Panic: UTF-8 byte boundary errors when rendering CJK text

[alterxyz]

Bug Description

Claude Code CLI crashes with Rust panics when rendering content containing Chinese (CJK) characters. Multiple crashes at different string positions indicate a systemic UTF-8 handling issue.

Error Messages

Crash 1:

thread '<unnamed>' panicked at library/core/src/str/mod.rs:833:21:
byte index 11 is not a char boundary; it is inside '职' (bytes 9..12)
fatal runtime error: failed to initiate panic, error 5, aborting
zsh: abort      claude

Crash 2:

thread '<unnamed>' panicked at library/core/src/str/mod.rs:833:21:
byte index 4 is not a char boundary; it is inside '里' (bytes 2..5)
fatal runtime error: failed to initiate panic, error 5, aborting
zsh: abort      claude

Analysis

• Both crashes occur at Rust's str::mod.rs:833 (string slicing)
• Different Chinese characters trigger the panic at different byte positions
• This indicates a common text truncation/display function has the bug
• The issue appears during CLI startup when processing CLAUDE.md

Root Cause

Byte-based string slicing without checking UTF-8 character boundaries. CJK characters are 3 bytes in UTF-8, so byte-index slicing frequently lands mid-character.

Reproduction

1. Create a CLAUDE.md containing Chinese text (especially in markdown tables)
2. Run claude in the directory
3. CLI crashes during startup or conversation

Environment

• OS: macOS Darwin 24.6.0
• Claude Code: Latest version

Suggested Fix

Use character-aware string operations:

• .chars().take(n) instead of &s[..n]
• Check s.is_char_boundary(n) before slicing
• Use s.floor_char_boundary(n) (Rust 1.80+) for safe truncation

[alterxyz]

Additional Information

The crash also occurs during active conversation, not just on startup:

1. Mid-conversation crash: While editing/inserting content during a conversation, the CLI crashed
2. Resume crash: Attempting to use /resume to restore the session also triggers the same panic

This indicates the bug affects multiple code paths:

• CLAUDE.md parsing (startup)
• Conversation rendering (during chat)
• Session restoration (/resume)

The issue is more pervasive than initially reported - essentially any text display/truncation operation involving CJK characters can trigger the panic.

[PeterZerg]

Same here: byte index 14 is not a char boundary; it is inside '息' (bytes 12..15)

This is a classic UTF-8 string slicing bug in Rust - the code attempted to slice a string at byte position 14, which falls in the middle of the Chinese character '息' (which spans bytes 12-15). Chinese characters are 3 bytes in UTF-8, and the code incorrectly assumed it could slice at any byte position.

thread '' (2001) panicked at /rustc/ed61e7d7e242494fb7057f2657300d9e77bb4fcb/library/core/src/str/mod.rs:833:21: byte index 14 is not a char boundary; it is inside '息' (bytes 12..15) of 粘稠的窒息。浓云压顶的夜空没有星月，唯有远方闪电在云层深处游走，瞬间照亮荒原上枯树狰狞的枝桠，又迅速被黑暗吞没。 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