Gmail OAuth Connection Fails with Error 400: redirect_uri_mismatch in n8n

Below is a checklist that might help resolve the issue:

Verify the URI Format Exactly Matches

• Protocol & Domain:
  Ensure that the URI you registered in the Google Cloud Console exactly matches what’s used by n8n. For example, if n8n is using http://localhost:5678/ then register that URI (including the trailing slash if necessary).

• Exact Path:
  The OAuth callback provided by n8n is usually something like:
  http://<your-n8n-domain>/rest/oauth2-credential/callback
  Double-check that this is exactly the same in your Google credentials.

Credential Type and Configuration

• Application Type:
  Make sure you created your credentials as a Web Application rather than a desktop application. The redirect URI setting in the Google Cloud Console differs by type.

• Multiple URIs:
  In some cases, there may be subtle differences (for example, an extra trailing slash or a slight mismatch in capitalization). Remove any ambiguity by copying the full redirect URI from your n8n node’s credentials settings and pasting it into the Google Cloud Console.

Environment Considerations in Docker

• Exposed Host and Port:
  If n8n runs in a Docker container, the internal URI might differ from the external address used by your browser. Confirm that the address in your credential settings is the same one you access n8n through.

• Network Settings:
  Verify that no networking or reverse-proxy settings are altering the effective callback URL (e.g., via HTTP vs. HTTPS).

Additional Steps

• Clear Cached OAuth Tokens:
  Sometimes stale or cached tokens may cause issues. If possible, revoke any previously issued tokens in the Google Cloud Console and try the OAuth flow again.

• Recheck Google Cloud Console Settings:
  It can help to remove and re-add the redirect URI in the Cloud Console. Make sure that after saving the changes, the configuration is active.

Following these points should help narrow down the cause of the "Error 400: redirect_uri_mismatch" error and get your Gmail node connected successfully in n8n. If you continue to have issues, consider reviewing your n8n log output for more detailed clues on the actual request URL being used.