What critical aspects need to be tested when checking API endpoints?

When testing API endpoints, it is important to check:

• Correctness of responses: compliance with format, structure, and data specifications.
• Error handling: correct status codes and messages for invalid requests.
• Performance: response time and stability under load.
• Security: authentication, authorization, and vulnerability protection.
• Boundary conditions: handling minimal, maximal, and incorrect values.
• Compatibility: support for different API versions and clients.

Example: verify that sending incorrect JSON results in a 400 Bad Request with a clear message.