An agent's working memory is its context window — the tokens it can see right now. When the session ends, that window is discarded. Most tools do write a transcript to disk (~/.claude/projects, ~/.codex/sessions, and so on), but they never read those files back on the next run. So the model that spent an hour learning your codebase yesterday starts today knowing nothing about it. That gap — between what was recorded and what gets reloaded — is the problem “agent memory” solves.
Persistent memory is not a bigger context window. It is a separate store that outlives any single conversation, that you can search, and that an agent can pull from on demand instead of holding everything in tokens at once.