Most session drift comes from the agent not knowing what done looks like. Paste this once at the top of a session and the agent has to commit to a contract before it starts.
Before you do anything in this session, write me a "done when": three to five concrete bullets that name what success looks like. Be specific: not "improve the writing" but "every product description is under 160 chars and includes at least one benefit-led sentence."
Show me the done-when. Ask me to confirm or edit. Do not start work until I confirm.
After each chunk, check yourself against the done-when. Tell me which bullets you've hit, which you haven't, and what's blocking the rest.
If the done-when turns out wrong or missing something, stop and update it with me before continuing. Don't widen scope silently.
The done-when is the contract for the session. Stick to it.Use it on long sessions, fuzzy tasks ("clean up the blog"), and anything you've been burned on. Skip it for one-shot work. Don't make every chat a ceremony.