feat(web): opt-in LaTeX math rendering in chat

[hkarlsen06]

Models write LaTeX all the time, and chat shows it raw. \(x\) even loses its delimiters to CommonMark escapes. Eight earlier attempts (#4633 → #12118) were closed, mostly for turning math on for everyone. This one follows the bar set in #1784: fully opt-in, with limited complexity.

Note

KaTeX already ships in main. Mermaid diagrams (#15067) load it as a lazy chunk. This PR reuses that exact chunk: the build output is byte-identical, with the same hash. It adds no new KaTeX package, and the lockfile only gains mdast-util-math and one small dependency of it. The engine is already paid for. This PR lets users opt in to using it in chat.

Settings → Appearance → Render math (off by default). Implements the request in #9641. Happy to adjust the scope.

Before (off, same as main)	After (on)

What this gets right

• Off means off. No math plugin is registered and KaTeX never loads. Raw HTML using the math classes stays plain code.
• Lazy. The shared KaTeX chunk and KaTeX's CSS (29 KB) are fetched only when a formula renders. The browser downloads only the font faces a formula uses. The parser adds 2.9 KB gzipped to the ChatMarkdown chunk.
• Fast path. With the setting on, messages without $, \( or \[ still skip math parsing.
• Parsed, not pre-processed. One micromark extension, no regex rewrite of the source. Code spans, fences, links and escapes keep their CommonMark meaning, and source offsets are untouched, so task-list checkboxes still toggle the right line.
• Prose stays prose. Dollar math follows Pandoc's rules: $5 and $10, $5-$10, $HOME and $skill chips stay text, and \[1\] citations stay literal.
• Blocks behave like fences. A $$ or \[ line runs to its closing line, so formula lines starting with - or # don't turn into lists or headings. Blockquotes and lists work too.
• Safe. KaTeX runs with trust: false on text that has already been through the sanitizer. The sanitizer only learns two class names.
• Readable failures. Unfinished or invalid TeX, such as text mid-stream, shows its source.
• Copy gives TeX. Selecting any part of a formula copies $…$ or $$…$$.
• Wide equations scroll instead of clipping.
• Surfaces. Web and desktop share the renderer, which includes Markdown file previews. Mobile is unchanged, and the setting says so.

Verification: focused tests for the parser, rendering through the real sanitize pipeline, task offsets, list recovery, the off path, and clipboard. Web typecheck and lint pass. Checked in the dev app (screenshots above).

Built with Claude Opus 5.5 in Claude Code.

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📝 Walkthrough

Walkthrough

Adds an opt-in setting for rendering LaTeX in chat Markdown. The change adds Markdown math parsing, KaTeX rendering, TeX-aware clipboard output, Appearance controls, and user documentation.

Changes

Markdown Math Rendering

Layer / File(s)	Summary
Math rendering setting packages/contracts/src/settings.ts, apps/web/src/components/settings/SettingsPanels.tsx, apps/web/src/components/settings/settingsSearch.ts	Adds mathRenderingEnabled, defaulting to false, and exposes it in Appearance settings, restore-defaults handling, and settings search.
Markdown math syntax apps/web/package.json, apps/web/src/markdown-math.ts, apps/web/src/markdown-math.test.ts	Adds math parsing extensions for inline and display delimiters. Tests cover Markdown contexts, source offsets, delimiter handling, and incomplete input.
Chat Markdown math rendering apps/web/src/components/ChatMarkdown.tsx, apps/web/src/components/chat/*, apps/web/src/components/ChatMarkdown.test.tsx, apps/web/src/index.css, apps/web/package.json	Selects math parsing when the setting is enabled and the text may contain math. Routes recognized math nodes to KaTeX rendering, with fallback TeX text, and adds rendering tests and equation styles.
Math copying and documentation apps/web/src/markdown-clipboard.ts, apps/web/src/markdown-clipboard.math.test.ts, docs/user/appearance.md	Uses the formula’s TeX as Markdown clipboard text when a selection is inside rendered math. Documents supported delimiters and copy behavior.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant ChatMarkdown
  participant remarkChatMath
  participant ReactMarkdown
  participant MarkdownMath
  participant MathTypeset
  participant KaTeX
  ChatMarkdown->>remarkChatMath: Selects the math plugin when enabled and math may be present
  remarkChatMath->>ReactMarkdown: Converts math delimiters into renderable nodes
  ReactMarkdown->>MarkdownMath: Routes recognized math nodes
  MarkdownMath->>MathTypeset: Supplies TeX and a fallback
  MathTypeset->>KaTeX: Renders TeX as HTML and MathML

Loading

Possibly related PRs

• pingdotgg/t3code#10698: Changes overlapping web math rendering, parsing, display styles, and clipboard serialization.
• pingdotgg/t3code#12118: Implements web math rendering in the same areas, including ChatMarkdown, KaTeX, and clipboard handling.

Suggested reviewers: maria-rcks

---

Merge Risk | 🔵 Low · up to 53301

Merge Risk: 🔵 Low · up to 53301

Some code in messages can be typeset as math instead of displayed as code. This is a bounded issue that can be fixed or accepted before merging.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 53301

Math rendering is off by default and uses explicit restrictions on trusted commands and formula expansion. No concrete security vulnerability was established. The new rendering path nevertheless handles potentially untrusted content, and the full range of content sources and access contexts was not verified.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The newly reachable sink operates in the viewing web or desktop renderer after local opt-in. Identified callers include file-preview text and environment/thread-scoped reasoning text. MathTypeset accepts TeX, display mode, and fallback content, not credentials or environment authority. The complete upstream authorization and tenant-provenance chain remains unresolved.

Security Findings and Attack Paths

• inferred — With math enabled, raw-HTML parsing enabled, and a math marker elsewhere in the text, a supplied code element carrying language-math and a math-mode class can reach the typesetter without parser-produced formula provenance. This expands input reachability compared with the ordinary rendering path, but the same KaTeX restrictions apply; it is not evidence of an HTML or authority bypass.

Trust Boundaries and Controls

• observed — Every examined typesetting call uses trust:false, throwOnError:true, maxSize:20, and maxExpand:1000. Exceptions retain escaped source fallback. The preference is read from client settings rather than supplied by message content, and parsing and renderer selection use the same computed gate.

Resilience and Maintainability Implications

• inferred — The examined rendering transition has no asynchronous formula-state writeback: output is derived synchronously from current inputs, while Suspense and the error boundary expose source fallback. Cache limits constrain retained markup, but do not establish a total CPU or allocation budget for adversarial formula input or many formulas.

Hardening Proposals

• proposed — Add an enabled-state regression case for raw HTML carrying math classes and disallowed trusted commands. This would protect the library-control boundary against future parser, sanitizer, or renderer drift; it is not remediation for a verified vulnerability.

---

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 17.95% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 39 functions across 11 files. (2 skipped:…	Write docstrings for the functions missing them to satisfy the coverage threshold.
Description check	⚠️ Warning	The description explains the problem, implementation, scope, and verification, but it does not document explicit maintainer approval for this scope. The provided context says that approval is still pe…	Resolve the pending scope decision. Add a link to the maintainer’s explicit approval comment and summarize the approved scope, or revise the change to match the approved direction and document that approval in the description.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely identifies the main change: opt-in LaTeX math rendering in chat.

---

Full details: Docstring Coverage

Explanation

Docstring coverage is 17.95% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 39 functions across 11 files. (2 skipped: 2 unsupported.)

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Full details: Description check

Explanation

The description explains the problem, implementation, scope, and verification, but it does not document explicit maintainer approval for this scope. The provided context says that approval is still pending, and the template requires it.

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Review comments at @apps/web/src/markdown-math.ts:
- Around line 210-318: Update `mathBlock` so reaching EOF before a closing fence
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📒 Files selected for processing (14)

• apps/web/package.json
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• apps/web/src/components/ChatMarkdown.tsx
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[juliusmarminge]

Note

This comment is posted by Julius' dot

Leaving this open for a maintainer scope decision. At 2144157, math is off by default and uses one custom four-delimiter parser with lazy KaTeX, clipboard support, and web/desktop rendering. The earlier maintainer response permits opt-in math only if the added complexity is acceptable; that condition needs a ruling for this implementation.

The before/after captures and focused parsing checks are useful. Under prior approval, please resolve the author's existing scope question in #9641: is this implementation within the approved direction, or should support be narrower? Review handoff is pending that answer.

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Review comments at @apps/web/src/components/ChatMarkdown.tsx:
- Around line 3376-3378: Update MarkdownCodeOrMath so mathKind class matches
render as MarkdownMath only when they come from parser-produced math; preserve
raw HTML code elements as code when parseRawHtml is enabled. Keep the existing
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[aidenwboudr]

Found a case while running this branch:

It costs $5 and $10, and `$HOME` stays code.

The $ of $5 opens a formula. The parser scans through the backtick and closes on the $ inside the code span, so the code span gets swallowed and the paragraph shows $5 and $10, and $as failed math source. The same happens withExport $PATH, then $HOME.CommonMark gives code spans precedence over a construct that would end inside them, and TeX math has no use for backticks. So the fix I used stops the formula at a backtick, indataCharacter`:

const GRAVE_ACCENT = 96;
// …
function dataCharacter(code: Code): State | undefined {
  // A backtick would start a code span, which outranks math.
  if (code === GRAVE_ACCENT) return nok(code);
  consume(code);
  return code === BACKSLASH ? escaped : inside;
}

With these cases added to "leaves code, escapes, links, and citations alone":

["It costs $5 and $10, and `$HOME` stays code.", "It costs $5 and $10, and  stays code."],
["Export $PATH, then `$HOME`.", "Export $PATH, then ."],
["\\(a `\\)` b", "(a  b"],

The rest of the math and web suites still pass with it.

(Found and written up with Claude Code.)

[hkarlsen06]

@aidenwboudr Good catch, thanks. It's fixed in the latest push, where a formula now stops at a backtick, and your three cases are in the tests. The same problem hit links and autolinks with a $ in their URL, which is common with OData APIs such as ?$select= on Microsoft Graph, so a formula now also stops at ://, which TeX never contains.

[hkarlsen06]

https://x.com/theo/status/2109543705090904237?s=46

@t3dotgg has confirmed LaTeX will never be supported in T3 code, and as such, all PRs and issues about it should now be closed.

The recommended workaround is to use the inline HTML renderer.

[hkarlsen06]

Note

Claude Opus 5.5 is posting this on behalf of @hkarlsen06.

For anyone who still wants math in chat: I made a skill that uses the inline HTML renderer for it. When a reply needs a formula, the agent puts the math on a small HTML page typeset with KaTeX, and T3 Code shows that page above the reply. The reply itself stays plain text. Each formula has a Copy button for its LaTeX, and the page follows the app's light and dark theme.

npx skills add hkarlsen06/skills --skill show-math

The agent has to run inside T3 Code, because the skill uses T3's html_render tool. The repo explains how it works and how to install it by hand: https://github.com/hkarlsen06/skills

[RVZO6]

Note

Claude Opus 5.5 is posting this on behalf of @hkarlsen06.

For anyone who still wants math in chat: I made a skill that uses the inline HTML renderer for it. When a reply needs a formula, the agent puts the math on a small HTML page typeset with KaTeX, and T3 Code shows that page above the reply. The reply itself stays plain text. Each formula has a Copy button for its LaTeX, and the page follows the app's light and dark theme.

npx skills add hkarlsen06/skills --skill show-math

The agent has to run inside T3 Code, because the skill uses T3's html_render tool. The repo explains how it works and how to install it by hand: https://github.com/hkarlsen06/skills

i suppose this works but its quite slow with models like 6.1 sol, as their tps is relatively slow and making an entire html page for one latex expression is quite verbose. i ended up just making a userscript for the web version. i can share it but its slop - might as well just make your own with how u like it.

[hkarlsen06]

i suppose this works but its quite slow with models like 6.1 sol, as their tps is relatively slow and making an entire html page for one latex expression is quite verbose. i ended up just making a userscript for the web version. i can share it but its slop - might as well just make your own with how u like it.

I don't use the web version though, like at all.

i really hope they ship the plugins or modifiers or whatever maria called them soon, so we can mod it with the behavior we want

[RVZO6]

well she said not soon which is not very promising. but at least it will be good. and probably have mobile support.

i honestly hope they ship some sort of userscript manager in the desktop app in the interim. but i know if i submit a PR for it theres a 0% chance of it getting merged lol