[BUG] Cowork: "CLI output was not valid JSON" when MCP tool result contains rich-text characters (runtime, not startup)

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?

In Cowork mode, when an MCP tool (Desktop Commander start_process) returns content containing rich-text characters (smart quotes, em dashes, unicode, etc.), the tool call fails with "CLI output was not valid JSON." The conversation continues but the tool result is lost.

This is a runtime serialization failure, not a startup error — distinct from all existing reports of this error message (#23519, #18809, #25025, #25482), which involve Bedrock API failures, sandbox initialization, or pre-flight check corruption.

What Should Happen?

The tool result should be properly escaped/serialized regardless of content characters.

Error Messages/Logs

CLI output was not valid JSON. This may indicate an error during start up. Output: {"type": "user", "message": {"role": "user", "content": [{"tool_ use_id": "toolu_01HFWkZARPoAHs1K1EZ3KNJY", "type": "tool_result", "conten
t":[{"type":"text", "text": "Process started with PID 28918 (shell: /bi n/zs...

Steps to Reproduce

1. In Cowork on macOS, use Desktop Commander's start_process to run an AppleScript that reads an email body from Apple Mail:

osascript -e 'tell application "Mail" ... get content of message ...'

1. The email body contains typical characters: smart quotes (" "), em dashes, unicode, embedded URLs
2. The tool result is returned through the MCP → Claude Code CLI → Cowork pipeline
3. Error: "This task didn't load properly — CLI output was not valid JSON"

Claude Model

Opus

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.63 (Claude Code)

Platform

Other

Operating System

macOS

Terminal/Shell

Other

Additional Information

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Workaround

Writing the content to a temp file via AppleScript's do shell script, then reading the file separately with Desktop Commander's read_file, avoids the crash. The issue is specifically with content flowing through start_process stdout → CLI JSON serialization.

Environment

• macOS (Apple Silicon)
• Cowork mode in Claude Desktop
• Desktop Commander MCP v0.2.37
• Trigger: Apple Mail email bodies via osascript (but likely reproducible with any MCP tool returning rich-text content)

Why this matters

Any MCP tool that returns user-generated content (email, documents, web scraping, etc.) could trigger this. It's likely underreported because most Cowork users aren't piping raw email through MCP tools yet — but as MCP adoption grows, this will surface more.