Add model-native search (prefer_native) for OpenAI/Codex (#1618)

* config: add prefer_native and NativeSearchCapable for model-native search

* providers: implement native web search for OpenAI and Codex

* agent: use provider-native search when prefer_native and supported

* tests: add coverage for model-native search

* fix: Golang lint errors

* fix: update the code based on the review

* fix: update codex_provider_test

pkg/agent/loop.go

@@ -1037,6 +1037,19 @@ func (al *AgentLoop) runLLMIteration(
 		// Build tool definitions
 		providerToolDefs := agent.Tools.ToProviderDefs()
 
+		// Determine whether the provider's native web search should replace
+		// the client-side web_search tool for this request. Only enable when web
+		// search is actually enabled and registered (so users who disabled web
+		// access do not get provider-side search or billing).
+		_, hasWebSearch := agent.Tools.Get("web_search")
+		useNativeSearch := al.cfg.Tools.Web.PreferNative &&
+			isNativeSearchProvider(agent.Provider) &&
+			hasWebSearch
+
+		if useNativeSearch {
+			providerToolDefs = filterClientWebSearch(providerToolDefs)
+		}
+
 		// Log LLM request details
 		logger.DebugCF("agent", "LLM request",
 			map[string]any{
@@ -1045,6 +1058,7 @@ func (al *AgentLoop) runLLMIteration(
 				"model":             activeModel,
 				"messages_count":    len(messages),
 				"tools_count":       len(providerToolDefs),
+				"native_search":     useNativeSearch,
 				"max_tokens":        agent.MaxTokens,
 				"temperature":       agent.Temperature,
 				"system_prompt_len": len(messages[0].Content),
@@ -1067,6 +1081,9 @@ func (al *AgentLoop) runLLMIteration(
 			"temperature":      agent.Temperature,
 			"prompt_cache_key": agent.ID,
 		}
+		if useNativeSearch {
+			llmOpts["native_search"] = true
+		}
 		// parseThinkingLevel guarantees ThinkingOff for empty/unknown values,
 		// so checking != ThinkingOff is sufficient.
 		if agent.ThinkingLevel != ThinkingOff {
@@ -1976,6 +1993,28 @@ func extractParentPeer(msg bus.InboundMessage) *routing.RoutePeer {
 	return &routing.RoutePeer{Kind: parentKind, ID: parentID}
 }
 
+// isNativeSearchProvider reports whether the given LLM provider implements
+// NativeSearchCapable and returns true for SupportsNativeSearch.
+func isNativeSearchProvider(p providers.LLMProvider) bool {
+	if ns, ok := p.(providers.NativeSearchCapable); ok {
+		return ns.SupportsNativeSearch()
+	}
+	return false
+}
+
+// filterClientWebSearch returns a copy of tools with the client-side
+// web_search tool removed. Used when native provider search is preferred.
+func filterClientWebSearch(tools []providers.ToolDefinition) []providers.ToolDefinition {
+	result := make([]providers.ToolDefinition, 0, len(tools))
+	for _, t := range tools {
+		if strings.EqualFold(t.Function.Name, "web_search") {
+			continue
+		}
+		result = append(result, t)
+	}
+	return result
+}
+
 // Helper to extract provider from registry for cleanup
 func extractProvider(registry *AgentRegistry) (providers.LLMProvider, bool) {
 	if registry == nil {

pkg/agent/loop_test.go

@@ -1426,3 +1426,84 @@ func TestResolveMediaRefs_MixedImageAndFile(t *testing.T) {
 		t.Fatalf("expected content %q, got %q", expectedContent, result[0].Content)
 	}
 }
+
+// --- Native search helper tests ---
+
+type nativeSearchProvider struct {
+	supported bool
+}
+
+func (p *nativeSearchProvider) Chat(
+	ctx context.Context, msgs []providers.Message, tools []providers.ToolDefinition,
+	model string, opts map[string]any,
+) (*providers.LLMResponse, error) {
+	return &providers.LLMResponse{Content: "ok"}, nil
+}
+
+func (p *nativeSearchProvider) GetDefaultModel() string { return "test-model" }
+
+func (p *nativeSearchProvider) SupportsNativeSearch() bool { return p.supported }
+
+type plainProvider struct{}
+
+func (p *plainProvider) Chat(
+	ctx context.Context, msgs []providers.Message, tools []providers.ToolDefinition,
+	model string, opts map[string]any,
+) (*providers.LLMResponse, error) {
+	return &providers.LLMResponse{Content: "ok"}, nil
+}
+
+func (p *plainProvider) GetDefaultModel() string { return "test-model" }
+
+func TestIsNativeSearchProvider_Supported(t *testing.T) {
+	if !isNativeSearchProvider(&nativeSearchProvider{supported: true}) {
+		t.Fatal("expected true for provider that supports native search")
+	}
+}
+
+func TestIsNativeSearchProvider_NotSupported(t *testing.T) {
+	if isNativeSearchProvider(&nativeSearchProvider{supported: false}) {
+		t.Fatal("expected false for provider that does not support native search")
+	}
+}
+
+func TestIsNativeSearchProvider_NoInterface(t *testing.T) {
+	if isNativeSearchProvider(&plainProvider{}) {
+		t.Fatal("expected false for provider that does not implement NativeSearchCapable")
+	}
+}
+
+func TestFilterClientWebSearch_RemovesWebSearch(t *testing.T) {
+	defs := []providers.ToolDefinition{
+		{Type: "function", Function: providers.ToolFunctionDefinition{Name: "web_search"}},
+		{Type: "function", Function: providers.ToolFunctionDefinition{Name: "read_file"}},
+		{Type: "function", Function: providers.ToolFunctionDefinition{Name: "exec"}},
+	}
+	result := filterClientWebSearch(defs)
+	if len(result) != 2 {
+		t.Fatalf("len(result) = %d, want 2", len(result))
+	}
+	for _, td := range result {
+		if td.Function.Name == "web_search" {
+			t.Fatal("web_search should be filtered out")
+		}
+	}
+}
+
+func TestFilterClientWebSearch_NoWebSearch(t *testing.T) {
+	defs := []providers.ToolDefinition{
+		{Type: "function", Function: providers.ToolFunctionDefinition{Name: "read_file"}},
+		{Type: "function", Function: providers.ToolFunctionDefinition{Name: "exec"}},
+	}
+	result := filterClientWebSearch(defs)
+	if len(result) != 2 {
+		t.Fatalf("len(result) = %d, want 2", len(result))
+	}
+}
+
+func TestFilterClientWebSearch_EmptyInput(t *testing.T) {
+	result := filterClientWebSearch(nil)
+	if len(result) != 0 {
+		t.Fatalf("len(result) = %d, want 0", len(result))
+	}
+}