Why did AI forget things after it summarised our chat?
When the desk gets cleared to make room, some things don't come back.
You've been working with Claude Code for a couple of hours. A message pops up saying it's compacting the conversation.
Afterwards it carries on, but something's off. It's forgotten the file you said not to touch. It's asking a question you answered an hour ago. A decision you made together has quietly been undone.
The desk was nearly full (see "Why long chats make AI worse"). So the app asked the model to write a summary of the conversation so far, cleared the desk, and put the summary back in place of the whole conversation.
Compaction: shrinking a long conversation into a short summary so the work can carry on. Claude Code does this automatically when it gets close to the limit, and other apps do something similar.
It's like swapping a 300-page meeting transcript for one page of minutes. Useful, but whoever wrote the minutes chose what to leave out.
The summary keeps the big picture: what the task is and what's been done.
The things that go missing tend to be small but important:
Users report this a lot. Several Claude Code bug reports describe it following a rule perfectly until the conversation was compacted, then forgetting it.
The summary is written by the same model, in the same way it writes everything else. It can misremember, mix things up or make a confident mistake.
Once that summary is the only record on the desk, a mistake in it becomes the new truth (see "Why AI gets stuck on its first idea").
The safest place for anything that must survive is a file, not the conversation.
A summary is a copy of what the model happened to keep. A file is exactly what you wrote.
Compact on your terms. In Claude Code you can run /compact yourself at a good stopping point, and tell it what to keep: "/compact keep the list of files we must not edit and the reason we chose option B."
After any summary, check the two or three things that matter most. Ask: "What are the rules for this task?" and see if they're still there.
For long work, keep a progress file. Then a fresh start loses almost nothing.