[BUG] Panic when editing files containing Korean (multi-byte UTF-8) text - byte index not a char boundary

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 Rust panic when attempting to edit a file containing Korean text. The error indicates an invalid byte index that falls in the middle of a multi-byte UTF-8 character.

The application aborts completely, requiring a restart.

What Should Happen?

Claude Code should properly handle multi-byte UTF-8 characters (Korean, Chinese, Japanese, etc.) when performing edit operations, without crashing.

Error Messages/Logs

thread '<unnamed>' (222254) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
  byte index 7 is not a char boundary; it is inside '말' (bytes 5..8) of `\n주말 근무는 8시간까지 1.5배, 초과분은 2배로 인정됩니다.',`
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
  fatal runtime error: failed to initiate panic, error 5, aborting
  [1]    21988 abort      claude -c

Steps to Reproduce

1. Open a Flutter/Dart project with a file containing Korean text strings
2. The file contains text like:

```dart
'\n주말 근무는 8시간까지 1.5배, 초과분은 2배로 인정됩니다.'

1. Request Claude to edit/update this file (e.g., "add numbered list to the Korean text")
2. When Claude attempts to perform the Edit operation, the application panics and aborts

The issue appears to occur during the diff/edit preview stage, possibly when calculating string boundaries for displaying changes.

Claude Model

Opus

Is this a regression?

No, this never worked

Last Working Version

2.1.1 (Claude Code)

Claude Code Version

2.1.2 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

  **Root Cause Analysis:**
  This is a classic Rust UTF-8 string slicing bug. Korean characters occupy 3 bytes in UTF-8:
  - '주' = bytes 1-3
  - '말' = bytes 5-8 (with newline at byte 0)

  The code attempts to slice at byte index 7, which falls inside the '말' character (bytes 5-8), causing the panic.

  **Suggested Fix:**
  Use character-aware string operations instead of byte-based slicing:
  - `str.chars()` iterator
  - `str.char_indices()` for position-aware iteration
  - Ensure all string slice indices align with `str.is_char_boundary(index)`

  **Affected Use Case:**
  Editing source files containing CJK (Chinese, Japanese, Korean) or other multi-byte Unicode characters.