A context window is a fixed number of cells, and a new message has to fit the ones left over. Turn the message into chunks, then number them into the empty cells one at a time. If the cells run out before the chunks do, the message does not fit and the agent has to drop or compact something to make room. If the chunks run out first, with cells to spare, it fits.
Paid members: the worksheet and its printable PDF are below ↓