Use contract tests for external APIs

Dependency inventory

List the fields, headers, status codes and ordering guarantees used by the client. Distinguish required behavior from examples observed once. Pin the API version or record the documented compatibility policy.

Contract suite

Include a valid read, an empty result, pagination termination, malformed input, denied access and a stale write token where supported. Verify content type and required field types before asserting business values. Permit documented optional response fields so harmless provider additions do not break the client unnecessarily.

Two layers

Use a local fake to test difficult failures deterministically. Also run a small authorized provider-sandbox smoke check, because a fake can agree perfectly with an incorrect client assumption. Never create disposable fixtures in production without explicit permission and a cleanup plan.

Acceptance and limits

Change the fake to omit a required field or repeat a cursor; the integration should detect the contract violation. A passing test is evidence for the tested version and cases, not a promise that the remote service never changes. This is an original testing checklist; retain sanitized contract failures without credentials or full private response bodies.