[BUG] Panic when selecting text with Chinese characters: byte index is 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?

Bug Description:

Claude Code crashes with a Rust panic when selecting text from a markdown file containing Chinese characters.

What Should Happen?

Claude should vim markdown successfully.

Error Messages/Logs

thread '<unnamed>' (4645862) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 19 is not a char boundary; it is inside '。' (bytes 17..20) of `演(胡琴方向)。这种音乐与技术的双重背景塑造了我对音乐计算研究的思考方式。`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5, aborting
Abort trap: 6

Steps to Reproduce

Steps to Reproduce:

1. Open a markdown file containing Chinese text
2. Select a portion of text that includes Chinese characters (specifically text containing Chinese punctuation like 。)
3. Claude Code crashes with the panic above

Root Cause:
The error indicates that the code is attempting to slice a UTF-8 string at a byte index (19) that falls in the middle of a multi-byte character (Chinese full stop 。 which occupies bytes 17-20). This violates UTF-8 character boundaries.

Environment:

• Platform: macOS (Darwin 25.2.0)
• Claude Code version: [your version]
• File: 动机信-英文.md (contains both English and Chinese text)

Expected Behavior:
Claude Code should handle multi-byte UTF-8 characters correctly when processing selected text, ensuring string operations respect character boundaries.

Additional Context:
The Chinese full stop 。 and other CJK characters occupy 3 bytes in UTF-8. String slicing operations need to use character-aware methods rather than raw byte indexing.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

v2.1.5

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_