Kotlin's runTest

Wed, 08 Jul 2026 00:00:00 +0000

The flaky test

Recently we had a test that would fail. The test would set up some mocks and run a call, then some assertions.

@Test
fun `non-retryable status does not retry`() = runTest {
 val engine = MockEngine { _ -> respondError(HttpStatusCode.Unauthorized) }

 val result = ...

 assertTrue(result.isFailure)
 ...
 assertEquals(401, ex.statusCode)
 assertEquals(1, engine.requestHistory.size)
}@Test
fun `non-retryable status does not retry`() = runTest {
 val engine = MockEngine { _ -> respondError(HttpStatusCode.Unauthorized) }

 val result = ...

 assertTrue(result.isFailure)
 ...
 assertEquals(401, ex.statusCode)
 assertEquals(1, engine.requestHistory.size)
}

It seemed benign, almost boring.

Time on Vegatron 🌈✨

Kotlin's runTest

The flaky test