[BUG] Panic: UTF-8 byte boundary error when processing Korean characters in file paths

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?

Description

Claude Code crashes with a panic when processing file paths or content containing Korean (Hangul) characters. The error indicates an incorrect UTF-8 byte boundary calculation when slicing strings.

### Error Message

byte index 15 is not a char boundary; it is inside '은' (bytes 13..16)

### Environment

• Claude Code version: 2.1.5
• OS: macOS (Darwin 25.2.0)
• Terminal: iTerm2 (also reproducible in other terminals)
• Shell: zsh

### Steps to Reproduce

1. Navigate to a directory with Korean characters in the path

• Example: /Users/username/Library/CloudStorage/GoogleDrive-.../내 드라이브/HIP 업무/PE본부 Work/

1. Run claude
2. Perform operations that involve reading files or processing text containing Korean characters
3. Crash occurs intermittently

### Root Cause Analysis

The Rust code is attempting to slice a UTF-8 string at a byte index (15) that falls in the middle of a multi-byte Korean character '은', which occupies bytes 13-16 (3 bytes). Korean Hangul characters are encoded as 3 bytes in UTF-8, and the string slicing operation is not respecting character boundaries.

### Expected Behavior

Claude Code should properly handle UTF-8 multi-byte characters (Korean, Japanese, Chinese, etc.) without panicking.

### Workaround

Using English-only paths reduces the frequency of crashes:
```bash
ln -s "/path/with/한글" ~/english_path
cd ~/english_path && claude

Additional Context

This is a critical issue for users working in non-ASCII environments. The panic is non-recoverable and requires restarting Claude Code.

What Should Happen?

-

Error Messages/Logs

Steps to Reproduce

### Workaround

Using English-only paths reduces the frequency of crashes:
```bash
ln -s "/path/with/한글" ~/english_path
cd ~/english_path && claude

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.5

Claude Code Version

2.1.5

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_