What criteria do you use when assessing the quality of testing a specific API endpoint?

When assessing the quality of API endpoint testing, I pay attention to the following criteria:

• Test coverage: Are all main usage scenarios covered, including positive and negative cases?
• Testing all HTTP methods: If the endpoint supports multiple methods (GET, POST, PUT, DELETE), tests should cover all of them.
• Input validation: Tests should verify how the API handles incorrect or missing parameters.
• Response validation: Status codes, response structure, and content should conform to the specifications.
• Boundary testing: For example, maximum parameter length, empty values, special characters.
• Performance and stability: If possible, response time and behavior under high load are tested.
• Test automation: Availability of automated tests that can be run regularly.

Example of a simple PHP test using PHPUnit and Guzzle for a GET request:

public function testGetUserEndpoint() {
    $client = new \GuzzleHttp\Client(['base_uri' => 'https://api.example.com']);
    $response = $client->request('GET', '/users/1');

    $this->assertEquals(200, $response->getStatusCode());
    $data = json_decode($response->getBody(), true);
    $this->assertArrayHasKey('id', $data);
    $this->assertEquals(1, $data['id']);
}