Before each model or tool call, durable-harness writes an intent to a log, and when the call finishes it writes the result. A queue message backs up the whole turn. If the function times out while, say, CI is still running, the queue hands the turn to a new function, which reads the log and carries on.
Finished model calls are read back from the log instead of re-run, so Vercel says you don't pay twice for the same tokens. Tools carry a replay policy. A tool marked replay: 'safe' (a file read, a CI status check) runs again on its own. Anything else goes back to the model flagged as possibly already run, and the model decides whether to check before retrying.
Claude Code and Codex always run in a Vercel Sandbox, since they expect a real shell. Pi and OpenCode can use a virtual workspace kept in the session log. The log lives in a Workflow SDK "World": Vercel's own on Vercel, files on your laptop in development, or Postgres, MongoDB, Redis or Turso if you self-host. Vercel published it on October 19.