A Durable Coding Agent – with Modal and Restate

We built this to explore what it takes to move a coding agent from "works on my laptop" to "handles millions of users in production."

The core insight: durability matters more than you'd think for agents. When an agent takes 5-10 minutes on a task, crashes become inevitable. Rate limits hit. Sandboxes timeout. Users interrupt mid-task. Traditional retry logic gets messy fast.

Our approach uses Restate for durable execution (workflows continue from the last completed step) and Modal for ephemeral sandboxes. We get automatic failure recovery, interruptions for new input, great scalability, and scale-to-zero without any custom retry code. The tradeoffs: coupling to Restate's execution model and requiring discipline around deterministic replay.

How are you handling long-running agent workflows to make them run reliably at scale?