Phase 9a (ADR-0014): Registry.RegisterResolver for dynamic tiers; DefineTool[Args] typed tools; Usage cache/reasoning detail fields wired through anthropic/openai/google; WithPromptCaching (Anthropic cache_control); agent supervision hooks (WithMaxStepsFunc, WithSteer, WithCompactor, WithToolErrorLimits + ErrToolLoop); health Bench/Unbench/Snapshot; ChainConfig.Observer failover events. Co-Authored-By: Claude Fable 5 <[email protected]>