Bun "bun test" Fails or Hangs in CI

Bun’s built-in test runner is Jest-like but not identical. A suite that passed under Jest can fail or hang under bun test because of API differences, a missing preload/setup, or async tests exceeding the default timeout.

What this error means

bun test fails with undefined test globals/matchers, an unloaded setup file, or tests that hang until the timeout. The same tests may pass under Jest, which points at a runner difference, not a logic bug.

bun output

error: Cannot find name 'jest'. Did you mean 'Bun'?
# or
1 tests timed out after 5000ms
  at test/api.test.ts:12

Common causes

Jest-specific APIs or globals

Bun implements much of the Jest API but not all of it. Code relying on Jest-only globals/matchers or config can fail under Bun’s runner.

Missing preload/setup or short timeout

A setup file (mocks, env) configured for Jest is not loaded by Bun unless declared, and Bun’s default test timeout can be shorter than a slow async test needs.

How to fix it

Use Bun’s test API and preload setup

Import from bun:test and declare a preload in bunfig.toml so setup runs.

bun:test / bunfig.toml

// import from bun:test
import { test, expect, beforeAll } from "bun:test";

// bunfig.toml
[test]
preload = ["./test/setup.ts"]

Raise the timeout for slow tests

Terminal

bun test --timeout 20000
# or per test:
# test("slow", async () => { ... }, 20000)

How to prevent it

Import test helpers from bun:test rather than relying on Jest globals.
Declare setup files via bunfig.toml [test] preload.
Set realistic timeouts for async/integration tests.