[BUG] Markdown italics rendered with Unicode math-alphabet codepoints — garbled on selection / copy / search

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?

Claude Code renders markdown italic / bold-italic (and possibly other styled spans) by substituting ASCII letters with characters from the Unicode Mathematical Alphanumeric Symbols block (U+1D400–U+1D7FF) instead of emitting ANSI italic escapes (ESC[3m). The styled text looks acceptable inline when the terminal font has glyphs for those codepoints, but the substitution silently breaks several things:

• Selection rendering: when italicized text is highlighted with the mouse, the terminal falls back to a different font for the selection foreground, exposing the math-alphabet glyphs. Entire italicized regions look like Zalgo / IPA-mashup text instead of italics.
• Copy/paste: selecting and copying italicized text yields 𝓲𝓼 instead of is, so pasted content is unreadable in other apps and invisible to grep.
• Search / grep: grep "is" against a saved transcript misses italicized occurrences because the codepoints aren't ASCII i/s.
• Accessibility: screen readers announce these codepoints character-by-character ("mathematical script small i, mathematical script small s") instead of reading the word.

Likely the same root cause as #59660 ("Session-summary text renders garbled (box-drawing/Cyrillic glyphs) in Cursor terminal — 2.1.142/2.1.143"). That issue describes the same symptom in the session-summary block on macOS + Cursor and explicitly hypothesises math-italic Unicode codepoints as the cause. This report extends the repro to Linux + VS Code integrated terminal and shows the corruption also affects markdown emphasis in the agent's response body — not just the summary block.

What Should Happen?

Emit ANSI italic SGR (ESC[3m / ESC[23m) for italics. Most modern terminals (iTerm2, WezTerm, Kitty, Alacritty, GNOME Terminal, VS Code's integrated terminal) support italics natively when paired with an italic-capable font. On terminals that don't, prefer leaving the text as plain ASCII over substituting non-ASCII codepoints — preserving copy/paste fidelity, search, and accessibility is more valuable than rendering the slant.

Error Messages/Logs

Steps to Reproduce

1. Launch claude inside VS Code's integrated terminal (TERM_PROGRAM=vscode, TERM=xterm-256color).
2. Run any prompt whose response uses markdown italics, e.g.:

``
claude "Explain Outlook vs Gmail connectors using *italic* for key terms."
``

1. Wait for the response. Italicized words render as styled Latin-looking glyphs.
2. With the mouse, click-drag to select a region of the response that includes italicized text. The selection-highlighted glyphs visibly change — italicized words become garbled.
3. Copy the selection and paste into another editor (e.g. a new file). Italicized words paste as 𝓲𝓽𝓪𝓵𝓲𝓬𝓼 etc.
4. Run grep -F "italics" <pasted-file>. No match.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_(unknown — see #59660 which reports 2.1.138 as last clean on the related session-summary surface)_

Claude Code Version

2.1.148 (Claude Code)

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

VS Code integrated terminal

Additional Information

• Claude Code: 2.1.148
• Terminal: VS Code integrated terminal (TERM_PROGRAM=vscode, TERM_PROGRAM_VERSION=1.115.0)
• TERM: xterm-256color
• OS: Linux 5.4.0-200-generic #220-Ubuntu SMP x86_64

Workarounds (current, neither acceptable as default):

• Ask the assistant not to use markdown emphasis.
• Pipe output through a normalizer that maps the U+1D400 range back to ASCII.

Screenshots illustrating the bug — italicized prose rendered "normally" vs. the same prose with mouse selection exposing math-alphabet glyphs — will be attached as a follow-up comment (drag-and-drop, since gh can't upload images inline).