diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 557f3c4..4698618 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -36,6 +36,9 @@ jobs:
       - name: Verify browser harness build cleanup
         run: node --test scripts/fixtures/replay-process.test.mjs
 
+      - name: Verify browser runner mirror preparation
+        run: node --test scripts/fixtures/browser-mirrors.test.mjs
+
       - name: Verify Docker cloud runner offline
         run: |
           pnpm exec tsx --test scripts/cloud/*.test.ts
@@ -193,7 +196,14 @@ jobs:
         with:
           name: package-dist
           path: dist/packages/
+      # Prefer the runner's existing Ubuntu archive mirrors over its slow Azure mirror.
+      # Preserve package sources, signatures and Azure as a final fallback.
+      - name: Prepare browser dependency mirrors
+        run: |
+          node scripts/fixtures/browser-mirrors.mjs /etc/apt/apt-mirrors.txt "${RUNNER_TEMP}/domscribe-apt-mirrors.txt"
+          sudo install -m 644 "${RUNNER_TEMP}/domscribe-apt-mirrors.txt" /etc/apt/apt-mirrors.txt
       - name: Install Chromium
+        timeout-minutes: 5
         run: pnpm --filter @domscribe/test-fixtures exec playwright install --with-deps chromium
       - name: Replay persisted captures in a fresh browser session
         run: >-
@@ -297,7 +307,12 @@ jobs:
 
       - run: pnpm install --frozen-lockfile
 
+      - name: Prepare browser dependency mirrors
+        run: |
+          node scripts/fixtures/browser-mirrors.mjs /etc/apt/apt-mirrors.txt "${RUNNER_TEMP}/domscribe-apt-mirrors.txt"
+          sudo install -m 644 "${RUNNER_TEMP}/domscribe-apt-mirrors.txt" /etc/apt/apt-mirrors.txt
       - name: Install Playwright browsers
+        timeout-minutes: 5
         run: pnpm --filter @domscribe/test-fixtures exec playwright install --with-deps chromium
 
       - name: Download registry storage
diff --git a/scripts/fixtures/browser-mirrors.mjs b/scripts/fixtures/browser-mirrors.mjs
new file mode 100644
index 0000000..fa8fc8d
--- /dev/null
+++ b/scripts/fixtures/browser-mirrors.mjs
@@ -0,0 +1,76 @@
+import { readFileSync, writeFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+import { pathToFileURL } from 'node:url';
+
+export function preferUbuntuMirrors(content) {
+  const order = [
+    'archive.ubuntu.com',
+    'security.ubuntu.com',
+    'azure.archive.ubuntu.com',
+  ];
+  const comments = [],
+    rows = [];
+  const unsupported = () => {
+    throw new Error('Unsupported Ubuntu runner mirror list');
+  };
+  for (const line of content.split(/\r?\n/)) {
+    if (!line.trim() || line.trimStart().startsWith('#')) {
+      comments.push(line);
+      continue;
+    }
+    const match = line.match(/^(https?:\/\/[^\s]+)\t(.+)$/);
+    if (!match) unsupported();
+    let uri;
+    try {
+      uri = new URL(match[1]);
+    } catch {
+      // Native URL errors retain input; do not echo unfamiliar runner data.
+      unsupported();
+    }
+    const metadata = match[2].trim().split(/\s+/);
+    const priorities = metadata.filter((value) =>
+      value.startsWith('priority:'),
+    );
+    if (
+      !order.includes(uri.hostname) ||
+      uri.username ||
+      uri.password ||
+      uri.search ||
+      uri.hash ||
+      uri.port ||
+      !/^\/ubuntu\/?$/.test(uri.pathname) ||
+      priorities.length !== 1 ||
+      !/^priority:\d+$/.test(priorities[0])
+    )
+      unsupported();
+    rows.push({ uri: match[1], host: uri.hostname, metadata });
+  }
+  if (
+    rows.length !== order.length ||
+    new Set(rows.map((row) => row.host)).size !== order.length
+  )
+    unsupported();
+  rows.sort((a, b) => order.indexOf(a.host) - order.indexOf(b.host));
+  return (
+    [
+      ...comments.filter((line) => line.trim()),
+      ...rows.map(
+        (row, i) =>
+          `${row.uri}\t${row.metadata.map((value) => (/^priority:\d+$/.test(value) ? `priority:${i + 1}` : value)).join(' ')}`,
+      ),
+    ].join('\n') + '\n'
+  );
+}
+
+if (
+  process.argv[1] &&
+  pathToFileURL(resolve(process.argv[1])).href === import.meta.url
+) {
+  const [input, output] = process.argv.slice(2);
+  if (!input || !output || resolve(input) === resolve(output)) {
+    throw new Error(
+      'Usage: browser-mirrors.mjs <runner-mirror-list> <prepared-list>',
+    );
+  }
+  writeFileSync(output, preferUbuntuMirrors(readFileSync(input, 'utf8')));
+}
diff --git a/scripts/fixtures/browser-mirrors.test.mjs b/scripts/fixtures/browser-mirrors.test.mjs
new file mode 100644
index 0000000..fc4461f
--- /dev/null
+++ b/scripts/fixtures/browser-mirrors.test.mjs
@@ -0,0 +1,127 @@
+import assert from 'node:assert/strict';
+import { spawnSync } from 'node:child_process';
+import { mkdtempSync, readFileSync, rmSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
+import test from 'node:test';
+import { preferUbuntuMirrors } from './browser-mirrors.mjs';
+
+const runnerList =
+  [
+    'http://azure.archive.ubuntu.com/ubuntu\tpriority:1',
+    'http://archive.ubuntu.com/ubuntu\tpriority:2',
+    'http://security.ubuntu.com/ubuntu\tpriority:3',
+  ].join('\n') + '\n';
+
+test('prefer existing official Ubuntu mirrors while retaining Azure fallback', () => {
+  const rows = preferUbuntuMirrors(runnerList).trim().split('\n');
+  assert.equal(rows[0], 'http://archive.ubuntu.com/ubuntu\tpriority:1');
+  assert.equal(rows[1], 'http://security.ubuntu.com/ubuntu\tpriority:2');
+  assert.equal(rows[2], 'http://azure.archive.ubuntu.com/ubuntu\tpriority:3');
+});
+
+test('preserve transport, metadata and comments without adding package sources', () => {
+  const source =
+    '# Hosted runner mirrors\n\n' +
+    runnerList
+      .replaceAll('http:', 'https:')
+      .replace('priority:2', 'priority:2 arch:amd64');
+  const prepared = preferUbuntuMirrors(source);
+  assert.match(
+    prepared,
+    /https:\/\/archive\.ubuntu\.com\/ubuntu\tpriority:1 arch:amd64/,
+  );
+  assert.match(prepared, /^# Hosted runner mirrors/m);
+  assert.equal((prepared.match(/https:/g) ?? []).length, 3);
+});
+
+test('refuse unfamiliar or incomplete runner lists', () => {
+  for (const source of [
+    runnerList.replace(
+      'archive.ubuntu.com/ubuntu\tpriority:2',
+      'mirror.example/ubuntu\tpriority:2',
+    ),
+    runnerList.replace(/^http:\/\/azure[^\n]+\n/m, ''),
+    runnerList + 'http://archive.ubuntu.com/ubuntu\tpriority:4\n',
+    runnerList.replace('priority:2', ''),
+  ])
+    assert.throws(
+      () => preferUbuntuMirrors(source),
+      /Unsupported Ubuntu runner mirror list/,
+    );
+});
+
+test('reject a malformed duplicate priority instead of preserving it', () => {
+  assert.throws(
+    () =>
+      preferUbuntuMirrors(
+        runnerList.replace('priority:2', 'priority:2 priority:invalid'),
+      ),
+    /Unsupported Ubuntu runner mirror list/,
+  );
+});
+
+test('malformed URLs use a generic error without retaining raw input', () => {
+  assert.throws(
+    () =>
+      preferUbuntuMirrors(
+        runnerList.replace(
+          'http://archive.ubuntu.com/ubuntu',
+          'http://[invalid]/ubuntu',
+        ),
+      ),
+    (error) =>
+      error.message === 'Unsupported Ubuntu runner mirror list' &&
+      !('input' in error),
+  );
+});
+
+test('CLI writes a prepared copy and leaves the runner input unchanged', () => {
+  const root = mkdtempSync(join(tmpdir(), 'domscribe-browser-mirrors-'));
+  try {
+    const input = join(root, 'input'),
+      output = join(root, 'output');
+    writeFileSync(input, runnerList);
+    const result = spawnSync(
+      process.execPath,
+      [
+        new URL('./browser-mirrors.mjs', import.meta.url).pathname,
+        input,
+        output,
+      ],
+      { encoding: 'utf8' },
+    );
+    assert.equal(result.status, 0, result.stderr);
+    assert.equal(readFileSync(input, 'utf8'), runnerList);
+    assert.match(
+      readFileSync(output, 'utf8'),
+      /^http:\/\/archive\.ubuntu\.com\/ubuntu\tpriority:1/m,
+    );
+  } finally {
+    rmSync(root, { recursive: true });
+  }
+});
+
+test('invalid CLI input fails without replacing an existing prepared copy', () => {
+  const root = mkdtempSync(join(tmpdir(), 'domscribe-browser-mirrors-'));
+  try {
+    const input = join(root, 'input'),
+      output = join(root, 'output');
+    writeFileSync(input, 'http://mirror.example/ubuntu\tpriority:1\n');
+    writeFileSync(output, 'unchanged');
+    const result = spawnSync(
+      process.execPath,
+      [
+        new URL('./browser-mirrors.mjs', import.meta.url).pathname,
+        input,
+        output,
+      ],
+      { encoding: 'utf8' },
+    );
+    assert.notEqual(result.status, 0);
+    assert.equal(readFileSync(output, 'utf8'), 'unchanged');
+    assert.doesNotMatch(result.stderr, /mirror\.example/);
+  } finally {
+    rmSync(root, { recursive: true });
+  }
+});