Rent the loop. Build the harness. The line runs exactly where your domain starts.

The build-versus-buy advice this year mostly cuts it wrong. It treats the agent as one object you buy or assemble. That object doesn’t exist. What exists is a generic loop with a harness around it, and they belong on opposite sides of the line.

The loop is a commodity. A model picks the next step, a tool runs, the result comes back. Anthropic will now run the whole thing for you, sandbox and session log included, for eight cents a session-hour. Rent it.

The harness is the part that knows your deploy gates, your blast radius, what correct means on your data. None of it is visible from where the SDK was written.

There’s a clean test for which side a thing sits on. If it’s knowable from outside your company, rent it. If you only learned it by being in your own production when something broke, build it.

Part two of the harness series, on where the line actually goes: https://joaofogoncalves.com/articles/2026/06/11-rent-the-loop-build-the-moat/