When a session approaches the window limit, the harness replaces older turns with a summary and continues. Without it, long autonomous runs simply stop; with it, they continue at the cost of detail — the summary keeps decisions and loses specifics.

The failure mode to know is silent loss. An agent that compacted away a constraint you stated an hour ago will not tell you it forgot; it will just violate the constraint. Restating the things that must hold, or writing them to a file the agent re-reads, is the defense.