fix: MCP Proxy call_backend_tool — LLM이 arguments를 JSON string으로 보내면 SDK validation 실패하는 버그 수정

근본 원인: inputSchema에서 arguments를 "type": "object"로만 선언 → MCP SDK의
jsonschema.validate()가 handler 도달 전에 string/null 거부.

수정:
- inputSchema: oneOf [object, string, null] 허용
- handler: string → json.loads() 파싱, None → {} 폴백
- 테스트 3케이스 추가 (string, null, dict)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: SonAIengine

graph_tool_call/mcp_proxy.py

@@ -559,8 +559,12 @@ async def list_tools() -> list[types.Tool]:
                             "description": "Exact tool name from search_tools results",
                         },
                         "arguments": {
-                            "type": "object",
-                            "description": "Arguments matching the tool's inputSchema",
+                            "oneOf": [
+                                {"type": "object"},
+                                {"type": "string"},
+                                {"type": "null"},
+                            ],
+                            "description": "Arguments for the tool (object or JSON string)",
                         },
                     },
                     "required": ["tool_name"],
@@ -629,7 +633,13 @@ async def call_tool(
         # --- Meta-tool: call_backend_tool (fallback) ---
         if name == "call_backend_tool":
             tool_name = arguments.get("tool_name", "")
-            tool_args = arguments.get("arguments", {})
+            tool_args = arguments.get("arguments") or {}
+            # LLM clients may serialize arguments as a JSON string
+            if isinstance(tool_args, str):
+                try:
+                    tool_args = json.loads(tool_args)
+                except (json.JSONDecodeError, TypeError):
+                    tool_args = {}
             result = await proxy.call_tool(tool_name, tool_args)
             return result.content
 

tests/test_mcp_proxy.py

@@ -224,6 +224,73 @@ class FakeTool:
     assert proxy.get_tool_schema("nonexistent") is None
 
 
+@pytest.mark.asyncio
+async def test_call_backend_tool_string_arguments():
+    """call_backend_tool should handle arguments serialized as JSON string."""
+    mcp_mod = pytest.importorskip("mcp", reason="mcp required")
+    types = mcp_mod.types
+
+    from graph_tool_call.mcp_proxy import create_proxy_server
+
+    proxy = MCPProxy([], top_k=5, passthrough_threshold=0)
+    proxy._build_tool_graph()
+    proxy._all_tools = {"my_tool": None}
+    proxy._tool_to_backend = {"my_tool": "backend1"}
+    proxy._gateway_mode = True
+
+    received_args = []
+
+    # Mock the backend connection
+    class FakeSession:
+        async def call_tool(self, name, arguments):
+            received_args.append(arguments)
+            return types.CallToolResult(content=[types.TextContent(type="text", text="ok")])
+
+    class FakeConn:
+        session = FakeSession()
+
+    proxy._connections = {"backend1": FakeConn()}
+
+    server = create_proxy_server(proxy)
+    handler = server.request_handlers[types.CallToolRequest]
+
+    # Case 1: arguments as JSON string (the bug this fix addresses)
+    request = types.CallToolRequest(
+        method="tools/call",
+        params=types.CallToolRequestParams(
+            name="call_backend_tool",
+            arguments={"tool_name": "my_tool", "arguments": '{"action": "check"}'},
+        ),
+    )
+    result = await handler(request)
+    assert not result.root.isError
+    assert received_args[-1] == {"action": "check"}
+
+    # Case 2: arguments as None
+    request2 = types.CallToolRequest(
+        method="tools/call",
+        params=types.CallToolRequestParams(
+            name="call_backend_tool",
+            arguments={"tool_name": "my_tool", "arguments": None},
+        ),
+    )
+    result2 = await handler(request2)
+    assert not result2.root.isError
+    assert received_args[-1] == {}
+
+    # Case 3: arguments as proper dict (should still work)
+    request3 = types.CallToolRequest(
+        method="tools/call",
+        params=types.CallToolRequestParams(
+            name="call_backend_tool",
+            arguments={"tool_name": "my_tool", "arguments": {"action": "check"}},
+        ),
+    )
+    result3 = await handler(request3)
+    assert not result3.root.isError
+    assert received_args[-1] == {"action": "check"}
+
+
 def test_create_gateway_server():
     """Gateway mode creates server with meta-tools."""
     pytest.importorskip("mcp", reason="mcp required")