Contract test
A check that a real outside service still behaves the way your test doubles say it does. If the provider renames a field, this fails, even while everything else stays green.
A double only remembers how the service worked on the day it was written. An agent trusts it completely, and will happily build on a shape the real service stopped sending months ago.
Say it to your agent
“Add tests/contract/payments.contract.test.ts. It calls the provider's sandbox and checks that the response has the same fields and types as our FakeGateway returns. Run it nightly, not on every commit.”