Lockstep is a concurrency test runner for the BEAM. You write what looks like an ExUnit test; it runs many times under controlled scheduling, and when it finds a race it gives you the exact schedule to replay.

Real bugs surfaced building it: Phoenix.Tracker pre-fc5686f, NimblePool pre-e18f45f, Hammer’s Atomic.FixWindow.inc/4.

Features:

• 5 scheduling strategies + trace replay + counterexample shrinking
• Drop-in OTP wrappers (GenServer, Task, Registry, Supervisor, ETS, Atomics, …)
• AST rewriters for Elixir + Erlang — works on unmodified upstream source
• Multi-node Lockstep.Cluster with partition / heal
• Optional Anthropic Claude integration to explain failing traces

https://github.com/b-erdem/lockstep