[BUG] Rust panic: byte index not a char boundary inside full-width parenthesis

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 crashed with a server-side Rust panic while editing a Japanese markdown file.

Severity: Critical - causes complete crash, no recovery possible

What Should Happen?

• Was updating DOCUMENT.md
• File contains mixed Japanese/English text with full-width punctuation
• The string ） (full-width right paren + space) triggered the issue

Error Messages/Logs

thread '<unnamed>' (2424040) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
  byte index 2 is not a char boundary; it is inside '）' (bytes 0..3) of `） `
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
  fatal runtime error: failed to initiate panic, error 5, aborting

Steps to Reproduce

1. Use Edit tool to update a markdown file containing Japanese text
2. File contains full-width parentheses: ） (U+FF09, 3-byte UTF-8)
3. Panic occurs during "accept edits" phase

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Claude Code Version: 2.0.76

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

Root Cause (suspected):
String slicing at byte index 2 within a 3-byte UTF-8 character (） = bytes 0-2).