Smart zone

The smart zone is the part of a session where the agent does its best work: context is small and relevant, instructions are fresh, and the model can hold the whole task in view. Everyone who uses coding agents has felt the other side of it, the dumb zone, where a session that has run for hours starts making mistakes it would never have made in its first ten minutes. The model did not change. The context did, and attention degradation did the rest.

Where the line falls depends on the model and on what is in the window, and it is fuzzy rather than sharp, but it usually arrives well before the context window is full. Plenty of free window is no guarantee; quality can be gone long before capacity is. The signs are behavioural: repeated corrections, drifting from the plan, confident claims about files it read a long time ago.

Treat the zone as the resource you are spending, not the token count. Start a task in a fresh session so it lands in the sharp part; when a job clearly exceeds one session's good stretch, break it at a natural seam and hand off with a written plan. The tempting shortcut, adding a second job to a session because it 'already has the context', is exactly what pushes the second job into the dumb zone.