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Durability & Resume

Long-running agents outlive the processes that start them. A run can take minutes or hours, span a deploy, or pause for a human approval that arrives tomorrow. Durability is how the harness survives that: checkpointing progress so a run can pause, crash, and resume exactly where it left off — without redoing work.

A stateless agent that crashes after step 40 of 50 starts over and pays for steps 1–40 twice — if it can recover at all. A durable one reloads its journal, skips the 40 completed steps instantly, and continues from 41. For anything long or expensive, this is the difference between viable and not.

Structure​

Each completed step is journaled. On resume, the harness replays the journal — completed steps return cached results instantly — and only the first incomplete step onward runs live.

How It Works​

  1. Checkpoint at step boundaries — after each completed step (a turn, a tool call, a sub-agent), append its result to a durable journal keyed to the session.
  2. Make steps addressable — each step has a stable identity so a replay can match a completed step to its cached result.
  3. Resume by replay — on restart, walk the journal: return cached results for completed steps, and execute the first incomplete one live.
  4. Pause as a first-class state — a run waiting on human approval or an external event suspends durably and wakes when the event arrives, holding no process.
  5. Handle determinism — replay assumes steps are reproducible; isolate nondeterministic inputs (time, randomness) so a resumed run matches the original.

Key Characteristics​

  • Journaled steps, not periodic snapshots — recording each step's result lets resume skip exactly the completed work, not "the last save point."
  • Resume is replay — completed steps return instantly from the journal; only new work executes. Same inputs, same prefix, zero rework.
  • Pause without holding resources — a durable suspend means a run waiting a day for approval consumes nothing while it waits.
  • Determinism is a precondition — uncontrolled time/randomness in steps breaks replay; pass them in or stamp them so the prefix stays stable.
  • Durability underpins long-horizon agents — without it, run length is capped by process uptime.

Pitfalls​

  • No checkpoints — a crash loses the entire run and reruns everything, doubling cost or losing the work outright.
  • Nondeterministic steps — calling now() or a random source mid-step makes replay diverge from the original run.
  • Snapshot-only persistence — coarse snapshots force re-execution of everything since the last one; journal at step granularity instead.