Ask HN: How should I test backing services that change frequently/break my code?

Hi folks,

I'm working on a nodeJS application that acts as an API gateway for a mobile app being actively developed. This API gateway doesn't have any database, and formats/forwards requests to their appropriate services, then returns the expected response to the mobile app.

Obviously testing my code and mocking the backing services is necessary to make sure it works as expected. However those services are complex, aren't always stable, and sometimes changes are introduced to them that break the functionality of my API.

I'm wondering what the best testing/monitoring strategy would be here that would allow me to reduce bugs in my API, and in case of breaking changes be notified in order to make the necessary changes?

Thanks