diff --git a/.github/scripts/pr-test-loc-summary.mjs b/.github/scripts/pr-test-loc-summary.mjs
new file mode 100644
index 00000000000..f7fa2a67bb0
--- /dev/null
+++ b/.github/scripts/pr-test-loc-summary.mjs
@@ -0,0 +1,228 @@
+import { appendFileSync, readFileSync } from 'node:fs'
+import { pathToFileURL } from 'node:url'
+import { mergeLocBlock, renderLocBlock, sumChangedFiles } from './pr-test-loc-table.mjs'
+
+export const PR_FILES_PAGE_LIMIT = 3000
+
+export function nextLink(linkHeader) {
+  if (linkHeader == null || linkHeader.length === 0) {
+    return undefined
+  }
+
+  for (const part of linkHeader.split(',')) {
+    const match = part.match(/<([^>]+)>\s*;\s*rel="next"/)
+    if (match != null) {
+      return match[1]
+    }
+  }
+
+  return undefined
+}
+
+function githubHeaders(token) {
+  return {
+    Accept: 'application/vnd.github+json',
+    Authorization: `Bearer ${token}`,
+    'User-Agent': 'orca-pr-test-loc',
+    'X-GitHub-Api-Version': '2022-11-28'
+  }
+}
+
+export async function listPullFiles({ owner, repo, pullNumber, token, fetchImpl = fetch }) {
+  const files = []
+  let url = `https://api.github.com/repos/${owner}/${repo}/pulls/${pullNumber}/files?per_page=100`
+
+  while (url != null) {
+    const response = await fetchImpl(url, { headers: githubHeaders(token) })
+    if (!response.ok) {
+      throw new Error(
+        `Failed to list PR #${pullNumber} files: ${response.status} ${response.statusText}`
+      )
+    }
+    const page = await response.json()
+    if (!Array.isArray(page)) {
+      throw new Error(`Unexpected PR files payload for #${pullNumber}`)
+    }
+    files.push(...page)
+    if (files.length >= PR_FILES_PAGE_LIMIT) {
+      console.log(
+        `PR #${pullNumber} file list hit GitHub's ${PR_FILES_PAGE_LIMIT}-file cap; totals may be short.`
+      )
+      return files.slice(0, PR_FILES_PAGE_LIMIT)
+    }
+    url = nextLink(response.headers.get('link'))
+  }
+
+  return files
+}
+
+function writeGithubOutput(totals) {
+  const block = `${renderLocBlock(totals)}\n`
+  const outputPath = process.env.GITHUB_OUTPUT
+  if (outputPath != null) {
+    appendFileSync(outputPath, `summary<<ORCA_PR_LOC_EOF\n${block}ORCA_PR_LOC_EOF\n`)
+  }
+  const summaryPath = process.env.GITHUB_STEP_SUMMARY
+  if (summaryPath != null) {
+    appendFileSync(summaryPath, `${block}\n`)
+  }
+}
+
+async function updatePullRequest({ owner, repo, pullNumber, token, totals, fetchImpl = fetch }) {
+  const headers = githubHeaders(token)
+  const url = `https://api.github.com/repos/${owner}/${repo}/pulls/${pullNumber}`
+  const response = await fetchImpl(url, { headers })
+  if (response.status === 403) {
+    console.log('Skipping PR body update: token cannot write (likely a fork PR).')
+    return 0
+  }
+  if (!response.ok) {
+    throw new Error(`Failed to read PR #${pullNumber}: ${response.status} ${response.statusText}`)
+  }
+
+  const pull = await response.json()
+  const nextBody = mergeLocBlock(pull.body, totals)
+  if (nextBody === (pull.body ?? '')) {
+    console.log(`PR #${pullNumber} LoC header already current.`)
+    return 0
+  }
+
+  const update = await fetchImpl(url, {
+    method: 'PATCH',
+    headers: {
+      ...headers,
+      'Content-Type': 'application/json'
+    },
+    body: JSON.stringify({ body: nextBody })
+  })
+  if (update.status === 403) {
+    console.log('Skipping PR body update: token cannot write (likely a fork PR).')
+    return 0
+  }
+  if (!update.ok) {
+    throw new Error(`Failed to update PR #${pullNumber}: ${update.status} ${update.statusText}`)
+  }
+
+  console.log(`Updated LoC header on PR #${pullNumber}.`)
+  return 0
+}
+
+function resolveRepository() {
+  const repository = process.env.GITHUB_REPOSITORY
+  if (repository == null || !repository.includes('/')) {
+    return undefined
+  }
+  const slash = repository.indexOf('/')
+  return { owner: repository.slice(0, slash), repo: repository.slice(slash + 1) }
+}
+
+function resolveToken() {
+  return process.env.GITHUB_TOKEN ?? process.env.GH_TOKEN
+}
+
+function printUsage() {
+  console.error(
+    `Usage: ${process.argv[1]} --from-pr <number> | --update-pr <number> [--files-json <file>] [--merge-body <file>]`
+  )
+}
+
+async function main(argv) {
+  let filesJsonPath
+  let mergeBodyPath
+  let fromPrNumber
+  let updatePrNumber
+
+  for (let i = 0; i < argv.length; i += 1) {
+    const arg = argv[i]
+    if (arg === '--files-json') {
+      filesJsonPath = argv[i + 1]
+      i += 1
+      continue
+    }
+    if (arg === '--merge-body') {
+      mergeBodyPath = argv[i + 1]
+      i += 1
+      continue
+    }
+    if (arg === '--from-pr') {
+      fromPrNumber = argv[i + 1]
+      i += 1
+      continue
+    }
+    if (arg === '--update-pr') {
+      updatePrNumber = argv[i + 1]
+      i += 1
+      continue
+    }
+    printUsage()
+    return 2
+  }
+
+  const pullNumber = updatePrNumber ?? fromPrNumber
+  if (
+    (argv.includes('--files-json') && filesJsonPath == null) ||
+    (argv.includes('--merge-body') && mergeBodyPath == null) ||
+    (argv.includes('--from-pr') && fromPrNumber == null) ||
+    (argv.includes('--update-pr') && updatePrNumber == null) ||
+    (filesJsonPath == null && pullNumber == null)
+  ) {
+    printUsage()
+    return 2
+  }
+
+  let files
+  if (filesJsonPath != null) {
+    files = JSON.parse(readFileSync(filesJsonPath, 'utf8'))
+  } else {
+    const repository = resolveRepository()
+    const token = resolveToken()
+    if (repository == null || token == null) {
+      console.error('GITHUB_REPOSITORY and GITHUB_TOKEN are required to read a pull request.')
+      return 2
+    }
+    files = await listPullFiles({
+      ...repository,
+      pullNumber: Number(pullNumber),
+      token
+    })
+  }
+
+  const totals = sumChangedFiles(files)
+  writeGithubOutput(totals)
+
+  if (mergeBodyPath != null) {
+    process.stdout.write(mergeLocBlock(readFileSync(mergeBodyPath, 'utf8'), totals))
+    return 0
+  }
+
+  console.log(renderLocBlock(totals))
+
+  if (updatePrNumber == null) {
+    return 0
+  }
+
+  const repository = resolveRepository()
+  const token = resolveToken()
+  if (repository == null || token == null) {
+    console.error('GITHUB_REPOSITORY and GITHUB_TOKEN are required with --update-pr.')
+    return 2
+  }
+
+  return updatePullRequest({
+    ...repository,
+    pullNumber: Number(updatePrNumber),
+    token,
+    totals
+  })
+}
+
+if (process.argv[1] != null && import.meta.url === pathToFileURL(process.argv[1]).href) {
+  main(process.argv.slice(2))
+    .then((code) => {
+      process.exitCode = code
+    })
+    .catch((error) => {
+      console.error(error instanceof Error ? error.message : error)
+      process.exitCode = 1
+    })
+}
diff --git a/.github/scripts/pr-test-loc-table.mjs b/.github/scripts/pr-test-loc-table.mjs
new file mode 100644
index 00000000000..2d0c73a4f1c
--- /dev/null
+++ b/.github/scripts/pr-test-loc-table.mjs
@@ -0,0 +1,86 @@
+export const LOC_BLOCK_START = '<!-- orca-pr-loc -->'
+export const LOC_BLOCK_END = '<!-- /orca-pr-loc -->'
+export const LOC_HANDS_OFF_COMMENT =
+  '<!-- Programmatic LoC summary. Do not edit by hand; rewritten on every commit. -->'
+
+const TEST_DIR_SEGMENT = /(?:^|\/)(?:__tests__|e2e|tests)(?:\/|$)/i
+const TEST_FILENAME = /\.(?:test|spec|e2e)\.[^/]+$/i
+
+export function isTestPath(path) {
+  const normalized = path.replaceAll('\\', '/')
+  return TEST_DIR_SEGMENT.test(normalized) || TEST_FILENAME.test(normalized)
+}
+
+export function emptyLocTotals() {
+  return {
+    test: { files: 0, added: 0, deleted: 0 },
+    nonTest: { files: 0, added: 0, deleted: 0 }
+  }
+}
+
+export function sumChangedFiles(files) {
+  const totals = emptyLocTotals()
+  for (const file of files) {
+    const path = file.filename
+    if (path == null) {
+      continue
+    }
+    const bucket = isTestPath(path) ? totals.test : totals.nonTest
+    bucket.files += 1
+    bucket.added += Number(file.additions ?? 0)
+    bucket.deleted += Number(file.deletions ?? 0)
+  }
+  return totals
+}
+
+function signed(count) {
+  if (count === 0) {
+    return '0'
+  }
+  return count > 0 ? `+${count}` : `−${Math.abs(count)}`
+}
+
+function locTableRow(label, bucket) {
+  return `| ${label} | ${bucket.files ?? 0} | ${signed(bucket.added)} | ${signed(-(bucket.deleted ?? 0))} | ${signed((bucket.added ?? 0) - (bucket.deleted ?? 0))} |`
+}
+
+export function formatLocTable({ test, nonTest }) {
+  return [
+    '| | Files | Added | Deleted | Net |',
+    '| :--- | ---: | ---: | ---: | ---: |',
+    locTableRow('Test', test),
+    locTableRow('Prod', nonTest)
+  ].join('\n')
+}
+
+export function renderLocBlock(totals) {
+  return [
+    LOC_BLOCK_START,
+    LOC_HANDS_OFF_COMMENT,
+    '',
+    formatLocTable(totals),
+    '',
+    LOC_BLOCK_END
+  ].join('\n')
+}
+
+export function mergeLocBlock(body, totals) {
+  const block = renderLocBlock(totals)
+  const current = body ?? ''
+  const start = current.indexOf(LOC_BLOCK_START)
+  const end = current.indexOf(LOC_BLOCK_END)
+
+  if (start !== -1 && end !== -1 && end > start) {
+    const rest = current.slice(end + LOC_BLOCK_END.length).replace(/^\r?\n/, '')
+    if (rest.trim().length === 0) {
+      return `${current.slice(0, start)}${block}\n`
+    }
+    return `${current.slice(0, start)}${block}\n\n${rest.replace(/^\r?\n+/, '')}`
+  }
+
+  if (current.trim().length === 0) {
+    return `${block}\n`
+  }
+
+  return `${block}\n\n${current.replace(/^\r?\n+/, '')}`
+}
diff --git a/.github/workflows/pr-test-loc.yml b/.github/workflows/pr-test-loc.yml
new file mode 100644
index 00000000000..438951ee3fa
--- /dev/null
+++ b/.github/workflows/pr-test-loc.yml
@@ -0,0 +1,36 @@
+name: PR test LoC
+
+on:
+  pull_request:
+    types:
+      - opened
+      - synchronize
+      - reopened
+      - ready_for_review
+
+concurrency:
+  group: pr-test-loc-${{ github.event.pull_request.number }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+  pull-requests: write
+
+jobs:
+  loc:
+    name: test vs non-test LoC
+    runs-on: ubuntu-latest
+    timeout-minutes: 2
+    steps:
+      # Why no checkout: the Files API already has per-file additions/deletions.
+      - name: Count test vs non-test LoC
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GITHUB_REPOSITORY: ${{ github.repository }}
+        run: |
+          for script in pr-test-loc-table.mjs pr-test-loc-summary.mjs; do
+            gh api "repos/${GITHUB_REPOSITORY}/contents/.github/scripts/${script}?ref=pull/${{ github.event.pull_request.number }}/head" \
+              --jq .content | base64 --decode > "$RUNNER_TEMP/${script}"
+          done
+          node "$RUNNER_TEMP/pr-test-loc-summary.mjs" --update-pr "${{ github.event.pull_request.number }}"
diff --git a/config/scripts/pr-test-loc-summary.test.mjs b/config/scripts/pr-test-loc-summary.test.mjs
new file mode 100644
index 00000000000..bc92e40eef7
--- /dev/null
+++ b/config/scripts/pr-test-loc-summary.test.mjs
@@ -0,0 +1,175 @@
+import { spawnSync } from 'node:child_process'
+import { mkdtempSync, readFileSync, rmSync, writeFileSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join, resolve } from 'node:path'
+import { afterEach, describe, expect, it } from 'vitest'
+import { parse } from 'yaml'
+import { listPullFiles, nextLink } from '../../.github/scripts/pr-test-loc-summary.mjs'
+import {
+  LOC_HANDS_OFF_COMMENT,
+  isTestPath,
+  mergeLocBlock,
+  renderLocBlock,
+  sumChangedFiles
+} from '../../.github/scripts/pr-test-loc-table.mjs'
+
+const projectDir = resolve(import.meta.dirname, '../..')
+const locScript = join(projectDir, '.github/scripts/pr-test-loc-summary.mjs')
+const tempDirs = []
+
+function runLoc(args, { env } = {}) {
+  return spawnSync(process.execPath, [locScript, ...args], {
+    cwd: projectDir,
+    encoding: 'utf8',
+    env: { ...process.env, ...env }
+  })
+}
+
+afterEach(() => {
+  while (tempDirs.length > 0) {
+    rmSync(tempDirs.pop(), { force: true, recursive: true })
+  }
+})
+
+describe('PR test LoC summary', () => {
+  it('classifies colocated tests and e2e/tests directories', () => {
+    expect(isTestPath('src/main/foo.test.ts')).toBe(true)
+    expect(isTestPath('src/main/foo.spec.tsx')).toBe(true)
+    expect(isTestPath('src/e2e/login.ts')).toBe(true)
+    expect(isTestPath('tests/tools/probe.mjs')).toBe(true)
+    expect(isTestPath('src/main/foo-test-setup.ts')).toBe(false)
+    expect(isTestPath('src/main/foo.ts')).toBe(false)
+  })
+
+  it('sums GitHub pull-file additions and deletions', () => {
+    const totals = sumChangedFiles([
+      { filename: 'src/app.ts', additions: 4, deletions: 1 },
+      { filename: 'src/app.test.ts', additions: 12, deletions: 3 },
+      { filename: 'icon.png', additions: 0, deletions: 0 }
+    ])
+
+    expect(totals).toEqual({
+      test: { files: 1, added: 12, deleted: 3 },
+      nonTest: { files: 2, added: 4, deleted: 1 }
+    })
+  })
+
+  it('reads the next page from a GitHub Link header', () => {
+    expect(
+      nextLink(
+        '<https://api.github.com/repos/stablyai/orca/pulls/1/files?page=2>; rel="next", <https://api.github.com/repos/stablyai/orca/pulls/1/files?page=3>; rel="last"'
+      )
+    ).toBe('https://api.github.com/repos/stablyai/orca/pulls/1/files?page=2')
+    expect(
+      nextLink('<https://api.github.com/repos/stablyai/orca/pulls/1/files?page=1>; rel="prev"')
+    ).toBe(undefined)
+  })
+
+  it('paginates pull files until Link rel=next is gone', async () => {
+    const pages = {
+      'https://api.github.com/repos/stablyai/orca/pulls/9/files?per_page=100': {
+        body: [{ filename: 'src/app.ts', additions: 2, deletions: 0 }],
+        link: '<https://api.github.com/repos/stablyai/orca/pulls/9/files?page=2>; rel="next"'
+      },
+      'https://api.github.com/repos/stablyai/orca/pulls/9/files?page=2': {
+        body: [{ filename: 'src/app.test.ts', additions: 5, deletions: 1 }],
+        link: null
+      }
+    }
+
+    const files = await listPullFiles({
+      owner: 'stablyai',
+      repo: 'orca',
+      pullNumber: 9,
+      token: 'test-token',
+      fetchImpl: async (url) => {
+        const page = pages[url]
+        if (page == null) {
+          throw new Error(`unexpected url ${url}`)
+        }
+        return {
+          ok: true,
+          json: async () => page.body,
+          headers: { get: (name) => (name === 'link' ? page.link : null) }
+        }
+      }
+    })
+
+    expect(sumChangedFiles(files)).toEqual({
+      test: { files: 1, added: 5, deleted: 1 },
+      nonTest: { files: 1, added: 2, deleted: 0 }
+    })
+  })
+
+  it('replaces an existing header and prepends when missing', () => {
+    const totals = {
+      test: { files: 1, added: 2, deleted: 1 },
+      nonTest: { files: 1, added: 4, deleted: 0 }
+    }
+    const block = renderLocBlock(totals)
+
+    expect(block).toContain(LOC_HANDS_OFF_COMMENT)
+    expect(block).toContain('| Test | 1 | +2 | −1 | +1 |')
+    expect(block).toContain('| Prod | 1 | +4 | 0 | +4 |')
+    expect(block).not.toContain('| Total |')
+    expect(mergeLocBlock('## ELI5\n\nHello\n', totals)).toBe(`${block}\n\n## ELI5\n\nHello\n`)
+    expect(mergeLocBlock(`${block}\n\n## ELI5\n`, totals)).toBe(`${block}\n\n## ELI5\n`)
+    expect(
+      mergeLocBlock(
+        '<!-- orca-pr-loc -->\n**LoC** · test **+1 / −0**\n<!-- /orca-pr-loc -->\n\n## ELI5\n',
+        totals
+      )
+    ).toBe(`${block}\n\n## ELI5\n`)
+  })
+
+  it('counts and merges from a files JSON fixture', () => {
+    const root = mkdtempSync(join(tmpdir(), 'orca-pr-test-loc-'))
+    tempDirs.push(root)
+    const filesPath = join(root, 'files.json')
+    const bodyPath = join(root, 'body.md')
+    writeFileSync(
+      filesPath,
+      JSON.stringify([
+        { filename: 'src/app.ts', additions: 2, deletions: 0 },
+        { filename: 'src/app.test.ts', additions: 6, deletions: 1 }
+      ])
+    )
+    writeFileSync(bodyPath, '## ELI5\n\nHello\n')
+
+    const result = runLoc(['--files-json', filesPath, '--merge-body', bodyPath])
+
+    expect(result.status).toBe(0)
+    expect(result.stdout.startsWith('<!-- orca-pr-loc -->')).toBe(true)
+    expect(result.stdout).toContain(LOC_HANDS_OFF_COMMENT)
+    expect(result.stdout).toContain('| Test | 1 | +6 | −1 | +5 |')
+    expect(result.stdout).toContain('| Prod | 1 | +2 | 0 | +2 |')
+    expect(result.stdout).not.toContain('| Total |')
+    expect(result.stdout).toContain('## ELI5\n\nHello\n')
+    expect(result.stdout.match(/<!-- orca-pr-loc -->/g)).toHaveLength(1)
+  })
+
+  it('exits 2 with usage when no PR or files JSON is supplied', () => {
+    const result = runLoc([])
+
+    expect(result.status).toBe(2)
+    expect(result.stderr).toContain('--from-pr')
+  })
+
+  it('is a no-checkout GitHub-hosted PR workflow', () => {
+    const workflow = parse(
+      readFileSync(join(projectDir, '.github/workflows/pr-test-loc.yml'), 'utf8')
+    )
+    const locJob = workflow.jobs.loc
+    const serialized = JSON.stringify(workflow)
+
+    expect(locJob['runs-on']).toBe('ubuntu-latest')
+    expect(locJob.steps).toHaveLength(1)
+    expect(locJob.steps[0].run).toContain('gh api')
+    expect(locJob.steps[0].run).toContain('pr-test-loc-table.mjs')
+    expect(locJob.steps[0].run).toContain('pr-test-loc-summary.mjs')
+    expect(locJob.steps[0].run).toContain('--update-pr')
+    expect(workflow.permissions['pull-requests']).toBe('write')
+    expect(serialized).not.toContain('actions/checkout')
+    expect(serialized).not.toContain('self-hosted')
+  })
+})