What
Compaction is when Claude Code shortens a long conversation by summarising it. With each request, Claude Code sends an attribution header, extra information attached to the request about where it came from. With first-party providers, that header can now include three new fields taken from the response before compaction:
cc_prev_msgcc_prev_prompt_indexcc_prev_turn_index
The pattern Claude Code uses to strip these attribution fields out of headers now also matches the three new fields. Before, it matched only prev_req, prompt_id, turn_origin, prompt_index, turn_index and workload.
Why
Requests sent after compaction can now be linked back to the conversation before it. If you inspect request headers, expect these extra fields.