Constrain the model upfront. "Don't add new state, don't add new dependencies, don't add new files unless I explicitly ask." Write it once in your AGENTS.md and reference it. Without constraints, the model defaults to "add everything that might be useful" which is the opposite of what shipped code needs.
Prompt to paste
Constraint for this whole session: don't add new state, new dependencies, new files, or new abstractions unless I explicitly ask. Implement exactly what I request and nothing more. If you think something extra is needed, propose it first and wait for my yes.