Answer.

Automating negative scenarios is an integral part of a comprehensive testing system. These are checks where the system's resilience to erroneous actions, incorrect data, service failures, and other abnormal situations is validated.

Background:

Previously, the main focus was on positive scenarios (happy paths) because they are easier to automate and maintain. However, quality requirements have increased, and more bugs are now arising at the boundaries with erroneous or not obviously incorrect conditions. Manual testing of these quickly becomes obsolete, whereas automation allows for tracking degradation.

Problems:

Identifying and covering all possible negative scenarios
Correctly validating error messages and the validity of the system's state after an operation fails
Maintaining tests amidst changes in business logic

Solutions:

Identifying and systematizing negative scenarios (Boundary Value Analysis, Equivalence Partitioning)
Using custom asserts and validators to check error texts, HTTP codes, exceptions
Automatically "cleaning" the system state after a negative test

Key features:

Explicit formulation of tests for errors, exceptions, and non-standard input data
Verification of error messages and the internal state of the system
Control of side effects: after a negative test, the system should remain in a consistent state

Trick Questions.

Is it enough to only check what errors are returned during negative testing?

No. It is important to check not only the content of the error but also that there was no change in the system's state after the error (for example, that an incorrect entry was not added to the database).

Should all possible negative scenarios be automated?

No. There can be infinitely many; it's necessary to highlight the most likely and critical ones (Boundary Value, Null/Empty, incorrect type, SQL injections, etc.), while handling others as bugs arise.

Can the same handling be used for all negative cases?

No. Different negative scenarios require different checks, exception handling, and rollbacks; template asserts are insufficient.

Common Mistakes and Anti-Patterns

Ignoring negative scenarios or covering them "for show"
Unconvincing error checks (for example, only checking for a 500 code but not checking the text or side effects)
Unclean test environment after a test failure

Real-Life Example

Negative Case

The system only tests valid registration scenarios. Registration with an empty email is not automated. As a result, the problem that a user can register with an empty email is noticed only in production.

Pros:

Easy test maintenance
Low number of automated tests and quick runs

Cons:

Critical bugs go live
Increased costs for fixing and rolling back

Positive Case

For each negative scenario (missing email, incorrect format, SQL injection), there is an automated test that explicitly checks for the absence of a new account and the content of the error message.