[BUG] Panic on UTF-8 emoji character during Edit operation

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 told me this is a bug and that it is reportable.

What Should Happen?

Claude Code crashes with a Rust panic when attempting to edit files containing multi-byte UTF-8 characters (specifically emojis like 🔲).

Error message:
thread '<unnamed>' (377726) panicked at .../library/core/src/str/mod.rs:833:21:
byte index 3 is not a char boundary; it is inside '🔲' (bytes 0..4) of 🔲 PLACEHOLDER
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5, aborting
Aborted (core dumped)

Root cause: The code is indexing into a UTF-8 string at byte position 3, but the 🔲 emoji is 4 bytes (U+1F532). Byte index 3 falls in the middle of this character, which is invalid in Rust's string slicing.

Reproduction:

1. Edit a markdown file containing the 🔲 emoji
2. Perform an edit operation that modifies text near or including the emoji

Expected: Handle multi-byte UTF-8 characters correctly by using character boundaries, not byte indices.

Error Messages/Logs

● Update(docs/IMPLEMENTATION_PLAN.md)

✽ Testing and committing changes… (esc to interrupt · 14m 45s · thinking)

────────────────────────────────────────────────────────────────────────────
────
> 
────────────────────────────────────────────────────────────────────────────
────
  ⏵⏵ accept edits on (shift+tab to cycle)  Context left until auto-compact: 
4%

thread '<unnamed>' (377726) panicked at /rustc/ed61e7d7e242494fb7057f2657300
d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 3 is not a char boundary; it is inside '🔲' (bytes 0..4) of `🔲 
PLACEHOLDER`
note: run with `RUST_BACKTRACE=1` environment variable to display a 
backtrace
fatal runtime error: failed to initiate panic, error 5, aborting
Aborted (core dumped)

Steps to Reproduce

I think it was a once off.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

Claude Opus 4.5 (model ID: claude-opus-4-5-20251101).

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Xterm

Additional Information

_No response_