The wrong instinct
The common beginner move is to keep piling material into the context until the model complains. That treats the window like a storage bin. Real work treats it like a budget.
Every token has an opportunity cost. A stale tool dump competes with fresh requirements. A pasted design doc competes with a failing test log. A long apology from the model competes with the next command output. The question is never "can it fit?" The question is "does it deserve attention right now?"
Safety margin is part of the design
Do not drive the window to the wall. Keep a margin for output, corrections, tool results, and the unexpected turn where the human says, "actually, this rule matters more." When the margin disappears, the model becomes brittle: it stops reasoning carefully, drops citations, truncates final answers, and starts agreeing with everything because there is no room left to push back.
Pre-allocate before loading
The mature pattern is to reserve output, reasoning, and a safety buffer first, then decide how much input fits. Most loops do this in reverse, then act surprised when the answer ends mid-sentence.