The gateway created a fresh AIAgent per message, rebuilding the system
prompt (including memory, skills, context files) every turn. This broke
prompt prefix caching — providers like Anthropic charge ~10x more for
uncached prefixes.
Now caches AIAgent instances per session_key with a config signature.
The cached agent is reused across messages in the same session,
preserving the frozen system prompt and tool schemas. Cache is
invalidated when:
- Config changes (model, provider, toolsets, reasoning, ephemeral
prompt) — detected via signature mismatch
- /new, /reset, /clear — explicit session reset
- /model — global model change clears all cached agents
- /reasoning — global reasoning change clears all cached agents
Per-message state (callbacks, stream consumers, progress queues) is
set on the agent instance before each run_conversation() call.
This matches CLI behavior where a single AIAgent lives across all turns
in a session, with _cached_system_prompt built once and reused.
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