feat(v2): populate CacheHints on provider.Request when caching enabled

buildProviderRequest now computes cache-breakpoint positions automatically
when the WithPromptCaching() option is set. It places up to 3 hints:
tools, system, and the index of the last non-system message. Providers
that don't support caching (OpenAI, Google) ignore the field.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

v2/llm.go

@@ -122,6 +122,26 @@ func buildProviderRequest(model string, messages []Message, cfg *requestConfig)
 		}
 	}
 
+	if cfg.cacheConfig != nil && cfg.cacheConfig.enabled {
+		hints := &provider.CacheHints{LastCacheableMessageIndex: -1}
+		if len(req.Tools) > 0 {
+			hints.CacheTools = true
+		}
+		for _, m := range messages {
+			if m.Role == RoleSystem {
+				hints.CacheSystem = true
+				break
+			}
+		}
+		for i := len(messages) - 1; i >= 0; i-- {
+			if messages[i].Role != RoleSystem {
+				hints.LastCacheableMessageIndex = i
+				break
+			}
+		}
+		req.CacheHints = hints
+	}
+
 	return req
 }
 

v2/request_test.go

@@ -154,3 +154,89 @@ func TestWithoutPromptCaching(t *testing.T) {
 		t.Error("expected cacheConfig to be nil when option not applied")
 	}
 }
+
+func TestBuildProviderRequest_CachingDisabled(t *testing.T) {
+	cfg := &requestConfig{}
+	msgs := []Message{SystemMessage("sys"), UserMessage("hi")}
+	req := buildProviderRequest("m", msgs, cfg)
+	if req.CacheHints != nil {
+		t.Errorf("expected nil CacheHints when caching disabled, got %+v", req.CacheHints)
+	}
+}
+
+func TestBuildProviderRequest_CachingEnabled_AllSections(t *testing.T) {
+	tool := DefineSimple("greet", "greet", func(ctx context.Context) (string, error) { return "ok", nil })
+	tb := NewToolBox(tool)
+	cfg := &requestConfig{
+		tools:       tb,
+		cacheConfig: &cacheConfig{enabled: true},
+	}
+	msgs := []Message{
+		SystemMessage("you are helpful"),
+		UserMessage("hello"),
+		AssistantMessage("hi"),
+		UserMessage("thanks"),
+	}
+	req := buildProviderRequest("m", msgs, cfg)
+	if req.CacheHints == nil {
+		t.Fatal("expected CacheHints to be set")
+	}
+	if !req.CacheHints.CacheTools {
+		t.Error("expected CacheTools=true")
+	}
+	if !req.CacheHints.CacheSystem {
+		t.Error("expected CacheSystem=true")
+	}
+	// Last non-system message index = 3 ("thanks")
+	if req.CacheHints.LastCacheableMessageIndex != 3 {
+		t.Errorf("expected LastCacheableMessageIndex=3, got %d", req.CacheHints.LastCacheableMessageIndex)
+	}
+}
+
+func TestBuildProviderRequest_CachingEnabled_NoTools(t *testing.T) {
+	cfg := &requestConfig{cacheConfig: &cacheConfig{enabled: true}}
+	msgs := []Message{SystemMessage("sys"), UserMessage("hi")}
+	req := buildProviderRequest("m", msgs, cfg)
+	if req.CacheHints == nil {
+		t.Fatal("expected CacheHints")
+	}
+	if req.CacheHints.CacheTools {
+		t.Error("expected CacheTools=false when there are no tools")
+	}
+	if !req.CacheHints.CacheSystem {
+		t.Error("expected CacheSystem=true")
+	}
+	if req.CacheHints.LastCacheableMessageIndex != 1 {
+		t.Errorf("expected LastCacheableMessageIndex=1, got %d", req.CacheHints.LastCacheableMessageIndex)
+	}
+}
+
+func TestBuildProviderRequest_CachingEnabled_NoSystem(t *testing.T) {
+	cfg := &requestConfig{cacheConfig: &cacheConfig{enabled: true}}
+	msgs := []Message{UserMessage("hi")}
+	req := buildProviderRequest("m", msgs, cfg)
+	if req.CacheHints == nil {
+		t.Fatal("expected CacheHints")
+	}
+	if req.CacheHints.CacheSystem {
+		t.Error("expected CacheSystem=false when there is no system message")
+	}
+	if req.CacheHints.LastCacheableMessageIndex != 0 {
+		t.Errorf("expected LastCacheableMessageIndex=0, got %d", req.CacheHints.LastCacheableMessageIndex)
+	}
+}
+
+func TestBuildProviderRequest_CachingEnabled_OnlySystem(t *testing.T) {
+	cfg := &requestConfig{cacheConfig: &cacheConfig{enabled: true}}
+	msgs := []Message{SystemMessage("sys")}
+	req := buildProviderRequest("m", msgs, cfg)
+	if req.CacheHints == nil {
+		t.Fatal("expected CacheHints")
+	}
+	if !req.CacheHints.CacheSystem {
+		t.Error("expected CacheSystem=true")
+	}
+	if req.CacheHints.LastCacheableMessageIndex != -1 {
+		t.Errorf("expected LastCacheableMessageIndex=-1 when no non-system messages, got %d", req.CacheHints.LastCacheableMessageIndex)
+	}
+}