mastra-ai/mastra · error · Error

Failed to fetch CDP version info from ${versionUrl}: ${respo

Error message

Failed to fetch CDP version info from ${versionUrl}: ${response.status} ${response.statusText}

What it means

When connecting to an external CDP endpoint given as an http(s) URL, the browser queries <host>/json/version to discover the webSocketDebuggerUrl. If the HTTP response is not ok, it throws with the status code and status text. This means the endpoint answered but rejected the request.

Source

Thrown at packages/core/src/browser/browser.ts:929

    }

    // HTTP URL - fetch /json/version to get the WebSocket URL
    if (url.startsWith('http://') || url.startsWith('https://')) {
      const baseUrl = url.replace(/\/$/, ''); // Remove trailing slash
      const versionUrl = `${baseUrl}/json/version`;

      this.logger.debug?.(`Resolving WebSocket URL from ${versionUrl}`);

      // Add timeout to prevent hanging on dead endpoints
      const controller = new AbortController();
      const timeoutId = setTimeout(() => controller.abort(), 10000);

      try {
        const response = await fetch(versionUrl, { signal: controller.signal });
        clearTimeout(timeoutId);

        if (!response.ok) {
          throw new Error(
            `Failed to fetch CDP version info from ${versionUrl}: ${response.status} ${response.statusText}`,
          );
        }

        const data = (await response.json()) as { webSocketDebuggerUrl?: string };
        if (!data.webSocketDebuggerUrl) {
          throw new Error(`No webSocketDebuggerUrl found in CDP version response from ${versionUrl}`);
        }

        this.logger.debug?.(`Resolved WebSocket URL: ${data.webSocketDebuggerUrl}`);
        return data.webSocketDebuggerUrl;
      } catch (error) {
        clearTimeout(timeoutId);
        if (error instanceof Error && error.name === 'AbortError') {
          throw new Error(`Timeout resolving WebSocket URL from ${versionUrl} (10s)`);
        }
        throw error;
      }

View on GitHub (pinned to 75dd419e61)

Solutions

1. Verify Chrome was launched with --remote-debugging-port=<port> and curl the same versionUrl from the app host.
2. Fix proxy/ingress rules so /json/version is forwarded and returns 200.
3. Use the correct protocol prefix (ws:// / wss:// for direct WebSocket, http(s) only for the version endpoint).
4. Check the port isn't claimed by another process.

Example fix

// before
await browser.connectToExternalCdp('https://my-proxy.example.com/chrome'); // 404 on /json/version
// after
await browser.connectToExternalCdp('ws://127.0.0.1:9222/devtools/browser/<id>'); // direct ws URL

// probe the version endpoint before attaching
const res = await fetch(versionUrl.replace(/\/$/, '') + '/json/version');
if (!res.ok) throw new Error(`CDP version endpoint unhealthy: ${res.status}`);

function isCdpVersionOk(d: unknown): d is { webSocketDebuggerUrl: string } {
  return typeof d === 'object' && d !== null && 'webSocketDebuggerUrl' in d && typeof (d as any).webSocketDebuggerUrl === 'string';
}

try {
  await browser.connectToExternalCdp(versionUrl);
} catch (err) {
  if (err instanceof Error && err.message.startsWith('Failed to fetch CDP version info')) {
    // fall back to constructing the ws URL directly
    await browser.connectToExternalCdp(versionUrl.replace(/^http/, 'ws').replace(/\/$/, '') + '/devtools/browser');
  } else throw err;
}

When it happens

Trigger: The CDP URL points at a host/port where /json/version returns 4xx/5xx — e.g. a proxy requiring auth, a DevTools endpoint bound to a different path, a port occupied by a non-Chrome service, or a remote debugging endpoint returning 502/503.

Common situations: Chrome launched without --remote-debugging-port but something else listens on that port; reverse proxy rules not forwarding /json/version; connecting to a containerized Chrome through an ingress that blocks the endpoint; TLS mismatch producing a gateway error.

Related errors

• Failed to fetch CDP version info from ${versionUrl}: ${respo
• No webSocketDebuggerUrl found in CDP version response from $
• Timeout resolving WebSocket URL from ${versionUrl} (10s)
• Slack OAuth HTTP error: ${tokenResponse.status} ${tokenRespo
• Failed to stream background tasks: ${response.statusText}

AI-assisted analysis of mastra-ai/mastra@75dd419e61 (2026-08-30). Data as JSON: /api/errors/9674dd2deef5f673. Report an issue: GitHub.