I don't think you understand what I'm saying. If, during a session, Claude makes any change to a source file, the stop hook script FORCES Claude to run the existing tests. There is literally no way Claude can get around running the tests because the prompt will not stop being processed until the stop hook script passes.

There is no contradiction. Stop hooks (as well as all the other hook types) are the only way to force Claude to work deterministically.

Hook scripts can be as simple or as complex as you like: you define the success criteria. For example, if Claude just added a new feature but didn't create a test for it, then a stop hook would prevent Claude from stopping until the test was written.