Korean character boundary panic in editor

Bug: Korean character boundary panic in editor

Environment

• Claude Code: 2.0.75
• OS: macOS Darwin 24.6.0 (Apple Silicon)

Error

thread '<unnamed>' (51374513) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 18 is not a char boundary; it is inside '요' (bytes 16..19) of ` 말씀해주세요.\n\n무엇을 도와드릴까요?"`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5, aborting
zsh: abort      claude

Context

• Occurred during file editing (Edit tool) with Korean text
• The string contains Korean characters which are 3 bytes each in UTF-8
• String indexing/truncation logic appears to not respect UTF-8 char boundaries
• The panic occurred in Rust's core string module when accessing byte index 18, which falls inside a multi-byte Korean character

Expected behavior

String operations should respect UTF-8 character boundaries when handling non-ASCII text.

Reproduction

• Edit a file containing Korean text
• The exact trigger is unclear, but it happened during an Edit tool operation on a markdown file with Korean content