Test First

Test First is the discipline of writing the test before the code it checks, so that the expectation is captured as an executable statement rather than something you get around to verifying afterward. It is the core idea underneath Test-driven Development and behavior-driven development: describe the desired behavior, watch it fail, then build until it passes. The name emphasizes the ordering — the test comes first, and the implementation exists to satisfy it.

• Expectation before implementation - You state what success looks like up front, which keeps you honest about requirements instead of rationalizing whatever the code happens to do.
• Tests as design tool - Writing the test first pushes you to design a clean, callable interface, since awkward-to-test code is a signal of awkward design.
• Regression safety net - Every expectation you write stays in the suite, catching the day a future change quietly breaks old behavior.
• Small, verifiable steps - Work proceeds in increments each guarded by a test, so you always know exactly what you just changed.

For APIs, test-first means the contract — endpoints, schemas, status codes, error payloads — gets asserted before the service is built, whether that lives in Test Scripts, a Postman collection, or a spec-first workflow driven off OpenAPI. Those assertions then run continuously through design, build, and production, and they double as the ground truth that governance rules and API-consuming agents check against. Teams that genuinely work test-first tend to ship APIs whose documented behavior and actual behavior match, which is exactly the property that makes an API dependable enough to build a business on.