diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8e124d9..3bb5a53 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,7 +5,7 @@ All notable changes to the AxonFlow Python SDK will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
-## [3.9.0] - 2026-03-05
+## [3.9.0] - 2026-03-06
 
 ### Added
 
@@ -15,6 +15,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
     - `operation`: Operation type forwarded to `mcp_check_input` (default: `"execute"`; use `"query"` for known read-only tool calls)
   - `MCPInterceptorOptions` and `WorkflowApprovalRequiredError` are now exported from `axonflow.adapters`
 
+### Changed
+
+- `mcp_check_input()` default `operation` changed from `"query"` to `"execute"` to better reflect the default MCP tool call pattern where side effects are unknown
+
+### Fixed
+
+- `mcp_tool_interceptor()` now uses JSON serialization (`json.dumps`) for `statement` and output `message` fields instead of Python `repr()`, ensuring the policy engine receives valid structured data
+
 ---
 
 ## [3.8.0] - 2026-03-03
diff --git a/axonflow/adapters/langgraph.py b/axonflow/adapters/langgraph.py
index 3522dd0..0f111d4 100644
--- a/axonflow/adapters/langgraph.py
+++ b/axonflow/adapters/langgraph.py
@@ -33,6 +33,7 @@
 from __future__ import annotations
 
 import asyncio
+import json
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, Any, Callable
 
@@ -549,7 +550,8 @@ def _default_connector_type(request: Any) -> str:
 
         async def _interceptor(request: Any, handler: Callable[..., Any]) -> Any:
             connector_type = resolve_connector_type(request)
-            statement = f"{connector_type}({request.args!r})"
+            args_str = json.dumps(request.args, default=str) if request.args else "{}"
+            statement = f"{connector_type}({args_str})"
 
             pre_check = await self.client.mcp_check_input(
                 connector_type=connector_type,
@@ -562,9 +564,13 @@ async def _interceptor(request: Any, handler: Callable[..., Any]) -> Any:
 
             result = await handler(request)
 
+            try:
+                result_str = json.dumps(result, default=str)
+            except (TypeError, ValueError):
+                result_str = str(result)
             output_check = await self.client.mcp_check_output(
                 connector_type=connector_type,
-                message=f"{{result: {result!r}}}",
+                message=result_str,
             )
             if not output_check.allowed:
                 raise PolicyViolationError(
diff --git a/tests/test_langgraph_adapter.py b/tests/test_langgraph_adapter.py
index 3e1929f..efd0b59 100644
--- a/tests/test_langgraph_adapter.py
+++ b/tests/test_langgraph_adapter.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import json
 from typing import Any
 from unittest.mock import AsyncMock, MagicMock
 
@@ -87,6 +88,9 @@ async def test_connector_type_derived_from_request(
         call_kwargs = client.mcp_check_input.call_args.kwargs
         assert call_kwargs["connector_type"] == "srv.tool"
         assert call_kwargs["parameters"] == request.args
+        # Statement uses JSON serialization, not Python repr
+        expected_args = json.dumps(request.args, default=str)
+        assert call_kwargs["statement"] == f"srv.tool({expected_args})"
 
     @pytest.mark.asyncio
     async def test_same_connector_type_sent_to_check_output(
@@ -100,6 +104,18 @@ async def test_same_connector_type_sent_to_check_output(
         call_kwargs = client.mcp_check_output.call_args.kwargs
         assert call_kwargs["connector_type"] == "srv.tool"
 
+    @pytest.mark.asyncio
+    async def test_output_message_uses_json_serialization(
+        self, adapter: AxonFlowLangGraphAdapter, client: AxonFlow
+    ) -> None:
+        result_data = {"rows": [{"id": 1, "name": "test"}]}
+        handler = AsyncMock(return_value=result_data)
+
+        await adapter.mcp_tool_interceptor()(_make_request(), handler)
+
+        call_kwargs = client.mcp_check_output.call_args.kwargs
+        assert call_kwargs["message"] == json.dumps(result_data, default=str)
+
     # --- operation ---
 
     @pytest.mark.asyncio