[quesera]

The startup costs are high, but once you're in the flow of doing it, it's not so bad.

I usually have a switch in our tests: default is to run against local mocks, but the test data also works against the third party sandbox environment. Periodically we revalidate our mocks against the sandbox responses to make sure the other API hasn't drifted.

CI always runs against local mocks, for speed and reliability.