[BUG] mcp serve: tools advertise outputSchema but return no structuredContent

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

When running as an MCP server, some tools advertise a outputSchema but return structuredContent: null for successful tools/call responses, for example Bash. This causes every session.call_tool with those tools to error.

This has been reported in #11354 but ignored for over 3 months and the issue is now closed.

What Should Happen?

The python MCP SDK enforces that tools with outputSchema must return structuredContent (see https://github.com/modelcontextprotocol/python-sdk/blob/v1.24.0/src/mcp/client/session.py#L416)

Error Messages/Logs

Tool Bash has an output schema but did not return structured content

Steps to Reproduce

Run:

import asyncio

from mcp import ClientSession, StdioServerParameters
from mcp.client.stdio import stdio_client


async def main() -> None:
    params = StdioServerParameters(
        command="npx",
        args=["-y", "@anthropic-ai/claude-code@2.1.38", "mcp", "serve"],
    )

    async with (
        stdio_client(params) as (read_stream, write_stream),
        ClientSession(read_stream, write_stream) as session,
    ):
        await session.initialize()

        try:
            result = await session.call_tool("Bash", {"command": "echo hi"})
            print(result)
        except Exception as e:
            print("Exception: ", e)


if __name__ == "__main__":
    asyncio.run(main())

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

1.0.119

Claude Code Version

2.1.37

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_