1.6The Testing Pyramid for APIs

Traditional testing pyramids emphasize unit tests at the base, integration tests in the middle, and end-to-end tests at the top. For API testing, we use a modified pyramid:

              /\
             /  \
            / E2E\
           / Flow \
          / Tests  \
         /----------\
        / Contract   \
       /   Tests      \
      /----------------\
     / Schema Validation\
    /   & Drift Detection\
   /----------------------\
  / Endpoint-Level Tests   \
 / (Happy path, errors,    \
/   boundaries, auth)       \
-----------------------------

Layer 1: Endpoint-Level Tests verify that each endpoint handles valid inputs, rejects invalid inputs, enforces authentication, and returns documented status codes. These are the most numerous tests and run fastest.

Layer 2: Schema Validation and Drift Detection verify that actual API responses match the documented schema. These catch structural regressions and documentation staleness.

Layer 3: Contract Tests verify that services communicate correctly across boundaries. Consumer-driven contracts ensure that API changes do not break downstream services.

Layer 4: End-to-End Flow Tests verify complete business flows that span multiple services and communication channels (synchronous APIs, events, webhooks).