feat: conversion-driven extensions — resolvers, DefineTool, hooks, ops controls

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 <noreply@anthropic.com>

provider/openai/model.go

@@ -130,10 +130,7 @@ func (m *model) apiError(httpResp *http.Response) error {
 func (m *model) toResponse(wire *chatResponse) *llm.Response {
 	resp := &llm.Response{Model: m.p.name + "/" + m.id, Raw: wire}
 	if wire.Usage != nil {
-		resp.Usage = llm.Usage{
-			InputTokens:  wire.Usage.PromptTokens,
-			OutputTokens: wire.Usage.CompletionTokens,
-		}
+		resp.Usage = wire.Usage.toUsage()
 	}
 	if len(wire.Choices) == 0 {
 		resp.FinishReason = llm.FinishOther

provider/openai/stream.go

@@ -104,10 +104,7 @@ func (s *stream) handleChunk(data []byte) error {
 		return apiErr
 	}
 	if chunk.Usage != nil {
-		s.usage = llm.Usage{
-			InputTokens:  chunk.Usage.PromptTokens,
-			OutputTokens: chunk.Usage.CompletionTokens,
-		}
+		s.usage = chunk.Usage.toUsage()
 	}
 	// Why the guard: the include_usage chunk arrives with an EMPTY choices
 	// array; indexing choices[0] unconditionally would panic on it.

provider/openai/wire.go

@@ -125,9 +125,32 @@ type wireRespMessage struct {
 }
 
 type wireUsage struct {
-	PromptTokens     int `json:"prompt_tokens"`
-	CompletionTokens int `json:"completion_tokens"`
-	TotalTokens      int `json:"total_tokens"`
+	PromptTokens            int               `json:"prompt_tokens"`
+	CompletionTokens        int               `json:"completion_tokens"`
+	TotalTokens             int               `json:"total_tokens"`
+	PromptTokensDetails     *wirePromptDetail `json:"prompt_tokens_details"`
+	CompletionTokensDetails *wireOutputDetail `json:"completion_tokens_details"`
+}
+
+type wirePromptDetail struct {
+	CachedTokens int `json:"cached_tokens"`
+}
+
+type wireOutputDetail struct {
+	ReasoningTokens int `json:"reasoning_tokens"`
+}
+
+// toUsage maps wire usage (with optional detail objects — absent on many
+// compat servers) onto the canonical Usage.
+func (u *wireUsage) toUsage() llm.Usage {
+	out := llm.Usage{InputTokens: u.PromptTokens, OutputTokens: u.CompletionTokens}
+	if u.PromptTokensDetails != nil {
+		out.CacheReadTokens = u.PromptTokensDetails.CachedTokens
+	}
+	if u.CompletionTokensDetails != nil {
+		out.ReasoningTokens = u.CompletionTokensDetails.ReasoningTokens
+	}
+	return out
 }
 
 type errorEnvelope struct {