Replacing older turns with a summary, so a session can continue in less space. Most tools do it automatically near the limit, or when you ask. Whatever the summary leaves out is gone, and nothing flags the loss.
It is a summary you didn't write, and it loses things. A rule you typed in chat can vanish, and a wrong assumption can survive as fact. So keep rules on disk, where they get reloaded, and if you do compact, say what to keep. Doing it yourself, early, beats waiting for the automatic one.
Say it to your agent
“/compact keep the decision not to add a date library, the failing test name, the exact error text, what we ruled out and the next step. Drop old test logs and file contents.”