[BUG] Undocumented breaking change in MCP tool output display (sometime between 2.0.10-2.0.22): structured content now prioritized over TextContent

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?

Issue

In 2.0.22 and possibly earlier, but after 2.0.9, MCP tool results are displayed differently:

• Before (≤2.0.9): TextContent was displayed cleanly for human readability
• After (in 2.0.22 possibly earlier): Structured content is wrapped in JSON format, even when TextContent is available

Impact

• MCP servers designed with pretty-printed output (code, formatted data) now show JSON-wrapped results
• Significantly degrades readability and user experience
• Affects all servers using FastMCP 2.10+ (which returns both formats per MCP spec)

Example

Expected (2.0.9):

f:{[x]
    :x+1
    }

Actual (2.0.22):

{
  "result": "f:{[x]\n    :x+1\n    }"
}

MCP Spec Context

The MCP 2025-06-18 spec states:

"For backwards compatibility, a tool that returns structured content SHOULD also return the serialized JSON in a TextContent block."

This suggests TextContent should be the primary display format for human readability.

What Should Happen?

Questions

1. Was this display change intentional?
2. If yes, why wasn't it documented in release notes?
3. Should Claude Code prioritize TextContent when both formats are available?

Workaround

Using structured_output=False in FastMCP forces text-only output.

Error Messages/Logs

Steps to Reproduce

Use a plain Python MCP server without the additional decorator described and return string

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.0.22

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

WSL (Windows Subsystem for Linux)

Additional Information

_No response_