agents-sdk/references/durable-execution.md

Durable Execution

Fetch https://developers.cloudflare.com/agents/api-reference/durable-execution/ for complete documentation.

Fibers let agent work survive Durable Object eviction. Progress is checkpointed to SQLite; on recovery, you decide what to do.

runFiber

export class MyAgent extends Agent<Env, State> {
  async onRequest(request: Request) {
    await this.runFiber("process-data", async (ctx) => {
      const step1 = await fetchData();
      ctx.stash({ step: 1, data: step1 });

      const step2 = await transform(step1);
      ctx.stash({ step: 2, result: step2 });

      this.setState({ result: step2 });
    });
    return new Response("Started");
  }

  async onFiberRecovered(ctx) {
    const checkpoint = ctx.stash;
    if (checkpoint.step === 1) {
      const step2 = await transform(checkpoint.data);
      this.setState({ result: step2 });
    }
  }
}

Key APIs

API	Purpose
this.runFiber(name, fn)	Start a named fiber
ctx.stash / this.stash	Read latest checkpoint
ctx.stash = data	Write checkpoint (JSON-serializable)
onFiberRecovered(ctx)	Called on DO restart if fiber was in-flight
keepAlive()	Prevent hibernation while fiber runs
keepAliveWhile(fn)	Keep alive for duration of async function

Important

• stash replaces the entire checkpoint — not a merge
• The lambda is NOT restored on recovery — only the stash data is. You must re-derive what to do in onFiberRecovered
• No auto-retry on throw — handle errors yourself
• For long-running pipelines with automatic retries, use Workflows instead
• Filter concurrent fibers by ctx.name in onFiberRecovered