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 {