[BUG] Panic on UTF-8 boundary when editing files with Chinese characters

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 performing an Edit operation on a file containing Chinese text.

## Error message
``
thread '<unnamed>' panicked at .../core/src/str/mod.rs:833:21:
byte index 26 is not a char boundary; it is inside '策' (bytes 24..27) of
五大功能 + 工程决策
fatal runtime error: failed to initiate panic, error 5, aborting
zsh: abort claude
``

## Context

• The Edit tool was updating a markdown file with mixed Chinese/English

content

• The string being processed was 五大功能 + 工程决策
• Byte index 26 falls inside the 3-byte UTF-8 encoding of '策'

## Version
2.0.76

## Expected behavior
String slicing should respect UTF-8 character boundaries.

## Suggested fix
Use char_indices() or similar UTF-8-safe string operations instead of raw byte indexing."

What Should Happen?

String slicing should respect UTF-8 character boundaries.

Error Messages/Logs

byte index 26 is not a char boundary; it is inside '策' (bytes 24..27) of
      `五大功能 + 工程决策`
      fatal runtime error: failed to initiate panic, error 5, aborting
      zsh: abort      claude

Steps to Reproduce

The crash occurs when the Edit tool's diff display logic tries to slice the string 五大功能 + 工程决策 at byte index 26, which falls inside the multi-byte character '策'.

Each Chinese character is 3 bytes in UTF-8:

• 五 (bytes 0-2)
• 大 (bytes 3-5)
• 功 (bytes 6-8)
• 能 (bytes 9-11)
• (space, byte 12)
• (byte 13)
• (space, byte 14)
• 工 (bytes 15-17)
• 程 (bytes 18-20)
• 决 (bytes 21-23)
• 策 (bytes 24-26) ← byte 26 is the last byte of this char, not a valid boundary

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.76

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_