[BUG] Fatal Rust panic: byte index 14 is not a char boundary in Unicode string handling

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 fatal Rust panic error related to Unicode string processing. The error appears to occur when the application attempts to slice a string containing Japanese characters at an invalid byte boundary.

Error Message:
thread '<unnamed>' (54985224) panicked at /rustc/ed61e7d7e242494fb7057f26
57300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 14 is not a char boundary; it is inside 'ト' (bytes 12..15) of

ップデート"

note: run with RUST_BACKTRACE=1 environment variable to display a
backtrace
fatal runtime error: failed to initiate panic, error 5, aborting
[1] 56621 abort claude

What Should Happen?

Description:

Claude Code crashed with a fatal Rust panic error related to Unicode string processing. The error appears to occur when the application attempts to slice a string containing Japanese characters at an invalid byte boundary.

Expected Behavior:
Claude Code should handle Unicode strings (including Japanese characters) properly without crashing.

Actual Behavior:
The application crashes with a fatal runtime error and aborts.

Error Message:
thread '<unnamed>' (54985224) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 14 is not a char boundary; it is inside 'ト' (bytes 12..15) of ップデート"
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5, aborting
[1] 56621 abort claude

Steps to Reproduce:
Unknown - the crash occurred during normal usage. May be related to processing files or text containing Japanese characters.

Environment:

• OS: macOS (Darwin 24.1.0)
• Claude Code Version: [Please add your version with claude --version]
• Shell: Zsh

Additional Context:

• The error message shows Japanese characters ("ト" from what appears to be "アップデート" meaning "update")
• This suggests the crash is related to Unicode/multi-byte character handling in Rust string operations
• The panic occurs at a low level in Rust's string handling code when trying to access byte index 14 within a multi-byte character sequence

Suggested Investigation:
This appears to be a string slicing operation that doesn't respect UTF-8 character boundaries. The code should use character-boundary-aware slicing methods instead of raw byte indexing when dealing with potentially multi-byte Unicode strings.

Error Messages/Logs

Steps to Reproduce

Steps to Reproduce:

1. Launch Claude Code (claude)
2. Ask Claude to perform Japanese translation (translate text to Japanese)
3. Application crashes immediately during or after processing the Japanese translation request

Context:

• The crash occurred while requesting Japanese translation services
• The application terminated abruptly without completing the translation
• Error message contains Japanese characters ("ト" from "アップデート"), suggesting the crash happened during Japanese text processing

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.0.73

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_