[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.

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

1. npx -y @anthropic-ai/claude-code@2.0.72 mcp serve
2. call tool Bash with { "command": "echo hi" } using the official Python MCP SDK (e.g. mcp==1.24.0)

...or run the following script:

repro_structured_content.py

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

1.0.119

Claude Code Version

2.0.72

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_