Modal and OpenAI Agents SDK: building a parallel coding agent harness with sandboxes and GPUs

Modal published a walkthrough on building a custom coding agent harness on top of the OpenAI Agents SDK, which OpenAI launched today, using Modal Sandboxes to give agents isolated Linux environments and GPUs. The guide starts from a minimal exec-based agent, then adds sessions for memory, an orchestrator plus subagent pool for parallel async work, GPU quotas, filesystem snapshots to deduplicate setup, and a skills subsystem, ending with a harness that runs OpenAI's Parameter Golf challenge across parallel GPU subagents. Modal cites Ramp's fleet of background coding agents on Modal, which it says now produces more than half of the company's PRs.