Panic when processing Chinese text: byte index not a char boundary in UTF-8 string

Description

Claude Code crashes with a Rust panic when processing text containing Chinese characters. The error occurs during string slicing where the byte index falls inside a multi-byte UTF-8 character.

Error Message

thread '<unnamed>' (19968778) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21:
byte index 95 is not a char boundary; it is inside '。' (bytes 93..96) of `易產生內在歸因（「我需要加強」）而非外在歸因（「老師針對我」）。`
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
fatal runtime error: failed to initiate panic, error 5, aborting
[1]    91024 abort      claude --dangerously-skip-permissions

Environment

• OS: macOS (Darwin 25.2.0)
• Claude Code version: Latest (as of 2024-12-30)
• Command: claude --dangerously-skip-permissions

Steps to Reproduce

1. Open Claude Code in a project containing Chinese text files
2. The CLI attempts to process/display Chinese content
3. Crash occurs when the internal string slicing hits a multi-byte character boundary

Root Cause Analysis

The error indicates that the code is using byte indexing instead of character indexing when slicing a UTF-8 string containing CJK characters:

• Chinese punctuation 。 (U+3002) is encoded as 3 bytes in UTF-8
• The code attempted to slice at byte index 95
• This index falls inside the character 。 (bytes 93..96)
• Rust's string safety correctly panics to prevent invalid UTF-8

Suggested Fix

The string slicing logic should use character-aware methods:

// Instead of:
&s[0..byte_index]

// Use:
s.char_indices()
    .take_while(|(i, _)| *i < target_char_count)
    .map(|(_, c)| c)
    .collect::<String>()

// Or use the `unicode-segmentation` crate for proper grapheme handling

Impact

This bug affects all users working with:

• Chinese (Traditional/Simplified)
• Japanese
• Korean
• Any other multi-byte UTF-8 text

Additional Context

The affected text was from an educational psychology document discussing attribution theory in a Taiwanese high school context. The project contains extensive Chinese documentation.