Rust panic: UTF-8 string boundary error when editing CJK content

Description

Claude Code crashes with a Rust panic when editing files containing CJK (Chinese/Japanese/Korean) characters. The crash occurs during Edit operations when the tool attempts to display or truncate text containing multi-byte Unicode characters.

Error Messages

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 '詞' (bytes 7..10) of `個word詞");`
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 '二' (bytes 0..3) of `二");`

Steps to Reproduce

1. Edit a file containing CJK characters (e.g., Objective-C test file with Chinese strings)
2. Make multiple small Edit operations on lines containing CJK content
3. The crash occurs when the edit preview/display truncates text at a byte boundary that falls inside a multi-byte character

Root Cause

The Rust code is using byte indices to slice UTF-8 strings, but CJK characters are 3 bytes each. When truncating for display, the code lands in the middle of a character, causing a panic.

Environment

• Claude Code v2.1.4
• macOS (Darwin 25.2.0)
• Editing Objective-C test files with Chinese character test strings

Suggested Fix

Use character-aware string slicing:

• str.chars().take(n) instead of byte slicing
• Or use str.char_indices() to find safe truncation points
• Or use floor_char_boundary() (Rust 1.73+) before slicing

Workaround

Using the Write tool for larger file sections instead of multiple small Edit operations reduces the frequency of crashes.