bombadil

Bombadil is property-based testing for web UIs — it randomly and systematically explores your app's state space, doing things you didn't think to test. Unexpected action sequences, weird timings, strange inputs. It finds the bugs your test suite misses because no human wrote a test for that exact combination.

Built by Antithesis (the deterministic simulation company), the killer feature is perfect replay — when it finds a bug, you get the exact nanosecond-level event sequence that caused it. Write specs in TypeScript, run locally or in CI. Compared to Playwright (scripted paths), Bombadil finds bugs you didn't know to look for. Compared to fuzzing tools, it understands web UI semantics.

Use this when your web app has complex state interactions and you've exhausted manual test scenarios. Skip this if your app is a simple CRUD — the setup overhead won't pay off.

The catch: nascent (913 stars), requires thinking in properties rather than assertions, and the Antithesis platform integration is where the real power lives — the open-source local runner is the demo, not the product.