[BUG] Edit tool crashes with Rust panic when editing Korean/CJK multi-byte UTF-8 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?

The Edit tool causes a Rust panic when editing files containing Korean (or other multi-byte UTF-8) text. The CLI crashes completely and terminates the session, losing any unsaved work.

The error occurs because string indexing calculates byte positions that fall in the middle of multi-byte UTF-8 characters (Korean characters are 3 bytes in UTF-8).

What Should Happen?

The Edit tool should correctly handle multi-byte UTF-8 characters by using character-aware indexing instead of byte-based indexing. Edits to Korean, Chinese, Japanese, emoji, and other Unicode text should work without crashing.

Error Messages/Logs

thread '<unnamed>' panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 9 is not a char boundary; it is inside '는' (bytes 7..10) of ` 평가는 익명 라벨로 진행되었습니다.`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5, aborting

---

thread '<unnamed>' panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 2 is not a char boundary; it is inside '이' (bytes 0..3) of `이`);`
fatal runtime error: failed to initiate panic, error 5, aborting

Steps to Reproduce

1. Open a project containing files with Korean text (JSX, JS, or any text file)
2. Ask Claude to edit Korean strings using the Edit tool
3. Example: "Change '상호 평가' to '크로스 심판' in the JSX file"
4. CLI crashes with Rust panic before or during the edit operation

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

Terminal.app (macOS)

Additional Information

<img width="752" height="1287" alt="Image" src="https://github.com/user-attachments/assets/019d7740-e590-4e4e-b30f-6f07e871be3b" />

• This bug is 100% reproducible when editing Korean text
• Likely affects all CJK (Chinese, Japanese, Korean) languages and emoji
• Workaround: Use Write tool to rewrite entire file instead of Edit tool
• Screenshot attached showing the crash in terminal