fix(openai_compat): gate prompt_cache_key to OpenAI endpoints

pkg/providers/openai_compat/provider.go

@@ -155,9 +155,9 @@ func (p *Provider) Chat(
 	// The key is typically the agent ID — stable per agent, shared across requests.
 	// See: https://platform.openai.com/docs/guides/prompt-caching
 	// Prompt caching is only supported by OpenAI-native endpoints.
-	// Gemini and other providers reject unknown fields, so skip for non-OpenAI APIs.
+	// Other OpenAI-compatible providers may reject unknown fields.
 	if cacheKey, ok := options["prompt_cache_key"].(string); ok && cacheKey != "" {
-		if !strings.Contains(p.apiBase, "generativelanguage.googleapis.com") {
+		if supportsPromptCacheKey(p.apiBase) {
 			requestBody["prompt_cache_key"] = cacheKey
 		}
 	}
@@ -195,6 +195,18 @@ func (p *Provider) Chat(
 	return parseResponse(body)
 }
 
+func supportsPromptCacheKey(apiBase string) bool {
+	host := ""
+	if parsed, err := url.Parse(apiBase); err == nil {
+		host = parsed.Hostname()
+	}
+	if host == "" {
+		host = apiBase
+	}
+	host = strings.ToLower(strings.TrimSpace(host))
+	return host == "openai.com" || strings.HasSuffix(host, ".openai.com")
+}
+
 func parseResponse(body []byte) (*LLMResponse, error) {
 	var apiResponse struct {
 		Choices []struct {

pkg/providers/openai_compat/provider_test.go

@@ -376,6 +376,92 @@ func TestProviderChat_AcceptsNumericOptionTypes(t *testing.T) {
 	}
 }
 
+func TestProviderChat_SkipsPromptCacheKeyForNonOpenAIEndpoint(t *testing.T) {
+	var requestBody map[string]any
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if err := json.NewDecoder(r.Body).Decode(&requestBody); err != nil {
+			http.Error(w, err.Error(), http.StatusBadRequest)
+			return
+		}
+		resp := map[string]any{
+			"choices": []map[string]any{
+				{
+					"message":       map[string]any{"content": "ok"},
+					"finish_reason": "stop",
+				},
+			},
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(resp)
+	}))
+	defer server.Close()
+
+	p := NewProvider("key", server.URL, "")
+	_, err := p.Chat(
+		t.Context(),
+		[]Message{{Role: "user", Content: "hi"}},
+		nil,
+		"gpt-4o",
+		map[string]any{"prompt_cache_key": "agent-123"},
+	)
+	if err != nil {
+		t.Fatalf("Chat() error = %v", err)
+	}
+
+	if _, ok := requestBody["prompt_cache_key"]; ok {
+		t.Fatalf("did not expect prompt_cache_key for non-OpenAI endpoint")
+	}
+}
+
+func TestSupportsPromptCacheKey(t *testing.T) {
+	tests := []struct {
+		name    string
+		apiBase string
+		want    bool
+	}{
+		{
+			name:    "openai endpoint",
+			apiBase: "https://api.openai.com/v1",
+			want:    true,
+		},
+		{
+			name:    "openai apex endpoint",
+			apiBase: "https://openai.com/v1",
+			want:    true,
+		},
+		{
+			name:    "nvidia endpoint",
+			apiBase: "https://integrate.api.nvidia.com/v1",
+			want:    false,
+		},
+		{
+			name:    "gemini endpoint",
+			apiBase: "https://generativelanguage.googleapis.com/v1beta",
+			want:    false,
+		},
+		{
+			name:    "openrouter endpoint",
+			apiBase: "https://openrouter.ai/api/v1",
+			want:    false,
+		},
+		{
+			name:    "local endpoint",
+			apiBase: "http://localhost:11434/v1",
+			want:    false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := supportsPromptCacheKey(tt.apiBase)
+			if got != tt.want {
+				t.Fatalf("supportsPromptCacheKey(%q) = %v, want %v", tt.apiBase, got, tt.want)
+			}
+		})
+	}
+}
+
 func TestNormalizeModel_UsesAPIBase(t *testing.T) {
 	if got := normalizeModel("deepseek/deepseek-chat", "https://api.deepseek.com/v1"); got != "deepseek-chat" {
 		t.Fatalf("normalizeModel(deepseek) = %q, want %q", got, "deepseek-chat")