The package records each step of a turn in a durable log, recording intent before model or tool calls and outcomes when they finish. A queue message backs the turn; if a function dies, a replacement reads the log and resumes from completed work rather than repeating completed calls.
The release also makes replay behavior explicit at the tool boundary: tools marked safe can run again after interruption, while tools marked never are not automatically repeated and the model is told the call may have occurred. Vercel says the same interface can use its hosted World, local files in development, or self-hosted backends including Postgres, MongoDB, Redis, and Turso. The package is presented as an adapter layer that adds durability without changing the harness itself. Its guarantees depend on tool replay policy and deployment configuration; the announcement does not establish a general guarantee that arbitrary external side effects are exactly-once.