AutoCompact.prepare_session runs on the turn hot path
(AgentLoop._compact_session) and read the persisted _last_summary metadata
with an unguarded meta['text'] and datetime.fromisoformat(meta['last_active']).
A _last_summary dict that was hand-edited or written by another version
(missing text/last_active, or a non-ISO last_active) raised KeyError/ValueError
out of the turn.
Sibling readers already tolerate the same data: estimate_session_prompt_tokens
uses .get('text') and _archive parses inside try/except. Mirror that tolerance:
skip when text is unusable, and fall back to the session's own updated_at (the
value the writer persists) when last_active is missing or unparseable, so the
archived summary is preserved instead of crashing the turn.
39bb20c76b