fix: stop KaTeX swallowing currency in assistant messages

[philmerrell]

The bug

Reported on a real conversation. The assistant wrote:

A table slide showing Q1 ($100K), Q2 ($115K), and the total ($215K)

and the SPA rendered:

Q1 (100K),Q2(115K), total ($215K)

The prose between the amounts disappeared, not just the numbers.

Cause

streaming-text.component.ts enabled ngx-markdown's katex plugin with no
katexOptions, so its DEFAULT_KATEX_OPTIONS applied. Those add
{ left: '$', right: '$' } — which KaTeX upstream deliberately leaves
commented out in auto-render.js, with the reason attached:

// LaTeX uses $…$, but it ruins the display of normal `$` in text:
// {left: "$", right: "$", display: false},

renderMathInElement pairs delimiters positionally within a text node, so
any two currency amounts on one line become one formula.

Why the obvious fixes don't work

Three things looked like answers and are not. Each is documented in the code
so nobody re-derives them:

$ does nothing. The system prompt already told the model to escape
$ as an HTML entity. marked passes the entity into innerHTML, the browser
decodes it to a literal $ in the text node, and KaTeX walks the DOM after
that — the entity form breaks identically. Verified against the app's own
KaTeX build. It only ever succeeded at leaking $100K into generated
.pptx/.xlsx cells (#1126's follow-on). Rule deleted.

\(…\) was never an available fallback. ( and [ are
CommonMark-escapable, so marked strips the backslash before KaTeX runs:

marked.parse('Let \\(x^2\\) hold.')  ->  '<p>Let (x^2) hold.</p>'

So $…$ and $$…$$ were the only working delimiters. Dropping $…$ alone
would have left no inline math at all.

Telling the model to write \(…\) doesn't hold. Measured, with the new
prompt confirmed live: Haiku 4.5 wrote $e^{i\pi} + 1 = 0$ anyway. $…$ is
too dominant in training data for one prompt line to override.

The fix

shared/utils/katex-math-markdown.ts — marked inline tokenizers registered at
bootstrap, which claim math spans before marked's escape and emphasis rules
reach them:

rule	what it does
\(…\) / \[…\]	re-emitted verbatim, so they reach the DOM at all
$…$	rewritten to \(…\), only where it cannot be currency
$$…$$	body passed through untouched
\begin{env}…\end{env}	body passed through untouched

shared/utils/katex-delimiters.ts binds explicit delimiters with bare $
removed. That is what makes the guarded rule safe: KaTeX never re-pairs
dollars in the DOM, so it cannot undo the decision made with full context.

The currency test is Pandoc's plus a digit check — opening $ not followed by
whitespace or a digit, closing $ not preceded by whitespace nor followed by
a digit, no line break. Revenue rose from $1M to $2M, i.e. $r = 2$. resolves
exactly right: both amounts skipped, only $r = 2$ becomes math.

Two more breakages found while hunting edge cases

Both pre-existing, both inside $$…$$ — the delimiter survived markdown, its
contents did not:

input	before	after
$$a*b*c$$	$$a<em>b</em>c$$ → renders nothing	renders
$$\begin{pmatrix} a \\ b \end{pmatrix}$$	\\→\ → broken matrix	renders

The emphasis case is the subtle one: <em> splits the text node in three, and
splitAtDelimiters works within a single text node, so the $$ stop pairing
entirely. The \\ case broke every matrix and every multi-row alignment.

Verification

Reproduced and re-verified in the running app against dev data — the reported
conversation renders correctly, and a fresh turn produced inline math, display
math, a 2×2 matrix and two currency amounts all correct in one response
(2 inline + 2 display KaTeX nodes, 0 errors, 0 leftover literal $…$).

streaming-text.katex.spec.ts renders end-to-end through the real pipeline
(marked → Angular sanitizer → KaTeX) rather than a mock. It includes a guard
test asserting the KaTeX globals are loaded, because angular.json's scripts
are emitted but not evaluated in the test build — without it every "no math
rendered" expectation would pass for the wrong reason.

• SPA: 3357 passed
• Backend: 8820 passed, 3 skipped
• tsc -p tsconfig.app.json --noEmit clean

Prompt-cache note

DEFAULT_SYSTEM_PROMPT changes, so the cacheable prefix re-writes once per
session on rollout. It is four lines shorter afterwards.

🤖 Generated with Claude Code