fix(agent): keep history when compaction cannot finish the turn

[cursor]

Summary

Compaction saved a summarized transcript before the user turn finished, and it could store two user turns in a row. A failed reply then deleted history the user still needed, and an odd max_context_messages made Anthropic reject that session on every later message.

• compact_session_via_llm called session_save as soon as the summary model returned. The follow-up chat had not run yet. A timeout, 429, or 5xx after that summary left the raw messages replaced by the summary, while the failed turn itself was not stored.
• The summary was always inserted as a new user message. With an odd max_context_messages (config allows 1–1000; example provider is Anthropic), the kept tail also starts on a user turn. The Messages API requires alternating roles and returns HTTP 400. Because the bad transcript was already saved, every later message on that session failed the same way until /reset.

The compacted JSON now stays in memory until agent_persist_session runs after a successful reply. If the kept tail already starts with user, the summary is folded into that turn instead of adding a second user message.

Testing

• make test_agent then ./build/test_agent — passed, including:
  • test_compaction_failure_keeps_original_session (summary call succeeds, follow-up chat fails, msg_0 remains and the summary is not stored)
  • test_compaction_merges_summary_when_tail_starts_with_user (max_context_messages = 5, roles alternate, summary is inside the leading user turn, success still persists compaction)
  • existing test_context_compaction_when_history_exceeds_max

  

[adriannoes]

Superseded by #121: consolidated into the operator-authored branch fix/agent-summary-compaction (same fix, re-validated with make test_agent, CI=true make test, make static). Closing in favor of the consolidation.