The per-message ephemeral context prompt re-renders every turn, and
any byte change (Discord auto-thread rename, reset notes, voice
channel state) both breaks the provider prompt-cache prefix at the
head of every request and changes the gateway agent-cache signature,
forcing a full agent rebuild per message. Pin the rendered block per
session keyed by a hash of exactly the fields it renders, so only a
real input change (rename, topic edit, /sethome, redact_pii flip)
re-renders; deliver one-shot per-turn facts (auto-reset note,
first-contact intro, voice-channel changes) on the current user
message via the api_content sidecar instead of the system prompt; sort
get_connected_platforms for byte-stable ordering.
