The first session with a coding agent is crisp: it reads the code, makes a clean change, writes a test. By the thirtieth, something has rotted — it re-discovers the same files, contradicts a decision it made three weeks ago, guesses the test command, and quietly stops writing tests because nobody made it. The fix isn’t a better model. It’s an operating system around the model : a constitution, an autonomy boundary, tiered context, durable memory, a review panel, and a post-edit hook.

A useful way to say it: the agent is the model plus its harness — the prompts, the tools, the context policies, the hooks, the sandboxes, the sub-agents, the observability wrapped around the raw model. The model is maybe a tenth of what decides whether you get working software over a quarter; the harness is the rest. The evidence is blunt.…