fix(web): disable single-dollar inline math in remark-math

web/package.json

@@ -58,10 +58,14 @@
         "@tailwindcss/postcss": "^4.1.18",
         "@vitejs/plugin-react": "^5.1.2",
         "autoprefixer": "^10.4.23",
+        "hast-util-to-html": "^9.0.5",
         "jsdom": "^26.1.0",
         "postcss": "^8.5.6",
+        "remark-parse": "^11.0.0",
+        "remark-rehype": "^11.1.2",
         "tailwindcss": "^4.1.18",
         "typescript": "^5.9.3",
+        "unified": "^11.0.5",
         "vite": "^7.3.0",
         "vitest": "^4.0.16"
     }

web/src/components/assistant-ui/markdown-text.test.ts

@@ -1,8 +1,16 @@
 import { describe, expect, it } from 'vitest'
+import { unified } from 'unified'
+import remarkParse from 'remark-parse'
+import remarkRehype from 'remark-rehype'
+import { toHtml } from 'hast-util-to-html'
 import remarkBreaks from 'remark-breaks'
 import remarkNonHttpsAutolink from '@/lib/remark-non-https-autolink'
 import remarkStripCjkAutolink from '@/lib/remark-strip-cjk-autolink'
-import { MARKDOWN_PLUGINS, MARKDOWN_PLUGINS_WITH_BREAKS } from '@/components/assistant-ui/markdown-text'
+import {
+    MARKDOWN_PLUGINS,
+    MARKDOWN_PLUGINS_WITH_BREAKS,
+    MARKDOWN_REHYPE_PLUGINS,
+} from '@/components/assistant-ui/markdown-text'
 
 describe('MARKDOWN_PLUGINS integration', () => {
     it('includes remarkNonHttpsAutolink', () => {
@@ -21,3 +29,47 @@ describe('MARKDOWN_PLUGINS integration', () => {
         expect(MARKDOWN_PLUGINS_WITH_BREAKS).toContain(remarkBreaks)
     })
 })
+
+function render(markdown: string): string {
+    const processor = unified()
+        .use(remarkParse)
+        .use(MARKDOWN_PLUGINS)
+        .use(remarkRehype)
+        .use(MARKDOWN_REHYPE_PLUGINS)
+    const tree = processor.runSync(processor.parse(markdown))
+    return toHtml(tree as never)
+}
+
+describe('MARKDOWN_PLUGINS — currency prose vs KaTeX', () => {
+    // Regression: prose with multiple "$N" amounts must NOT be eaten by KaTeX.
+    // remarkMath is configured with `singleDollarTextMath: false` so single
+    // dollar signs are treated as literal text, matching GitHub markdown.
+
+    it('does not render single-$ currency amounts as KaTeX math', () => {
+        const md = "The plan is $200/mo and the bill is $80 — total $400 saved."
+        const html = render(md)
+        expect(html).not.toContain('class="katex"')
+        expect(html).not.toContain('<math')
+        expect(html).toContain('$200')
+        expect(html).toContain('$80')
+        expect(html).toContain('$400')
+    })
+
+    it('does not render the reported real-world prose as KaTeX', () => {
+        // Lifted (paraphrased) from the bug report: paragraph with multiple
+        // "$N" amounts and apostrophes that previously collapsed into a single
+        // KaTeX block and stripped whitespace from the running text.
+        const md = "Cursor's UI quotes the ratio: at least $400 of API usage on a $200 plan. That's 2:1."
+        const html = render(md)
+        expect(html).not.toContain('class="katex"')
+        expect(html).not.toContain('<math')
+        expect(html).toContain('$400')
+        expect(html).toContain('$200')
+    })
+
+    it('still renders block math with $$...$$ on its own lines', () => {
+        const md = "Before\n\n$$\nE = mc^2\n$$\n\nAfter"
+        const html = render(md)
+        expect(html).toContain('class="katex"')
+    })
+})

web/src/components/assistant-ui/markdown-text.tsx

@@ -34,10 +34,17 @@ import type { MarkdownTextPrimitiveProps } from '@assistant-ui/react-markdown'
 // from them. Both must come before remarkMath (to avoid treating TeX as URI).
 // remarkFilePathLinks runs last to convert file paths → links after all other
 // transforms have settled.
+//
+// remarkMath is configured with `singleDollarTextMath: false` so that prose
+// containing currency amounts (e.g. "$200/mo ... $80 bill") is not garbled
+// into KaTeX output. Block math `$$...$$` (on its own line) still works.
+// This matches GitHub-flavored markdown behavior. The option lives on the
+// shared TAIL so both MARKDOWN_PLUGINS (default) and MARKDOWN_PLUGINS_WITH_BREAKS
+// (user-prompt rendering with hard breaks) inherit the fix.
 const MARKDOWN_PLUGIN_TAIL = [
     remarkNonHttpsAutolink,
     remarkStripCjkAutolink,
-    remarkMath,
+    [remarkMath, { singleDollarTextMath: false }],
     remarkDisableIndentedCode,
     remarkFilePathLinks,        // upstream — file path → link conversion, runs last
 ] satisfies NonNullable<MarkdownTextPrimitiveProps['remarkPlugins']>