[BUG] Bash & other Tools in Claude as MCP server: Validation Error: Tool has an output schema but did not return structured content

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 I run Claude as an MCP server and call the Bash tool, I get an error that states that the Tool has an output schema but did not return structured content.

The MCP protocol states that "Servers MUST provide structured results that conform to this schema."

See https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content

You can replicate this trivially using

npx @modelcontextprotocol/inspector claude mcp serve

I would expect that this also occurs with other tools than the Bash tool. At least this also occurs for Glob as well.

What Should Happen?

Claude running as MCP server MUST provide structured results that conform to this schema. according to the MCP spec

Error Messages/Logs

Steps to Reproduce

1. Run 'npx @modelcontextprotocol/inspector claude mcp serve'
2. Select tools
3. Select Bash
4. Fill in 'ls -la' in the command window
5. Press Run Tool
6. Error shows up in Red

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.36

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_