From the source
Vercel introduced durable-harness, an open-source package that adds crash recovery to coding agent harnesses such as Pi, OpenCode, Codex and Claude Code without changing the harness itself.
- Harnesses keep long-lived sessions in memory and agent tasks often run for minutes or hours, so a function timeout or dropped connection in the cloud can lose a turn's work. durable-harness writes each step of a turn to a durable log in a World, the Workflow SDK's backend: it records an intent before a model or tool call and the outcome after, and a queue message hands a dead turn to a new function that reads the log and continues.
- Completed calls are read from the log rather than re-run, so Vercel says you are never billed twice for a finished model call's tokens, and interrupted tools marked replay: 'safe' run again while others return to the model as possibly run.
- Harnesses with their own session files, including Claude Code's transcript, Codex's rollout and OpenCode's event store, have every record mirrored into the log so a new function or Sandbox can rebuild them.
- The same agent can move between Worlds without code changes: Vercel World in production, files locally, or Postgres, MongoDB, Redis and Turso on your own servers, with two environment variables redirecting it to Postgres.
- Agent Runs on Vercel shows the trace of every session, with resumed turns appearing in the same trace.
