Function compact_materialized_transcript_through 

pub fn compact_materialized_transcript_through(
    session_id: &str,
    event_frontier: u64,
) -> Result<TranscriptRetention>

Drop tool output that a verified checkpoint already holds.

The projection only ever grew: the only deletes were a per-item remove, a whole-session wipe, and the sessions cascade. One measured session reached 28,066 items and 635 MiB, of which 561 MB was tool-call content.

A checkpoint archive carries the complete transcript up to its event frontier, and one checkpoint per session is retained, so every item at or below event_frontier is durably recorded elsewhere. What stays here is what the transcript still shows: which tool ran, on what, with what result, and each edit’s diffstat. See mj_transcript::transcript::compact_tool_call_for_retention.