diff --git a/.github/workflows/frontend-typecheck.yml b/.github/workflows/frontend-typecheck.yml
new file mode 100644
index 000000000000..566e44c93913
--- /dev/null
+++ b/.github/workflows/frontend-typecheck.yml
@@ -0,0 +1,117 @@
+name: Frontend Type Check
+
+# Reports how a pull request changes the number of TypeScript errors.
+# Main keeps its own count as an artifact; pull requests compare against it.
+# Not a required check: it calls attention to new errors without blocking.
+
+on:
+  push:
+    branches:
+      - main
+    paths:
+      - frontend/**
+      - .github/workflows/frontend-typecheck.yml
+  pull_request:
+    types: [opened, synchronize, reopened, ready_for_review]
+    paths:
+      - frontend/**
+      - .github/workflows/frontend-typecheck.yml
+
+permissions:
+  contents: read
+
+# A newer push to a pull request replaces its older report. Main's counts are
+# never cancelled, since pull requests compare against them.
+concurrency:
+  group: frontend-typecheck-${{ github.event.pull_request.number || github.sha }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
+jobs:
+  typescript-errors:
+    name: TypeScript errors
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      actions: read # download main's count
+      pull-requests: write # post the report
+
+    defaults:
+      run:
+        working-directory: frontend
+
+    env:
+      BASE_SHA: ${{ github.event.pull_request.base.sha }}
+
+    steps:
+      - uses: actions/checkout@v5
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version-file: frontend/.nvmrc
+          cache: npm
+          cache-dependency-path: frontend/package-lock.json
+
+      - name: Install dependencies
+        run: npm ci
+
+      - name: Count errors
+        run: node scripts/typecheck-report/index.mjs count "$GITHUB_SHA" typecheck-counts.json
+
+      - name: Keep main's count
+        if: github.event_name == 'push'
+        uses: actions/upload-artifact@v4
+        with:
+          name: typecheck-counts
+          path: frontend/typecheck-counts.json
+          retention-days: 90
+
+      # Main only runs this when frontend files change, so its latest count
+      # usually belongs to an older commit than the base. The next step checks
+      # that the frontend is the same in both.
+      - name: Download main's latest count
+        if: github.event_name == 'pull_request'
+        continue-on-error: true
+        uses: dawidd6/action-download-artifact@v6
+        with:
+          workflow: frontend-typecheck.yml
+          branch: main
+          name: typecheck-counts
+          path: frontend/base
+          if_no_artifact_found: warn
+
+      - name: Check the count matches the base
+        if: github.event_name == 'pull_request'
+        id: base
+        run: |
+          if [ ! -f base/typecheck-counts.json ]; then
+            echo "state=missing" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+          count_sha=$(node -p "require('./base/typecheck-counts.json').sha")
+          state=current
+          if [ "$count_sha" != "$BASE_SHA" ]; then
+            git fetch --no-tags --depth=1 origin "$count_sha" "$BASE_SHA"
+            git diff --quiet "$count_sha" "$BASE_SHA" -- . || state=stale
+          fi
+          echo "state=$state" >> "$GITHUB_OUTPUT"
+
+      - name: Compare with main
+        if: github.event_name == 'pull_request'
+        id: compare
+        run: |
+          action=$(node scripts/typecheck-report/index.mjs compare base/typecheck-counts.json typecheck-counts.json "${{ steps.base.outputs.state }}" report.md)
+          echo "action=$action" >> "$GITHUB_OUTPUT"
+          cat report.md >> "$GITHUB_STEP_SUMMARY"
+
+      # Comments when the total moves or any file gains errors. Otherwise it
+      # only refreshes an existing comment, so quiet pull requests get none.
+      # Fork pull requests can't comment; the job summary still has the report.
+      - name: Report on the pull request
+        if: github.event_name == 'pull_request'
+        continue-on-error: true
+        uses: marocchino/sticky-pull-request-comment@v2
+        with:
+          header: typescript-errors
+          path: frontend/report.md
+          only_update: ${{ steps.compare.outputs.action == 'update' }}
diff --git a/frontend/scripts/typecheck-report/compare.test.ts b/frontend/scripts/typecheck-report/compare.test.ts
new file mode 100644
index 000000000000..164a3c9a13c5
--- /dev/null
+++ b/frontend/scripts/typecheck-report/compare.test.ts
@@ -0,0 +1,84 @@
+import { compareCounts, ErrorCounts } from './compare'
+
+const counts = (files: Record<string, number>): ErrorCounts => ({
+  files,
+  sha: 'abc',
+})
+
+describe('compareCounts', () => {
+  it('reports more errors in red and comments', () => {
+    const report = compareCounts(
+      counts({ 'a.ts': 2 }),
+      counts({ 'a.ts': 3, 'b.ts': 1 }),
+      'current',
+    )
+    expect(report.action).toBe('comment')
+    expect(report.markdown).toBe(
+      [
+        '## TypeScript errors',
+        '🔴 2 → 4 (+2)',
+        '',
+        '| File | Change |',
+        '| --- | --- |',
+        '| `a.ts` | +1 |',
+        '| `b.ts` | +1 |',
+        '',
+      ].join('\n'),
+    )
+  })
+
+  it('reports fewer errors in green and comments', () => {
+    const report = compareCounts(
+      counts({ 'a.ts': 3 }),
+      counts({ 'a.ts': 1 }),
+      'current',
+    )
+    expect(report.action).toBe('comment')
+    expect(report.markdown).toContain('🟢 3 → 1 (-2)')
+  })
+
+  it('comments when a file goes up even if the total does not', () => {
+    const report = compareCounts(
+      counts({ 'a.ts': 2, 'b.ts': 2 }),
+      counts({ 'a.ts': 4 }),
+      'current',
+    )
+    expect(report.action).toBe('comment')
+    expect(report.markdown).toContain('⚪️ 4 → 4 (0)')
+  })
+
+  it('only updates an existing comment when nothing goes up', () => {
+    const report = compareCounts(
+      counts({ 'a.ts': 2 }),
+      counts({ 'a.ts': 2 }),
+      'current',
+    )
+    expect(report.action).toBe('update')
+    expect(report.markdown).toBe('## TypeScript errors\n⚪️ 2 → 2 (0)\n')
+  })
+
+  it('does not compare with a stale count', () => {
+    const report = compareCounts(
+      counts({ 'a.ts': 9 }),
+      counts({ 'a.ts': 1 }),
+      'stale',
+    )
+    expect(report.action).toBe('update')
+    expect(report.markdown).toContain("Main's count is out of date")
+  })
+
+  it('says when main has no count yet', () => {
+    const report = compareCounts(null, counts({ 'a.ts': 1 }), 'missing')
+    expect(report.action).toBe('update')
+    expect(report.markdown).toContain('No count from main')
+  })
+
+  it('lists at most 30 files', () => {
+    const head = Object.fromEntries(
+      Array.from({ length: 32 }, (_, i) => [`f${i}.ts`, 1]),
+    )
+    const report = compareCounts(counts({}), counts(head), 'current')
+    expect(report.markdown.match(/^\| `/gm)).toHaveLength(30)
+    expect(report.markdown).toContain('And 2 more files.')
+  })
+})
diff --git a/frontend/scripts/typecheck-report/compare.ts b/frontend/scripts/typecheck-report/compare.ts
new file mode 100644
index 000000000000..2376bd5770cd
--- /dev/null
+++ b/frontend/scripts/typecheck-report/compare.ts
@@ -0,0 +1,92 @@
+export type ErrorCounts = {
+  sha: string
+  files: Record<string, number>
+}
+
+export type BaseState = 'current' | 'stale' | 'missing'
+
+// `comment` posts or updates the PR comment; `update` only refreshes one
+// that already exists, so quiet PRs get no comment.
+export type ReportAction = 'comment' | 'update'
+
+export type Report = {
+  action: ReportAction
+  markdown: string
+}
+
+// Keeps the comment well under GitHub's 65536 character limit.
+const MAX_ROWS = 30
+
+export const total = (files: Record<string, number>) =>
+  Object.values(files).reduce((sum, n) => sum + n, 0)
+
+const signed = (n: number) => (n > 0 ? `+${n}` : `${n}`)
+
+const icon = (delta: number) => {
+  if (delta > 0) return '🔴'
+  if (delta < 0) return '🟢'
+  return '⚪️'
+}
+
+export const compareCounts = (
+  base: ErrorCounts | null,
+  head: ErrorCounts,
+  baseState: BaseState,
+): Report => {
+  const heading = '## TypeScript errors'
+  if (!base || baseState === 'missing') {
+    return {
+      action: 'update',
+      markdown: `${heading}\n${total(
+        head.files,
+      )} errors. No count from main to compare with yet.\n`,
+    }
+  }
+  if (baseState === 'stale') {
+    return {
+      action: 'update',
+      markdown: `${heading}\n${total(
+        head.files,
+      )} errors. Main's count is out of date, so there is nothing to compare with.\n`,
+    }
+  }
+
+  const delta = total(head.files) - total(base.files)
+  const changed = [
+    ...new Set([...Object.keys(base.files), ...Object.keys(head.files)]),
+  ]
+    .map((file) => ({
+      delta: (head.files[file] || 0) - (base.files[file] || 0),
+      file,
+    }))
+    .filter((row) => row.delta !== 0)
+    .sort((a, b) => b.delta - a.delta || a.file.localeCompare(b.file))
+
+  const lines = [
+    heading,
+    `${icon(delta)} ${total(base.files)} → ${total(head.files)} (${signed(
+      delta,
+    )})`,
+  ]
+  if (changed.length) {
+    lines.push(
+      '',
+      '| File | Change |',
+      '| --- | --- |',
+      ...changed
+        .slice(0, MAX_ROWS)
+        .map((row) => `| \`${row.file}\` | ${signed(row.delta)} |`),
+    )
+    if (changed.length > MAX_ROWS) {
+      lines.push('', `And ${changed.length - MAX_ROWS} more files.`)
+    }
+  }
+
+  // A file going up is worth a comment even when fixes elsewhere hide it in
+  // the total.
+  const anyFileUp = changed.some((row) => row.delta > 0)
+  return {
+    action: delta !== 0 || anyFileUp ? 'comment' : 'update',
+    markdown: `${lines.join('\n')}\n`,
+  }
+}
diff --git a/frontend/scripts/typecheck-report/index.mjs b/frontend/scripts/typecheck-report/index.mjs
new file mode 100644
index 000000000000..3500d787cb9b
--- /dev/null
+++ b/frontend/scripts/typecheck-report/index.mjs
@@ -0,0 +1,57 @@
+#!/usr/bin/env node
+// Counts TypeScript errors per file and compares two counts for the PR report.
+//
+//   node scripts/typecheck-report/index.mjs count <sha> <out.json>
+//   node scripts/typecheck-report/index.mjs compare <base.json> <head.json> <current|stale|missing> <out.md>
+//
+// `compare` prints `comment` or `update` for the workflow to act on.
+// Never fails on type errors: the report calls attention, it doesn't block.
+
+import { existsSync, readFileSync, writeFileSync } from 'node:fs'
+import { spawnSync } from 'node:child_process'
+import { compareCounts, total } from './compare.ts'
+
+const WITH_FILE = /^(.+?)\(\d+,\d+\): error TS\d+:/
+const WITHOUT_FILE = /^error TS\d+:/
+
+const count = (sha, out) => {
+  const tsc = spawnSync(
+    'npx',
+    ['tsc', '--noEmit', '--pretty', 'false', '--incremental', 'false'],
+    { encoding: 'utf8', maxBuffer: 64 * 1024 * 1024 },
+  )
+  // tsc exits 0 when clean and 1 or 2 when it reports errors; anything else
+  // means it didn't run.
+  if (tsc.error || ![0, 1, 2].includes(tsc.status)) {
+    process.stderr.write(`${tsc.stdout}${tsc.stderr}`)
+    process.exit(1)
+  }
+  const files = {}
+  for (const line of tsc.stdout.split('\n')) {
+    const file = line.match(WITH_FILE)?.[1] ?? (WITHOUT_FILE.test(line) && '(no file)')
+    if (file) files[file] = (files[file] || 0) + 1
+  }
+  writeFileSync(out, `${JSON.stringify({ files, sha }, null, 2)}\n`)
+  process.stdout.write(
+    `${total(files)} errors in ${Object.keys(files).length} files\n`,
+  )
+}
+
+const read = (path) =>
+  existsSync(path) ? JSON.parse(readFileSync(path, 'utf8')) : null
+
+const compare = (basePath, headPath, baseState, out) => {
+  const report = compareCounts(read(basePath), read(headPath), baseState)
+  writeFileSync(out, report.markdown)
+  process.stdout.write(`${report.action}\n`)
+}
+
+const [mode, ...args] = process.argv.slice(2)
+if (mode === 'count') count(...args)
+else if (mode === 'compare') compare(...args)
+else {
+  process.stderr.write(
+    'Usage: index.mjs count <sha> <out> | compare <base> <head> <state> <out>\n',
+  )
+  process.exit(1)
+}
diff --git a/frontend/scripts/typecheck-report/package.json b/frontend/scripts/typecheck-report/package.json
new file mode 100644
index 000000000000..3dbc1ca591c0
--- /dev/null
+++ b/frontend/scripts/typecheck-report/package.json
@@ -0,0 +1,3 @@
+{
+  "type": "module"
+}