list_messages re-scanned every event in the thread on each call (category
filter + seq filter) — O(total events) per paginated request on the default
run-events backend. Maintain a messages-only, seq-sorted projection of _events
(shared dict refs, no copies) and locate the seq window with bisect:
list_messages drops to O(log m + page) and count_messages to O(1). The index is
kept in lockstep at every mutation site (put / put_batch via _put_one,
delete_by_run, delete_by_thread).
Externally observable behavior is unchanged — the full RunEventStore contract
suite passes across memory/db/jsonl.
Add a test covering pagination over non-contiguous message seqs (messages
interleaved with trace events), including in-gap and exact-boundary cursors.
Co-authored-by: ly-wang19 <ly-wang19@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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