[BUG] Panic on Korean UTF-8 string boundary when displaying edit UI

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?

Description

Claude Code crashes with a Rust panic when handling Korean (한글) text in the edit acceptance UI. The error occurs due to incorrect UTF-8 byte indexing on multi-byte characters.

Root Cause

I don't know. This report is generated by claude-code.

The panic occurs because the code attempts to index into a Korean UTF-8 string at byte position 5, which falls in the middle of the character '집' (bytes 3-6). Korean characters in UTF-8 are 3 bytes each, and the string 편집, 삭제 | is being incorrectly sliced at a non-character boundary.

Environment

• OS: macOS (Darwin 24.6.0)
• Platform: darwin
• Node.js: v22.14.0
• Claude Code Version: 2.0.76

Additional Context

This appears to be a string slicing issue in the Rust portion of Claude Code where character boundaries are not being respected for non-ASCII text. The fix would involve using character-aware string operations (e.g., .chars() iterator) instead of byte indexing.

What Should Happen?

Expected Behavior

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

Error Messages/Logs

thread '<unnamed>' (37091) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
  byte index 5 is not a char boundary; it is inside '집' (bytes 3..6) of `편집, 삭제 |`
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
  fatal runtime error: failed to initiate panic, error 5, aborting
  zsh: abort

Steps to Reproduce

Steps to Reproduce

1. Run Claude Code with a Korean locale or Korean UI text
2. Trigger the edit acceptance flow (shift+tab to cycle through options)
3. The application crashes when trying to render/process the Korean text "편집, 삭제 |"

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.76

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Other

Additional Information

I use ghostty.