effect-testing

Use it.effect or it.live from @effect/vitest to run Effect-returning tests directly with yield* inside Effect.gen. Provide dependencies with Effect.provide and layers, and assert with standard vitest expect or utilities like assertSome and assertSuccess from @effect/vitest/utils.

Use it.live as the default for tests touching services, databases, HTTP, or filesystems, since it uses real clock and random services. Reserve it.effect with TestClock only for tests that explicitly simulate time advancement, because TestClock causes hangs in integration tests.

Use Effect.flip to convert a failure into a success value you can assert on, or Effect.exit to capture the full Exit and inspect the Cause. In Effect v4, use Cause.hasFails and Cause.findErrorOption to check failure types.

Yes, but @effect/vitest helpers are unavailable, so build a custom harness using Effect.runPromise with Layer.provideMerge to supply TestClock and TestConsole layers. As of beta.70, TestClock.layer works with runPromise without an ambient Scope.

TestClock intercepts time-dependent operations like Effect.sleep, so real I/O combined with the test clock never advances and the test hangs. Switch those tests to it.live or exclude test services, keeping TestClock only for explicit time-simulation tests.

Prefer an HTTP mock server over service fakes: define a TestServer service with semantic helpers, implement it with Layer.effect and NodeHttpServer on port 0, and use Deferred-based request counting for synchronization. This tests serialization, status codes, and SSE framing through the real service layer.