[BUG] Broken syntax highlighting with non-ASCII characters (diacritics) in terminal output

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?

Any colored terminal output containing non-ASCII characters (Czech diacritics: č, ř, ž, š, ě, ů, etc.) displays with broken syntax highlighting. Individual characters within words receive random colors (red, green, blue, yellow), making the text very hard to read.

This is not limited to plan mode — it happens in any colored output (code diffs, markdown rendering, XAML code blocks, etc.) whenever the text contains multi-byte UTF-8 characters.

The coloring appears to be applied per-character rather than following any syntax highlighting rules. This likely points to the same underlying issue as #23851 (multi-byte UTF-8 character offset problem in the highlight.js → ANSI conversion layer).

Possibly related to #20799 and #23851.

What Should Happen?

Readable formatting with consistent colors based on syntax rules, regardless of whether the text contains ASCII-only or multi-byte UTF-8 characters (diacritics, Cyrillic, CJK, etc.).

Error Messages/Logs

No error messages - this is a visual rendering issue only.

Steps to Reproduce

1. Run claude in Windows Terminal (PowerShell)
2. Trigger any colored output containing non-ASCII characters — e.g. write a plan with Czech text and XAML code blocks, or ask Claude to display code with comments containing diacritics
3. Observe the terminal output
4. Individual characters within words have random colors (red, green, blue, yellow) instead of consistent syntax highlighting

Claude Model

Opus

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.38 (Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

Screenshot showing the broken highlighting (XAML code blocks with Czech text):

<img width="1066" height="572" alt="Broken syntax highlighting with Czech diacritics" src="https://github.com/user-attachments/assets/4f13aa49-b319-4690-be98-476582fd22b2" />