An agent working on one goal for hours or days, usually across many sessions that cannot remember each other. What lets it last is outside the model: a plan on disk, notes on progress, tests it can run, and a clean way to start again.
Nothing goes well for long on its own. Each session starts cold, and long ones drift or declare victory early. A run this long is decided by the harness you set up before it began.
Say it to your agent
“This will take more than one session. Before you code anything, write the plan to PLAN.md and a feature list with a test for each. In each session, read those first and work on one item only.”