End-to-End Testing vs API Testing: Which Should You Prioritize? (2026)

End-to-end testing and API testing both validate that software works correctly, but they operate at different layers, speeds, and costs. The evidence strongly favors API testing as the primary investment: faster, cheaper, more reliable, and catching 80-90% of defects before they require expensive UI reproduction. This guide provides a comparison table and makes the case for an API-first testing strategy.