For me one of the most interesting aspects is how compaction works. It turns out compaction still preserves the full original pre-compaction conversation in the session jsonl file, and those are marked as "not to be sent to the API". Which means, even after compaction, if you think something was lost, you can tell CC to "look in the session log files to find details about what we did with XYZ". I knew this before the leak since it can be seen from the session logs. Some more details:
The full conversation is preserved in the JSONL file, and messages
are filtered before being sent to the API.
Key mechanisms:
1. JSONL is append-only — old pre-compaction messages are never deleted. New messages (boundary
marker, summary, attachments) are appended after compaction.
2. Messages have flags controlling API visibility:
- isCompactSummary: true — marks the AI-generated summary message
- isVisibleInTranscriptOnly: true — prevents a message from being sent to the API
- isMeta — another filter for non-API messages
- getMessagesAfterCompactBoundary() returns only post-compaction messages for API calls
3. After compaction, the API sees only:
- The compact boundary marker
- The summary message
- Attachments (file refs, plan, skills)
- Any new messages after compaction
4. Three compaction types exist:
- Full compaction — API summarizes all old messages
- Session memory compaction — uses extracted session memory as summary (cheaper)
- Microcompaction — clears old tool result content when cache is cold (>1h idle)