Full context is an emergency state
When the window is nearly full, the model has less room to absorb corrections, inspect tool results, or produce a careful answer. It may also become more vulnerable to stale instructions competing with current ones — there is simply no spare attention for the human to redirect with.
Preempt, do not react
The correct move is preemptive. Checkpoint before the margin is gone. Summarize decisions while both the model and the human still have enough working space to verify the summary. Reactive checkpointing — the kind that happens after the first truncation or hallucination — is already too late; the corrupt state is already in the recent context.
Checkpoint is not failure
In serious work, stopping to preserve state is professionalism. The opposite is gambling that the last 5% of the window will somehow carry the hardest part of the job. Cute. Also how bugs breed.