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Bug and impact

Session compaction can fold a summary into an assistant message and leave that role in place. With an odd max_context_messages, the stored count on the first compaction is max_ctx + 1, so the folded head is inside the sent window. Anthropic rejects an assistant-first transcript with HTTP 400. The router treats 4xx as terminal, agent_run returns before session_save, and every later turn rebuilds the same request. The session accepts no new turns until /reset.

Trigger

max_context_messages is any odd value from 3 to 39 (the agent clamps above 40). The oldest stored user line is about 60 KiB (CLI -m or an ASAP task) and the following assistant reply is a few KiB, so the two summary lines do not fit in the 64 KiB source. After enough later turns that the stored count exceeds the limit, the next message sticks.

Root cause

compact_session_via_llm sets fold_into_oversized when a non-user line stops the summary pass at index 1, then fold_summary_into_message rewrites only content. The kept head stays assistant. The user-tail path already folds into a user message to avoid this Anthropic 400. The assistant-head path did not.

Fix

Retag that head to user, drop tool_use_id, and merge one following user turn so the roles still alternate. The oversized body stays in the head for the next stub pass.

Validation

test_compaction_fold_keeps_alternating_user_head failed before the fix (folded head role is assistant) and passes after it. CI=true make test_agent passes.