Http Contract Regression Testing

Make the old behavior fail loudly

• Configure the “old path” to fail in the test so fallback behavior is visible.
• Example: if the fix should stop locally downloading a snapshot, make any local snapshot fetch return 500 and assert it is never called.

Use direct controller tests for health/validation logic

• If validation is private but reachable through a controller action, instantiate the controller directly with:
  • in-memory EF AppDbContext
  • fake IHttpClientFactory
  • mocked logger
• This is faster and more surgical than a full host integration test.

Examples

• src/tests/Farm.Web.Api.Tests/Services/FailureDetection/ObicoFailureDetectionServiceTests.cs
  • proves the service should use GET /p/?img=...
  • proves local snapshot re-fetch is not part of the upstream contract
• src/tests/Farm.Web.Api.Tests/Controllers/ObicoServerControllerTests.cs
  • proves create/health validation must probe the GET prediction endpoint

Anti-Patterns

• Testing only deserialization — misses request-method/path regressions.
• Testing only the runtime service — misses setup/health validation mismatches.
• Using guessed payload objects — creates green tests against the wrong contract.
• Allowing fallback behavior in the happy-path test — hides that the new contract still is not truly supported.