How do you keep the agent to the plan you approved?

Every agent I use has some form of plan mode. I read the plan, say yes, and then somewhere in implementation it drifts — a file it was not meant to touch, a step it decided was unnecessary, a "done" that covers half the plan. On one feature I asked it to check its own work against the plan roughly thirty times.

What I do now: write the spec first, from the actual repo, approve that, and give the agent only the approved document. Review becomes a comparison. It mostly holds, but the last check is still me.

What do you do — tighter prompts, smaller tasks, a second agent as reviewer, or have you just accepted the drift?