The most common Ractor error is trying to access a non-shareable object (e.g. a plain Hash) from a non-main Ractor. Mitigations: pass the object in via Ractor#send, call .freeze on it so the Ractor knows it cannot mutate, or call Ractor.make_shareable (which accepts options like deep_freeze). Advice: while prototyping, just freeze or make_shareable.