
diff --git a/tests/test_agent_api_gateway.py b/tests/test_agent_api_gateway.py
index 7993dd91a..cea600e17 100644
--- a/tests/test_agent_api_gateway.py
+++ b/tests/test_agent_api_gateway.py
@@ -198,8 +198,12 @@ def test_mcp_tools_list_preserves_all_moviepilot_api_operation_branches() -> Non
     assert operation_ids == set(API_OPERATION_ROUTES)
 
 
-def test_local_agent_tool_rejects_json_encoded_object_bodies() -> None:
-    """本地 Agent 工具 schema 不得把 JSON 对象字符串化作为合法请求体。"""
+def test_local_agent_tool_restores_json_encoded_object_bodies() -> None:
+    """本地 Agent 工具 schema 不得把 JSON 对象字符串化作为合法请求体。
+
+    部分模型在 body 声明为 anyOf 联合类型时会把整个请求体编码成 JSON 字符串；
+    规范化阶段将其还原为原生对象，非 JSON 的非法字符串仍按合同报错。
+    """
     tool = MoviePilotApiTool(session_id="session", user_id="api_user")
     schema = tool.tool_call_schema
     assert not isinstance(schema, dict)
@@ -223,11 +227,22 @@ def test_local_agent_tool_rejects_json_encoded_object_bodies() -> None:
     canonical = tool.canonical_arguments({"operation_id": "download.add", "body": body})
     assert canonical["body"]["torrent_in"]["enclosure"] == body["torrent_in"]["enclosure"]
 
+    # 模型把 body 编码为 JSON 字符串时，规范化阶段还原为原生对象。
+    restored = tool.canonical_arguments(
+        {
+            "operation_id": "download.add",
+            "body": json.dumps(body, ensure_ascii=False),
+        }
+    )
+    assert isinstance(restored["body"], dict)
+    assert restored["body"]["torrent_in"]["enclosure"] == body["torrent_in"]["enclosure"]
+
+    # 无法还原为对象/数组的非法字符串仍按合同报错。
     with pytest.raises(ValueError):
         tool.canonical_arguments(
             {
                 "operation_id": "download.add",
-                "body": json.dumps(body, ensure_ascii=False),
+                "body": "not-a-json-object",
             }
         )