Housing wants confidence in SCD helper functions before they enter the eligibility pipeline. What testing step should come first?

Housing teams that want confidence in SCD helper functions before those helpers enter the eligibility pipeline should add unit tests for the helpers first. Unit tests exercise Type 1/2 update logic, effective dating, and edge cases (null keys, same-day changes, late arrivals) in isolation—fast feedback before the helpers are bolted into the broader eligibility job. Catching helper bugs with fast unit tests in isolation is far cheaper than discovering them only after running integration tests for the SCD helpers, which require dependencies and give slower first feedback. Adding integration tests for the SCD helper functions before integrating them into the eligibility pipeline fails as the first step because it skips isolated unit validation of Type 1/2 logic, effective dating, and edge cases in favor of heavier dependency-involved checks. Adding performance tests for the SCD helper functions before integrating them into the eligibility pipeline fails because it measures speed and throughput rather than validating the correctness of Type 1/2 update logic and effective dating the stem asks for. Adding end-to-end tests for the SCD helper functions before integrating them into the eligibility pipeline fails as the first step because it exercises broader flows before the helper logic is unit tested in isolation, so helper defects cannot be isolated quickly. Exam caveat: unit tests target helpers/pure logic; integration and E2E tests cover landing-to-silver or extract-to-gold paths. The exam often asks which test type fits which stage—here the first step for helpers is unit testing. Tests belong in CI on the PR, not only on a laptop. Operational check: add unit tests for SCD helpers covering insert, update, no-op, and boundary dates; run them in CI on the feature branch; only then integrate helpers into the eligibility pipeline tasks. Confirm integration tests for the helpers are not the first check where helper behavior is validated — unit tests must come first.