Three test types that check completely different things, at completely different costs — and why the shape of your test suite matters as much as its size.
"We have good test coverage" is a claim that hides an important question: coverage of what kind, at what layer? A unit test that mocks every dependency can hit 100% line coverage on a function while never once proving that function actually works when wired to the real database it calls in production. Unit, integration, and end-to-end (E2E) tests each verify a different scope of the system — a single function in isolation, a handful of real components working together, or an entire user-facing flow through the deployed app — and each comes with a different speed and cost. Understanding that tradeoff, not just writing "more tests," is what the testing pyramid is actually describing.