The persisted <available_skills> index is reused byte-for-byte across turns so
the provider prefix cache stays warm (#104414). A skill that lands after the
prompt was built (hub install, skill_manage from another session, org sync,
curator) therefore never reached the index the model actually routes on until
compaction rebuilt the prompt: skills_list is live, but the prompt tells the
model to scan the index, not to call it. cloudflare-os#267 found the same fault
in their chat-cached catalog ("a catalog states what a session can reach now,
which is not a fact a chat can hold").
Hermes' idiom for stored-prompt drift is a one-shot note behind the cached
prefix, not a rebuild, so agent/skills_index_delta.py stages the delta on the
same per-turn user-message channel the surface-switch note rides: the added
skills' own index lines (name + description, the routing signal), the removed
names, cumulative against the stored prompt and re-staged only when the delta
changes (read back from the api_content sidecar, so the gateway's fresh agent
per turn does not stack copies). An undone delta retires the stale note once.
The current index comes from the same cached builder the prompt did, so an
unchanged turn costs one LRU hit.
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