Hey there! Let's walk through the most common pitfalls and fixes when you're stuck exchanging an auth code for an access token with Google's OpenID Connect:

• Double-check your client credentials
  Make sure your client_id and client_secret are exactly what's listed in your Google Cloud Console OAuth 2.0 client ID configuration. It's easy to accidentally copy extra spaces or miss a character in the client secret. Also, confirm that the client ID is tied to the correct project and your OAuth consent screen is properly set up and published (if needed for external users).

• Ensure the auth code is valid and unused
  Google's auth codes are single-use only and expire after roughly 10 minutes. If you're reusing a code from a previous test run, that's guaranteed to throw an error. Grab a fresh auth code by going through the full authentication flow again and try the exchange right away.

• Verify your request method and content type
  The token exchange endpoint only accepts POST requests with the application/x-www-form-urlencoded content type. Sending parameters via GET or as JSON will fail immediately. Your request body should follow this structure (replace placeholders with your actual values):

  client_id=YOUR_CLIENT_ID&client_secret=YOUR_CLIENT_SECRET&code=YOUR_AUTH_CODE&grant_type=authorization_code&redirect_uri=YOUR_REDIRECT_URI
  

• Match the redirect URI exactly
  The redirect_uri you use in the token exchange must be identical to the one you included when generating the initial authentication URL. This means checking for exact matches in protocol (http vs https), path, trailing slashes, and even capitalization. Don't forget to add this exact URI to the "Authorized redirect URIs" list in your Google Cloud Console client settings.

• Confirm required scopes are included
  At minimum, your authentication request must include the openid scope. If you omitted this, the auth code won't be valid for OpenID Connect token exchange. Common additional scopes like email or profile are fine, but openid is non-negotiable here.

• Check for network/proxy restrictions
  If your app server is behind a firewall or proxy, ensure it can reach Google's token endpoint. You can test this directly from your server using a command like:

  curl -X POST https://oauth2.googleapis.com/token -d "client_id=YOUR_CLIENT_ID&client_secret=YOUR_CLIENT_SECRET&code=YOUR_AUTH_CODE&grant_type=authorization_code&redirect_uri=YOUR_REDIRECT_URI"
  

  If this command fails, your network setup is blocking the request to Google's endpoint.

If you're still hitting issues, share the exact error response you're getting (like the error and error_description fields from Google's response) — that will help narrow down the problem even faster!

内容的提问来源于stack exchange，提问作者Michael Ekoka