Catching API regressions with snapshot testing

Summary

API development is a high-wire act. You’re constantly balancing new feature delivery against the terrifying possibility of breaking existing functionality. Testing, for example manually writing assertions for every field you expect back, is your safety harness. But what happens when the response payload is 500 lines of complex nested JSON? Your harness becomes a tangled mess of brittle code that takes longer to maintain than the feature itself. This is where snapshot testing shines, but it also has drawbacks of its own. Let's take a look. What is snapshot testing?​ Snapshot testing (sometimes called "Golden Master" testing) is straightforward: You take a system in a known, correct state. You capture its output (the "snapshot") and save it to a file. This is your baseline. In future tests, you run the system again and compare the new output against the saved baseline. If they match exactly, the test passes. If they differ by even a single character, the test fails, and you are presented with a "diff" highlighting the variance. Unlike traditional unit tests that ask, "Is X equal to Y?", snapshot tests ask, "Has anything changed since last time?" Why snapshot test HTTP APIs?​ While snapshot testing is popular in UI frameworks (like React), it also works pretty well in the world of APIs. Be it HTTP, REST, gRPC or GraphQL, all types of APIs can be made to work with snapshot testing. API responses, particularly large JSON or XML payloads, are notoriously difficult to test comprehensively using traditional assertions. Imagine an e-commerce API returning a product details object. It might contain nested categories, arrays of variant images, pricing structures, and localized metadata. Writing individual assertions for every one of those fields is tedious, error-prone, and results in massive test files that developers hate updating. Often, developers end up just asserting the HTTP 200 OK status and maybe checking some fields of the top-level object, leaving 90% of the payload...