bombadil

Bombadil explores your UI on its own and checks that your correctness rules still hold. It is property-based testing pointed at interfaces: you define what "correct" means, and it hunts for every way the app violates that. MIT licensed, written in Rust, from the team at Antithesis.

Traditional end-to-end tests replay paths you scripted. Bombadil takes paths you never thought of. It clicks, types, and navigates, checking your invariants at every step, and when something breaks it hands back the exact sequence of actions that broke it. That reproduction case is the part that saves you the afternoon.

Free and open source with no paid tier. It covers web and terminal UIs now, and it is moving fast, with releases through 2026 and a version number still in the zeros. Solo and small teams: worth pointing at one flow you actually care about. Larger teams: treat it as a supplement to the existing suite rather than a replacement, at least until the API settles.

The catch is that property-based testing makes you do the thinking. You have to define properties like "no console errors", "this element always exists", "this form never submits empty". Writing good ones is a skill, and bad ones either cry wolf or quietly miss real bugs. Autonomous exploration of a large app is also slow, so a full run is not something you drop into every pull request.