Support structured responses from MCP Tool should make text content optional

[mrecouly]

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

While testing an MCP Tool which returns only structured content, I got this error:

Failed to call tool 'my_tool_returning_structured_content'.

Traceback (most recent call last):
  File "/my_project/.venv/Lib/site-packages/semantic_kernel/connectors/mcp.py", line 422, in call_tool
    return _mcp_call_tool_result_to_kernel_contents(await self.session.call_tool(tool_name, arguments=kwargs))
                                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/my_project/.venv/Lib/site-packages/mcp/client/session.py", line 293, in call_tool
    result = await self.send_request(
             ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/my_project/.venv/Lib/site-packages/mcp/shared/session.py", line 288, in send_request
    return result_type.model_validate(response_or_error.result)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/my_project/.venv/Lib/site-packages/pydantic/main.py", line 703, in model_validate
    return cls.__pydantic_validator__.validate_python(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.ValidationError: 1 validation error for CallToolResult
content
  Field required [type=missing, input_value={'isError': False, 'struc...', 'my_json_string_1']}}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/my_project/.venv/Lib/site-packages/semantic_kernel/kernel.py", line 454, in _inner_auto_function_invoke_handler
    result = await context.function.invoke(
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/my_project/.venv/Lib/site-packages/semantic_kernel/functions/kernel_function.py", line 259, in invoke
    raise e
  File "/my_project/.venv/Lib/site-packages/semantic_kernel/functions/kernel_function.py", line 251, in invoke
    await stack(function_context)
  File "/my_project/.venv/Lib/site-packages/semantic_kernel/functions/kernel_function_from_method.py", line 107, in _invoke_internal
    result = await result
             ^^^^^^^^^^^^
  File "/my_project/.venv/Lib/site-packages/semantic_kernel/connectors/mcp.py", line 426, in call_tool
    raise FunctionExecutionException(f"Failed to call tool '{tool_name}'.") from ex

Further testing unveiled that the error was due to the presence of only the structuredContent element in the result element of the JSON response:

{
  "jsonrpc" : "2.0",
  "id" : 3,
  "result" : {
    "isError" : false,
    "structuredContent" : {
      "my_json_array" : [ "my_json_string_1", "my_json_string_2", "my_json_string_3"]
    }
  }
}

Whereas the presence of the element content for text content is expected systematically as per definition of MCP type CallToolResult in the SDK.
Thus a valid response according to the code would be:

{
  "jsonrpc" : "2.0",
  "id" : 3,
  "result" : {
    "isError" : false,
    "content" : [
      {
        "type": "text",
        "text": "{\"my_json_array\" : [\"my_json_string_1\", \"my_json_string_2\", \"my_json_string_3\"]}"
      }
    ],
    "structuredContent" : {
      "my_json_array" : [ "my_json_string_1", "my_json_string_2", "my_json_string_3"]
    }
  }
}

However the MCP specification 2025-06-18 about structured content says: For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block..

Hence the content element should be made optional when the Tool definition comes with outputSchema, since a valid response according to the specification could contain only the structuredContent element ...

Example Code

Python & MCP Python SDK

Tested using Python v3.12 and MCP Python SDK v1.12.3

[jwhandy3]

Yes it would be awesome if the content block was optional. Responses get quite bloated if you turn on structured-output right now. But turn it off prevents list/tools from describing your output parameters. Also, you can't control structured output as a config right now it must be set on individual decorators which is pretty verbose. If you could turn off 'content' as a config parameter we may get best of all solutions.

[felixweinberger]

Hi @mrecouly thanks for reporting this - could you provide the code snippet you used to generate the errors you're seeing? Would help us a lot with debugging.