diff --git a/internal/translator/codex/openai/chat-completions/codex_openai_request.go b/internal/translator/codex/openai/chat-completions/codex_openai_request.go
index 272037da0..309c974e8 100644
--- a/internal/translator/codex/openai/chat-completions/codex_openai_request.go
+++ b/internal/translator/codex/openai/chat-completions/codex_openai_request.go
@@ -275,7 +275,15 @@ func ConvertOpenAIRequestToCodex(modelName string, inputRawJSON []byte, stream b
 		arr := tools.Array()
 		for i := 0; i < len(arr); i++ {
 			t := arr[i]
-			if t.Get("type").String() == "function" {
+			toolType := t.Get("type").String()
+			// Pass through built-in tools (e.g. {"type":"web_search"}) directly for the Responses API.
+			// Only "function" needs structural conversion because Chat Completions nests details under "function".
+			if toolType != "" && toolType != "function" && t.IsObject() {
+				out, _ = sjson.SetRaw(out, "tools.-1", t.Raw)
+				continue
+			}
+
+			if toolType == "function" {
 				item := `{}`
 				item, _ = sjson.Set(item, "type", "function")
 				fn := t.Get("function")
@@ -304,6 +312,37 @@ func ConvertOpenAIRequestToCodex(modelName string, inputRawJSON []byte, stream b
 		}
 	}
 
+	// Map tool_choice when present.
+	// Chat Completions: "tool_choice" can be a string ("auto"/"none") or an object (e.g. {"type":"function","function":{"name":"..."}}).
+	// Responses API: keep built-in tool choices as-is; flatten function choice to {"type":"function","name":"..."}.
+	if tc := gjson.GetBytes(rawJSON, "tool_choice"); tc.Exists() {
+		switch {
+		case tc.Type == gjson.String:
+			out, _ = sjson.Set(out, "tool_choice", tc.String())
+		case tc.IsObject():
+			tcType := tc.Get("type").String()
+			if tcType == "function" {
+				name := tc.Get("function.name").String()
+				if name != "" {
+					if short, ok := originalToolNameMap[name]; ok {
+						name = short
+					} else {
+						name = shortenNameIfNeeded(name)
+					}
+				}
+				choice := `{}`
+				choice, _ = sjson.Set(choice, "type", "function")
+				if name != "" {
+					choice, _ = sjson.Set(choice, "name", name)
+				}
+				out, _ = sjson.SetRaw(out, "tool_choice", choice)
+			} else if tcType != "" {
+				// Built-in tool choices (e.g. {"type":"web_search"}) are already Responses-compatible.
+				out, _ = sjson.SetRaw(out, "tool_choice", tc.Raw)
+			}
+		}
+	}
+
 	out, _ = sjson.Set(out, "store", false)
 	return []byte(out)
 }
diff --git a/internal/translator/openai/openai/responses/openai_openai-responses_request.go b/internal/translator/openai/openai/responses/openai_openai-responses_request.go
index 687c2a304..86cf19f88 100644
--- a/internal/translator/openai/openai/responses/openai_openai-responses_request.go
+++ b/internal/translator/openai/openai/responses/openai_openai-responses_request.go
@@ -163,6 +163,14 @@ func ConvertOpenAIResponsesRequestToOpenAIChatCompletions(modelName string, inpu
 		var chatCompletionsTools []interface{}
 
 		tools.ForEach(func(_, tool gjson.Result) bool {
+			// Built-in tools (e.g. {"type":"web_search"}) are already compatible with the Chat Completions schema.
+			// Only function tools need structural conversion because Chat Completions nests details under "function".
+			toolType := tool.Get("type").String()
+			if toolType != "" && toolType != "function" && tool.IsObject() {
+				chatCompletionsTools = append(chatCompletionsTools, tool.Value())
+				return true
+			}
+
 			chatTool := `{"type":"function","function":{}}`
 
 			// Convert tool structure from responses format to chat completions format
diff --git a/test/builtin_tools_translation_test.go b/test/builtin_tools_translation_test.go
new file mode 100644
index 000000000..b4ca7b0da
--- /dev/null
+++ b/test/builtin_tools_translation_test.go
@@ -0,0 +1,54 @@
+package test
+
+import (
+	"testing"
+
+	_ "github.com/router-for-me/CLIProxyAPI/v6/internal/translator"
+
+	sdktranslator "github.com/router-for-me/CLIProxyAPI/v6/sdk/translator"
+	"github.com/tidwall/gjson"
+)
+
+func TestOpenAIToCodex_PreservesBuiltinTools(t *testing.T) {
+	in := []byte(`{
+		"model":"gpt-5",
+		"messages":[{"role":"user","content":"hi"}],
+		"tools":[{"type":"web_search","search_context_size":"high"}],
+		"tool_choice":{"type":"web_search"}
+	}`)
+
+	out := sdktranslator.TranslateRequest(sdktranslator.FormatOpenAI, sdktranslator.FormatCodex, "gpt-5", in, false)
+
+	if got := gjson.GetBytes(out, "tools.#").Int(); got != 1 {
+		t.Fatalf("expected 1 tool, got %d: %s", got, string(out))
+	}
+	if got := gjson.GetBytes(out, "tools.0.type").String(); got != "web_search" {
+		t.Fatalf("expected tools[0].type=web_search, got %q: %s", got, string(out))
+	}
+	if got := gjson.GetBytes(out, "tools.0.search_context_size").String(); got != "high" {
+		t.Fatalf("expected tools[0].search_context_size=high, got %q: %s", got, string(out))
+	}
+	if got := gjson.GetBytes(out, "tool_choice.type").String(); got != "web_search" {
+		t.Fatalf("expected tool_choice.type=web_search, got %q: %s", got, string(out))
+	}
+}
+
+func TestOpenAIResponsesToOpenAI_PreservesBuiltinTools(t *testing.T) {
+	in := []byte(`{
+		"model":"gpt-5",
+		"input":[{"role":"user","content":[{"type":"input_text","text":"hi"}]}],
+		"tools":[{"type":"web_search","search_context_size":"low"}]
+	}`)
+
+	out := sdktranslator.TranslateRequest(sdktranslator.FormatOpenAIResponse, sdktranslator.FormatOpenAI, "gpt-5", in, false)
+
+	if got := gjson.GetBytes(out, "tools.#").Int(); got != 1 {
+		t.Fatalf("expected 1 tool, got %d: %s", got, string(out))
+	}
+	if got := gjson.GetBytes(out, "tools.0.type").String(); got != "web_search" {
+		t.Fatalf("expected tools[0].type=web_search, got %q: %s", got, string(out))
+	}
+	if got := gjson.GetBytes(out, "tools.0.search_context_size").String(); got != "low" {
+		t.Fatalf("expected tools[0].search_context_size=low, got %q: %s", got, string(out))
+	}
+}