[BUG] Panic when displaying Korean (UTF-8 multibyte) text: "byte index is not a char boundary"

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/displaying Korean text.

## Error Message

❯ thread '<unnamed>' (65878105) panicked at
/rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 2 is not a char boundary; it is inside '에' (bytes 0..3) of 에 있는 아기 여우가 보인다 - 푹신한 주황빛
털, 큰 뾰족한 귀, 눈물로 반짝이는 호박색 눈. 남자를 보자마자
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5, aborting
[1] 720 abort claude

What Should Happen?

❯ thread '<unnamed>' (65878105) panicked at
/rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 2 is not a char boundary; it is inside '에' (bytes 0..3) of 에 있는 아기 여우가 보인다 - 푹신한 주황빛
털, 큰 뾰족한 귀, 눈물로 반짝이는 호박색 눈. 남자를 보자마자
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5, aborting
[1] 720 abort claude

Error Messages/Logs

Steps to Reproduce

❯ thread '<unnamed>' (65878105) panicked at
/rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 2 is not a char boundary; it is inside '에' (bytes 0..3) of 에 있는 아기 여우가 보인다 - 푹신한 주황빛
털, 큰 뾰족한 귀, 눈물로 반짝이는 호박색 눈. 남자를 보자마자
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5, aborting
[1] 720 abort claude

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

2.1.3

Claude Code Version

2.1.3 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Warp

Additional Information

_No response_