fix: redact MCP invalid JSON errors when tool logging is disabled

src/agents/mcp/util.py

@@ -376,16 +376,21 @@ async def invoke_mcp_tool(
         meta: dict[str, Any] | None = None,
     ) -> ToolOutput:
         """Invoke an MCP tool and return the result as ToolOutput."""
+        json_decode_error: Exception | None = None
         try:
             json_data: dict[str, Any] = json.loads(input_json) if input_json else {}
         except Exception as e:
+            json_decode_error = e
+
+        if json_decode_error is not None:
+            error_message = f"Invalid JSON input for tool {tool.name}"
             if _debug.DONT_LOG_TOOL_DATA:
-                logger.debug(f"Invalid JSON input for tool {tool.name}")
+                logger.debug(error_message)
+                raise ModelBehaviorError(error_message)
             else:
-                logger.debug(f"Invalid JSON input for tool {tool.name}: {input_json}")
-            raise ModelBehaviorError(
-                f"Invalid JSON input for tool {tool.name}: {input_json}"
-            ) from e
+                error_message = f"{error_message}: {input_json}"
+                logger.debug(error_message)
+            raise ModelBehaviorError(error_message) from json_decode_error
 
         if _debug.DONT_LOG_TOOL_DATA:
             logger.debug(f"Invoking MCP tool {tool.name}")

tests/mcp/test_mcp_util.py

@@ -9,6 +9,7 @@
 from mcp.types import CallToolResult, ImageContent, TextContent, Tool as MCPTool
 from pydantic import BaseModel, TypeAdapter
 
+import agents._debug as _debug
 from agents import Agent, FunctionTool, RunContextWrapper, default_tool_error_function
 from agents.exceptions import AgentsException, MCPToolCancellationError, ModelBehaviorError
 from agents.mcp import MCPServer, MCPUtil
@@ -222,6 +223,54 @@ async def test_mcp_invoke_bad_json_errors(caplog: pytest.LogCaptureFixture):
     assert "Invalid JSON input for tool test_tool_1" in caplog.text
 
 
+@pytest.mark.asyncio
+async def test_mcp_invoke_bad_json_redacts_payload_when_dont_log_tool_data(
+    monkeypatch: pytest.MonkeyPatch, caplog: pytest.LogCaptureFixture
+):
+    caplog.set_level(logging.DEBUG)
+    monkeypatch.setattr(_debug, "DONT_LOG_TOOL_DATA", True)
+
+    server = FakeMCPServer()
+    server.add_tool("test_tool_1", {})
+
+    ctx = RunContextWrapper(context=None)
+    tool = MCPTool(name="test_tool_1", inputSchema={})
+    bad_json = '{"secret":"SECRET_TOKEN_123"'
+
+    with pytest.raises(ModelBehaviorError) as exc_info:
+        await MCPUtil.invoke_mcp_tool(server, tool, ctx, bad_json)
+
+    assert str(exc_info.value) == "Invalid JSON input for tool test_tool_1"
+    assert exc_info.value.__cause__ is None
+    assert exc_info.value.__context__ is None
+    assert "SECRET_TOKEN_123" not in str(exc_info.value)
+    assert "SECRET_TOKEN_123" not in caplog.text
+
+
+@pytest.mark.asyncio
+async def test_mcp_invoke_bad_json_includes_payload_when_tool_logging_enabled(
+    monkeypatch: pytest.MonkeyPatch, caplog: pytest.LogCaptureFixture
+):
+    caplog.set_level(logging.DEBUG)
+    monkeypatch.setattr(_debug, "DONT_LOG_TOOL_DATA", False)
+
+    server = FakeMCPServer()
+    server.add_tool("test_tool_1", {})
+
+    ctx = RunContextWrapper(context=None)
+    tool = MCPTool(name="test_tool_1", inputSchema={})
+    bad_json = '{"secret":"SECRET_TOKEN_123"'
+
+    with pytest.raises(ModelBehaviorError) as exc_info:
+        await MCPUtil.invoke_mcp_tool(server, tool, ctx, bad_json)
+
+    assert str(exc_info.value) == f"Invalid JSON input for tool test_tool_1: {bad_json}"
+    assert isinstance(exc_info.value.__cause__, json.JSONDecodeError)
+    assert exc_info.value.__cause__.doc == bad_json
+    assert "SECRET_TOKEN_123" in str(exc_info.value)
+    assert "SECRET_TOKEN_123" in caplog.text
+
+
 class CrashingFakeMCPServer(FakeMCPServer):
     async def call_tool(
         self,