fix: Panic on CJK character truncation in MCP prompt descriptions #3227
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mzkmnk
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evanliu048
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Oct 22, 2025
| // If we found a valid boundary, use it; otherwise use the last character start | ||
| if truncate_at == 0 && !text.is_empty() { | ||
| // Edge case: even the first character is too long | ||
| truncate_at = text.char_indices().next().map(|(i, _)| i).unwrap_or(0); |
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qq: Should we delete this if check? char_indices().next() always returns the first character at index 0, so this line doesn't modify truncate_at
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You're absolutely right. Removed in c029a35. All tests still pass. 
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I've made the corrections. Please review 🙇 |
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- Consolidate ASCII and CJK test cases into single test function - Reduces diff size while maintaining comprehensive coverage - Ensures backward compatibility verification
Add comprehensive test coverage for edge cases: - Very small max_length values - CJK characters that don't fit in target length - Emoji (4-byte UTF-8 characters) - Mixed ASCII and CJK text - Single CJK character within limit All tests verify UTF-8 safe truncation behavior.
Remove the unnecessary if-check that was doing nothing. char_indices().next() always returns index 0 for the first character, so this code was just reassigning truncate_at = 0 without any effect. All tests pass without this code, confirming it was redundant.
Updated Cargo.lock to enable local testing and verification of the fix.
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Replace custom truncate logic in truncate_description with the existing truncate_safe_in_place utility function to ensure consistency across the codebase and leverage tested UTF-8 safe truncation logic.
mzkmnk
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Oct 25, 2025
| } | ||
| let mut result = text.to_string(); | ||
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| truncate_safe_in_place(&mut result, max_length, "..."); |
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I found that truncate_safe_in_place already exists in the codebase, so I used that instead
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Summary
Fixes panic when truncating MCP prompt descriptions containing CJK (Chinese, Japanese, Korean) characters.
Issues
Closes #3117
Closes #3170
Related to #3136, #3086
Problem
The
truncate_descriptionfunction inprompts.rswas using unsafe byte-index slicing (&text[..n]), which panics when the index falls in the middle of a multibyte UTF-8 character. CJK characters typically use 3 bytes in UTF-8, causing crashes when truncating at byte boundaries.Error example:
byte index 37 is not a char boundary; it is inside '한' (bytes 36..39)
Solution
char_indices()iterationChanges
truncate_description()function incrates/chat-cli/src/cli/chat/cli/prompts.rsNote
Testing
Verified with test cases from reported issues:
All tests pass without panics, respecting character boundaries.
Impact
/prompts listcommand)