[BUG] Thinking does not work upon Custom slash command

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 executing custom slash commands, Claude's thinking blocks (interleaved thinking mode) don't appear to be generated, even though they work normally in regular conversation.

What Should Happen?

Claude should generate thinking blocks (when thinking_mode is set to interleaved or auto) during slash command execution, just as it does during normal conversation.

Error Messages/Logs

Steps to Reproduce

1. Configure a custom slash command in .claude/commands/
2. Execute the slash command
3. Observe Claude's response

Claude Model

None

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.1

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Xterm

Additional Information

This affects the quality of responses for complex custom commands, as the thinking process helps Claude work through multi-step problems more effectively.