What causes "error fetching ${url}: ${response.status} ${response.statusText}" in GoogleChrome/lighthouse?

Check the URL printed before the fetch and verify it is reachable in a browser/curl.

error fetching ${url}: ${response.status} ${response.statusT

Error message

error fetching ${url}: ${response.status} ${response.statusText}

What it means

Thrown by fetchString() in the Lantern collection script when a fetch() to a URL completes but returns a non-ok HTTP status. This is the generic HTTP-fetch guard used by the WPT integration to download pages, API JSON, and trace files.

Source

Thrown at core/scripts/lantern/collect/collect.js:50

/**
 * @param {string} filename
 * @param {string} data
 */
function saveData(filename, data) {
  fs.mkdirSync(common.collectFolder, {recursive: true});
  fs.writeFileSync(`${common.collectFolder}/${filename}`, data);
  return filename;
}

/**
 * @param {string} url
 * @return {Promise<string>}
 */
async function fetchString(url) {
  const response = await fetch(url);
  if (response.ok) return response.text();
  throw new Error(`error fetching ${url}: ${response.status} ${response.statusText}`);
}

/**
 * @param {string} url
 */
async function startWptTest(url) {
  if (!WPT_KEY) throw new Error('missing WPT_KEY');

  const apiUrl = new URL('/runtest.php', WPT_URL);
  apiUrl.search = new URLSearchParams({
    k: WPT_KEY,
    f: 'json',
    url,
    location: 'gce-us-east4-linux:Chrome.3GFast',
    runs: '1',
    lighthouse: '1',
    mobile: '1',
    // Make the trace file available over /getgzip.php.

View on GitHub (pinned to 9515cd4e58)

Solutions

Check the URL printed before the fetch and verify it is reachable in a browser/curl.
If fetching a trace too early, ensure the polling loop (pollTestUntilComplete) has confirmed test completion first.
Retry transient WPT errors after a short delay.

Defensive patterns

Strategy: retry

Validate before calling

async function fetchStringWithRetry(url, retries = 3) {
  for (let i = 0; i < retries; i++) {
    const res = await fetch(url);
    if (res.ok) return res.text();
    if (res.status < 500 && res.status !== 429) throw new Error(`error fetching ${url}: ${res.status} ${res.statusText}`);
    await new Promise(r => setTimeout(r, 2000 * (i + 1)));
  }
  throw new Error(`error fetching ${url} after ${retries} retries`);
}

Try / catch

try {
  const data = await fetchString(url);
} catch (e) {
  if (/error fetching/.test(e.message)) { /* log and retry or skip */ }
  throw e;
}

Prevention

Verify URLs are well-formed and reachable before bulk fetching.
Only fetch trace URLs after confirming the WPT test is complete.
Retry transient 5xx/429 errors with backoff.

When it happens

Trigger: Fetching a URL (WPT test JSON, trace file, or page HTML) that returns 4xx/5xx — e.g. 404 for a not-yet-available trace, 500 from WPT, or network proxy errors.

Common situations: WPT test not finished when trace URL is fetched; WPT service outage; incorrect URL construction; expired test results.

Related errors

AI-assisted analysis of GoogleChrome/lighthouse@9515cd4e58 (2026-08-13). Data as JSON: /api/errors/bfd18adb17477d12. Report an issue: GitHub.