[BUG] Panic when handling Japanese UTF-8 strings - invalid byte boundary slice

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?

Claude Code crashes with a panic when processing Japanese text.

Error message:
thread '<unnamed>' (3848961) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 17 is not a char boundary; it is inside 'ル' (bytes 15..18) of コントロール（シンクロ）
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5, aborting
zsh: abort claude

Environment:

• OS: macOS (Darwin 25.2.0)
• Shell: zsh
• 2.0.76 (Claude Code)

Root cause:
The code appears to be slicing a UTF-8 string at a byte index that falls in the middle of a multi-byte character (Japanese ル occupies bytes 15-18, but slice attempted at byte 17).

Expected behavior:
Claude Code should correctly handle multi-byte UTF-8 characters and only slice at valid character boundaries.

What Should Happen?

Claude Code should correctly handle multi-byte UTF-8 characters (such as Japanese, Chinese, Korean, emoji, etc.) without crashing. When truncating or displaying strings, it should slice at valid character boundaries, not in the middle of multi-byte characters.

Error Messages/Logs

>  
────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────
  ? for shortcuts

thread '<unnamed>' (3848961) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 17 is not a char boundary; it is inside 'ル' (bytes 15..18) of `コントロール（シンクロ）`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5, aborting
zsh: abort      claude

Steps to Reproduce

1. Open Claude Code in a project containing Japanese text in source files
2. Ask Claude to edit a TypeScript/React file (e.g., AdminWindow.tsx)
3. While Claude is processing/thinking ("Noodling…" or "Clauding…" phase),

the application crashes with UTF-8 panic

Context: The project appears to contain Japanese game-related text
including コントロール（シンクロ） (Control/Synchro) in the codebase.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.0.76

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_