Game rules stability is the property where, if you change the game code, you don't break how cards behave. To help ensure this, code design on Artifact is heavily test-driven. If we add a mechanic we can auto-check that we didn't break a bunch of card interactions. New cards are implemented through tests even before the card is playable on the client. When we find a rules bug, we also write a test for that case so we'll know if we break it again in the future. We like self-documenting tests, so we end up with some crazy test names.