The #45966 cross-process coherence guard snapshots a session's on-disk
message_count next to the cached agent and rebuilds the agent when the
count changes. But the snapshot is taken at agent-BUILD time — before
the turn writes its own user + assistant (+ tool) rows — and the cache
entry is never rewritten on a reuse. So this process's OWN turn grows
message_count, and the very next turn sees a mismatch and rebuilds the
agent. That happens every turn, for every conversation, silently
destroying the per-conversation prompt caching the cache exists to
protect (AGENTS.md: prompt caching is sacred).
Add _refresh_agent_cache_message_count(): after a turn completes and the
agent has flushed its rows to the SessionDB, re-baseline the stored count
to the now-current value. The guard then fires ONLY when a DIFFERENT
process changes the transcript — preserving the #45966 fix while keeping
the cache warm for normal single-process operation.
Tests drive the real SessionDB + the real guard condition: 5 consecutive
same-process turns now all REUSE the cached agent (0 before the fix); a
cross-process append still invalidates; and the re-baseline is fail-safe
(no DB, falsy session_id, raising probe, legacy 2-tuple, pending sentinel
all no-op).
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