You Can't Review an Agent. You Can Review a Plan.

A harness for AI-era Terraform. I'm building one. For a while now I've been developing a harness for infrastructure-as-code as a private SDK and compiler — the layer that sits between whoever proposes a change (a person, an agent, CI) and whatever actually reaches production. This post isn't the tool. It's the thinking underneath it, and the few pieces I've become most convinced by while building it. (Notes from inside the work — where I've landed so far, not advice.) The problem that sent me down this road is easy to state and easy to underrate. A version of it happened recently. An agent fixed some Terraform; the PR read clean — tidy diff, sensible resource names, a plan output that looked exactly like what I'd asked for. It got approved. And then, at apply time, a different plan ran than the one that was reviewed: apply had re-planned against state that moved in between, and the diff that touched production wasn't quite the diff anyone had read. Nothing broke, that time. But that near-miss is the whole reason the harness exists. Because the danger was never "the agent writes bad HCL." Agents write perfectly good HCL; I let them. The danger is the distance between the plan a human reviewed and the plan that actually runs — and once agents are the ones proposing changes at volume, that distance is the thing I most want to nail shut. Where I've landed for now (and expect to keep revising): What AI-era IaC needs isn't AI that can apply . It's a structure where every change — human or agent — is evaluated at the same boundary , and only a reviewed plan ships. The unit of trust isn't the agent. It's a specific, reviewed plan , bound byte for byte. You can't review an agent. You can only review a plan. Instructions to an agent can be broken. A CI gate can't be talked out of it. Put guidance in the prompt; put the guarantee in the gate. Terraform/OpenTofu don't go away. You wrap them in a harness; you don't replace them. Your repo has non-human authors now For years IaC