[BUG] Rust panic on UTF-8 char boundary when processing Korean text files

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?

Claude Code crashes with a Rust panic when processing text files containing Korean (or other multi-byte UTF-8) characters. The panic occurs because the code attempts to slice a string at a byte index that falls in the middle of a multi-byte character.

What Should Happen?

Claude Code should properly handle UTF-8 multi-byte characters by ensuring string slicing only occurs at valid character boundaries, not arbitrary byte positions.

Error Messages/Logs

thread '<unnamed>' (79760) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb\library\core\src\str\mod.rs:833:21:
  byte index 86 is not a char boundary; it is inside '요' (bytes 84..87) of `엔 달력 보면서 손으로 계산했는데, 계산이 틀린 적도 있었거든요.`
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Steps to Reproduce

1. Create a text file containing Korean text, for example:

옛날엔 달력 보면서 손으로 계산했는데, 계산이 틀린 적도 있었거든요.

1. Open Claude Code in the directory containing this file
2. Attempt to update or process the file (e.g., editing, reading with truncation)
3. The Rust panic occurs when the string is sliced at a non-character boundary

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.0.76 (Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Terminal.app (macOS)

Additional Information

This is a common Rust UTF-8 handling issue. The fix typically involves using .chars() iterator or .char_indices() instead of direct byte indexing, or using safe slicing methods that respect character boundaries.

The bug appears to occur in a string truncation or display formatting context, possibly when showing file update notifications or content previews.

Affected text was from a Korean lecture script file: 2026-01-07_강의스크립트_30분_초안.txt