[cursor]

Session compaction deleted most of an oversized message when earlier lines had already filled the 64 KiB summary source.

Bug and impact

compact_session_via_llm summarizes the oldest turns once a session grows past max_context_messages. A single message larger than the 64 KiB summary source cannot be copied whole. The recent stub path still accepted that line when only a sliver of the buffer was free, then dropped it from the kept tail.

Concrete trigger: an ASAP task.request whose input is about 50 KiB, a short reply, then another task whose input is about 65 KiB, then enough short turns to exceed the default 40-message window. Both pastes fit in the 128 KiB session blob. Compaction summarized the first paste, cut the second to the leftover few kilobytes, and agent_persist_session wrote that result back. The rest of the second task was gone from the transcript and was never shown to the summarizer.

Root cause

summary_source_append_truncated returns success whenever the role wrapper and the [truncated] marker fit in the remaining bytes. The caller treated that success as "this message is fully consumed." A later line that can never fit a fresh source was therefore deleted after the model had seen only the leftover sliver.

Fix

Stub a never-fit line only when the summary source is still empty. Otherwise leave it in the session. This pass summarizes the prefix, and the next pass stubs the line against a full 64 KiB buffer. A never-fit first line is still stubbed, so compaction cannot stall with an empty source. Lines that would fit a fresh source keep the existing prefer-keep path.

Validation

make test_agent passes. The updated later-line regression uses a 50 KiB predecessor and a 65 KiB message: after the first turn the oversized end marker is still in the session and the predecessor is not, and a later turn still brings the session back down to the context cap. The existing first-line oversized test still requires the empty-source stub.

  

[adriannoes]

Superseded by #133: consolidated into the operator-authored branch fix/agent-compaction-empty-stub (same fix, re-validated with make test_agent, CI=true make test, make static). Closing in favor of the consolidation.