[BUG] Panic on UTF-8 boundary when processing Chinese text

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 crashed with a Rust panic while processing Chinese text during a normal session. The error indicates improper byte-level string slicing on multi-byte UTF-8 characters.

The crash occurred while working on a Chinese novel project. Chinese characters are 3 bytes each in UTF-8, so byte index 44 falls inside the character '铁' (bytes 42-45). The code is using byte indexing (e.g., &s[0..44]) instead of character-aware methods like .chars(), .char_indices(), or the unicode-segmentation crate. This will affect any user working with CJK text. Environment: Ubuntu 22.04 WSL2, Claude Code CLI.

What Should Happen?

Claude Code should handle Chinese/CJK text without crashing. When processing,
displaying, or truncating multi-byte UTF-8 strings, the application should:

1. Complete the session normally without panicking
2. Correctly handle string operations on Chinese text by respecting UTF-8

character boundaries

1. If text truncation is needed (for display, context limits, etc.), truncate

at valid character boundaries, not arbitrary byte positions

The session should continue working with Chinese text files the same way it
works with ASCII/Latin text.

Error Messages/Logs

thread '<unnamed>' (928) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
  byte index 44 is not a char boundary; it is inside '铁' (bytes 42..45) of `在很远的地方用砂纸打磨生锈的铁。声音从我们两侧山坡传来，不止一个方向。`
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
  fatal runtime error: failed to initiate panic, error 5, aborting
  Aborted (core dumped)

Steps to Reproduce

1. Open Claude Code in a project containing Chinese/CJK text files
2. Work with files containing Chinese prose (in my case, a Chinese novel in markdown)
3. The crash occurred during normal session operation - exact trigger unknown as it happened mid-session
4. Cannot reliably reproduce, but the crash is deterministic once the specific text is being processed internally

Note: The problematic string was: 在很远的地方用砂纸打磨生锈的铁。声音从我们两侧山坡传来，不止一个方向。

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

claude-opus-4-5-20250101

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

WSL (Windows Subsystem for Linux)

Additional Information

_No response_