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New: Replays now compare against the last 90 days. Read the changelog

Senior backend engineer, Runs

Own the system that runs every agent step, from scheduling and retries to budgets and the write gate.

Every Capstan agent is a run: a plan of steps, each one a model call or a tool call, some of them waiting for a person. The Runs team owns the system underneath. It schedules steps, retries the ones that fail, stops a run that hits its budget, and makes sure no write leaves Capstan unless the agent's owner allowed it.

We run around 40 million steps a month today, and that number doubles every five months. The system is TypeScript on Node, Postgres, and a queue we wrote ourselves after outgrowing two we didn't.

What you'll do

  • Own the step scheduler: fair queuing across workspaces, priorities for runs waiting on a person, and backpressure when a tool rate-limits us.
  • Make the write gate faster and harder to get wrong. Every write a step makes passes through it, is checked against the agent's permissions, and is either sent, held for approval or recorded during a replay.
  • Design how budgets stop a run mid-step without leaving half-finished work in a customer's tools.
  • Work with the Replays team on recording writes instead of sending them, at the volume of a 90-day replay.
  • Be on call one week in six, with a second person always beside you.

What you'll bring

  • Five or more years building backend systems that other people rely on, in any language. We use TypeScript; you'll learn it if you need to.
  • Experience with queues, retries and idempotency in production, and the scars that come with them.
  • Comfort with Postgres beyond an ORM: query plans, locks, migrations on large tables.
  • Clear writing. Most of our design work happens in documents before it happens in code.

Nice to have

  • You've worked on a workflow engine, a job system or a payments platform.
  • You've run something on call that woke you up, and fixed it so it didn't again.

How the first months look

Your first week is spent with a customer's support team, watching the work their agents do. By the end of your first month you'll have shipped a change to the scheduler. By the third, you'll own one part of it.

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Apply for this role

Five fields, no account and no cover letter. Here's what happens after you send it:

  1. 1A conversation · 30 minutes
  2. 2A paid work sample · 3 hours, $300
  3. 3A day with the team · 4 conversations
  4. 4A decision · Within 5 working days

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