Panic on Chinese character boundary when processing UTF-8 strings

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?

Version: 2.0.76
OS: macOS Darwin 25.1.0

Error:
byte index 2 is not a char boundary; it is inside '户' (bytes 0..3)

What Should Happen?

The CLI crashes intermittently when processing conversations
containing Chinese characters.

Error Messages/Logs

Steps to Reproduce

1. Start Claude Code CLI: claude
2. Have a conversation that includes Chinese characters
3. The crash occurs intermittently during the conversation,

particularly when:

• The conversation becomes longer
• Switching between messages
• Status bar or UI elements are being updated

## Environment

• Claude Code version: 2.0.76
• OS: macOS Darwin 25.1.0
• Terminal: [你用的终端，如 iTerm2 / Terminal.app / Warp]
• Shell: zsh

## Frequency
The crash happens multiple times during extended conversations
containing Chinese text. Not every session, but frequently enough
to disrupt workflow.

## Additional Context
The error message specifically mentions '户' but this appears to be
whichever Chinese character happens to be at the truncation point.
The root cause seems to be byte-indexing instead of char-boundary
indexing when slicing UTF-8 strings.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

v2.0.76

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_