This is a great question about Ory Hydra's error handling behavior in OAuth2 flows. The difference in error handling is related to security considerations and OAuth2 specifications. When you make an OAuth2 request with invalid parameters, Ory Hydra handles errors differently based on the type of error: 1. Invalid

client_id

or

redirect_uri

: These errors redirect to

hydra_urls_error

because: - With an invalid

client_id

, Hydra cannot identify a legitimate client to redirect to - With an invalid

redirect_uri

, Hydra cannot safely redirect to an unverified URL (this is a security measure to prevent open redirectors) As mentioned in the documentation, Hydra validates pre-registered redirect URIs as part of its security principles: "Validate Pre-Registered 'redirect_uri'" [Hydra Security Architecture](https://www.ory.sh/docs/hydra/security-architecture). 2. Invalid

scope

or

response_type

: These errors redirect back to the client with error parameters because: - The client is valid and authenticated - The redirect URI is valid and verified - The error can be safely communicated back to the legitimate application This behavior aligns with OAuth2 specifications where the authorization server should redirect back to the client with appropriate error codes when possible, but must not redirect to unverified destinations. In the troubleshooting documentation, you can see examples of error responses that get returned to the client when parameters are invalid but the client and redirect URI are valid: [Hydra Debug](https://www.ory.sh/docs/hydra/debug). If you're developing an OAuth2 client, you should handle both scenarios: 1. Direct error responses from Hydra with error parameters in the query string 2. Cases where users might be redirected to the Hydra error page instead of your application This dual approach to error handling is an important security feature that prevents potential phishing or redirect attacks while still providing useful error information when it's safe to do so.