[BUG] Thai text input in Claude CLI interactive mode is broken

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?

Dear Anthropic Support Team,

I am a paying Claude Pro customer and I need to report a CRITICAL bug in Claude CLI that makes it completely unusable for Thai language input.

ISSUE SUMMARY:
Thai text input in Claude CLI interactive mode is broken. Characters display incorrectly, text doesn't wrap properly, and continues invisibly beyond screen edge.

SEVERITY: HIGH

• Affects all Thai language users (~70 million speakers)
• Makes CLI completely unusable for Thai input
• This impacts paying customers who expect a working product

EVIDENCE:

1. Regular terminal (kitty/gnome-terminal): Thai displays perfectly ✓
2. Claude CLI prompt: Thai characters broken, wrong spacing, no line wrap ✗
3. Fonts installed: Noto Sans Thai, TH Sarabun - working correctly ✓
4. Locale: th_TH.UTF-8 - configured correctly ✓

ENVIRONMENT:

• OS: Ubuntu 24.04 LTS
• Terminal: kitty (also tested with gnome-terminal, alacritty)
• Claude CLI version: v2.1.19
• Locale: th_TH.UTF-8

ROOT CAUSE:
Claude CLI's input handler does not properly support Unicode combining characters used in Thai language (U+0E34-U+0E3A, U+0E47-U+0E4E).

EXPECTED BEHAVIOR:
Thai input should work identically to English - proper display, proper line wrapping, proper character editing.

ACTUAL BEHAVIOR:
Thai text with vowels/tone marks above/below base characters:

• Displays with incorrect spacing
• Doesn't wrap to new line at terminal edge
• Continues invisibly beyond screen width
• Makes interactive mode completely unusable

SCREENSHOTS:
[Attach screenshots from your uploads showing the problem]

REPRODUCTION STEPS:

1. Launch: claude
2. Type Thai text with combining characters: "ภาษาไทยพิมพ์ยาวๆไม่ขึ้นบรรทัดใหม่"
3. Observe: characters display incorrectly, no proper line wrapping

IMPACT:
As a paying customer, I expect Claude CLI to work with my native language. This is not a minor UI issue - it's a complete blocker for Thai language users.

REQUEST:
Please prioritize fixing this bug. Similar issues likely affect other languages with combining characters (Vietnamese, Arabic, Hebrew, etc.).

TECHNICAL SUGGESTION:
The input library (likely readline or prompt_toolkit) needs proper wcwidth() implementation for Unicode combining characters.

Thank you for your attention to this matter.

Best regards,

What Should Happen?

Thai language input should work exactly like English input:

1. Characters with combining marks (vowels/tones above/below) should display correctly with proper spacing
2. Text should wrap to new line when reaching terminal edge
3. Long Thai sentences should be editable normally
4. Cursor movement should work correctly with Thai characters
5. Backspace/delete should work properly with combining characters

Expected: Thai text input works identically to English - proper display, proper wrapping, proper editing in Claude CLI interactive prompt.

Error Messages/Logs

Steps to Reproduce

1. Open terminal (tested with kitty, gnome-terminal, alacritty)
2. Verify locale is set to th_TH.UTF-8: locale
3. Verify Thai fonts are installed (Noto Sans Thai, TH Sarabun New)
4. Run: claude
5. In the interactive prompt, type Thai text with vowels/tone marks above/below:

Example: "ภาษาไทยพิมพ์ยาวๆไม่ขึ้นบรรทัดใหม่ ทดสอบการแสดงผลกับสระบนและวรรณยุกต์"

1. Observe:

• Characters display with incorrect spacing
• Text doesn't wrap at terminal edge
• Text continues invisibly beyond screen width
• Cursor position is wrong

Note: The SAME Thai text displays perfectly in the regular terminal prompt before launching claude.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

claude --version 2.1.19 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_