Ask HN: How do services like Zapier, IFTTT test API integrations at scale?

Services like Zapier, IFTTT, etc have to provide integrations with thousands of other services. The common suggestion is to mock the 3rd part APIs for integration tests. But mocks do have their limitations. Like,

- It's possible to add breaking changes on your end but mock responses might still be happy with it, so your tests will pass.

- In large enough numbers of integrations, some 3rd party services will eventually introduce breaking changes without following due/expected procedures.

If you are developing a service that stands in the middle of so many other services as an integration point, how would you make sure things stay in place proactively?