Adam — Modal engineer, maintainer of Prefect and FastMCP — on Hamel Husain’s channel, making the case that environment engineering, not agent design, is the hard part of background agents. ~27 minutes.

The thesis

  • Agents have converged on a universal design (harness + context); the differentiator is the environment they operate in
  • An homage to Anthropic’s “Don’t Build Agents, Build Skills” — one layer up: “the hard part of background agents is the background part, not the agent part”

Dev boxes, not sandboxes

  • A bare sandbox can’t run a server, take screenshots, or profile on a GPU — an agent is doomed without a machine prepped for its task
  • CI/CD treats environments as jobs: clone, run deterministic tests, throw away
  • Agents need sessions: resumable state (snapshot the filesystem, resume mid-thought), an environment that’s alive (repo drifted, ffmpeg now needed), a different security boundary, and cold starts that don’t make you wait 2-10 minutes every spin-up

What Ramp and others converged on

  • Per-repo images defined in code, rebuilt on a 30-minute schedule so agents boot warm in under a second
  • Secrets kept out of the environment — agents submit requests to a proxy sidecar that attaches credentials deterministically
  • Result: ~1M sandboxes, ~70% of Ramp’s PRs from background agents
  • Async image builds mean humans never pay for the expensive rebuild

Control plane vs data plane

  • The old pattern (agent + tools in one process) means one aberrant tool call kills the agent holding the state
  • Put a fire door between the planner and the executor: the agent lives in the control plane and executes work in a throwaway dev box — at worst the data-plane environment gets corrupted, never the agent
  • Same principle as Airflow never running Spark itself

Build vs buy

  • The dev box supply chain is where to invest; Modal’s docs/examples cover single coding agents, a Lovable-style builder, LangGraph, computer-use, code interpreters
  • Observability of “software factories” — instrumenting the impact of hundreds of agents, not one agent — is an underexplored open problem

“The hard part of the background agents is always the background part and not the agent part.”