Compaction

What is Compaction

Compaction is what happens when a session's context window starts running out of room: instead of hitting a hard wall, older turns get condensed into a shorter summary, freeing space for the session to keep going. It is a deliberate tradeoff, some detail from the compacted portion is lost, in exchange for the session being able to continue at all.

This matters most on long sessions: an extended debugging effort, a multi-hour build, a research task that reads dozens of files. Without compaction, a session either has to stop the moment its window fills, or every new turn has to fight for shrinking room against a fixed and growing history. Compaction lets sessions run much longer than the raw token budget would otherwise allow.

Compaction is not the same as clearing a session, which discards history rather than summarizing it, and it is not free: whatever gets compressed away is genuinely gone in its original form, only the summary remains. This is why handoffs matter for anything that must survive with full fidelity, a compacted summary is good enough for most continuity but not a substitute for a deliberately written record of something important.