diff --git a/backend/internal/handler/openai_gateway_handler.go b/backend/internal/handler/openai_gateway_handler.go
index 80e00b7fc4..65bf172ecd 100644
--- a/backend/internal/handler/openai_gateway_handler.go
+++ b/backend/internal/handler/openai_gateway_handler.go
@@ -47,6 +47,30 @@ func resolveOpenAIMessagesDispatchMappedModel(apiKey *service.APIKey, requestedM
 	return strings.TrimSpace(apiKey.Group.ResolveMessagesDispatchModel(requestedModel))
 }
 
+type openAIModelBodyReplaceFunc func([]byte, string) []byte
+
+func openAIModelMappedBody(body []byte, mapped bool, mappedModel string, replace openAIModelBodyReplaceFunc) []byte {
+	if !mapped || replace == nil {
+		return body
+	}
+	return replace(body, mappedModel)
+}
+
+func newOpenAIModelMappedBodyCache(body []byte, replace openAIModelBodyReplaceFunc) func(bool, string) []byte {
+	replacedBodies := make(map[string][]byte)
+	return func(mapped bool, mappedModel string) []byte {
+		if !mapped {
+			return body
+		}
+		if cachedBody, ok := replacedBodies[mappedModel]; ok {
+			return cachedBody
+		}
+		replacedBody := openAIModelMappedBody(body, true, mappedModel, replace)
+		replacedBodies[mappedModel] = replacedBody
+		return replacedBody
+	}
+}
+
 func usageRecordContext(parent context.Context, base context.Context) context.Context {
 	if base == nil {
 		base = context.Background()
@@ -241,6 +265,7 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 
 	// 解析渠道级模型映射
 	channelMapping, _ := h.gatewayService.ResolveChannelMappingAndRestrict(c.Request.Context(), apiKey.GroupID, reqModel)
+	forwardBody := openAIModelMappedBody(body, channelMapping.Mapped, channelMapping.MappedModel, h.gatewayService.ReplaceModelInBody)
 
 	// 提前校验 function_call_output 是否具备可关联上下文，避免上游 400。
 	if !h.validateFunctionCallOutputRequest(c, body, reqLog) {
@@ -353,11 +378,6 @@ func (h *OpenAIGatewayHandler) Responses(c *gin.Context) {
 		// Forward request
 		service.SetOpsLatencyMs(c, service.OpsRoutingLatencyMsKey, time.Since(routingStart).Milliseconds())
 		forwardStart := time.Now()
-		// 应用渠道模型映射到请求体
-		forwardBody := body
-		if channelMapping.Mapped {
-			forwardBody = h.gatewayService.ReplaceModelInBody(body, channelMapping.MappedModel)
-		}
 		writerSizeBeforeForward := c.Writer.Size()
 		result, err := func() (*service.OpenAIForwardResult, error) {
 			defer func() {
@@ -660,6 +680,7 @@ func (h *OpenAIGatewayHandler) Messages(c *gin.Context) {
 
 	// 解析渠道级模型映射
 	channelMappingMsg, _ := h.gatewayService.ResolveChannelMappingAndRestrict(c.Request.Context(), apiKey.GroupID, reqModel)
+	mappedBodyForMessages := newOpenAIModelMappedBodyCache(body, h.gatewayService.ReplaceModelInBody)
 
 	// 绑定错误透传服务，允许 service 层在非 failover 错误场景复用规则。
 	if h.errorPassthroughService != nil {
@@ -758,10 +779,7 @@ func (h *OpenAIGatewayHandler) Messages(c *gin.Context) {
 
 		defaultMappedModel := strings.TrimSpace(effectiveMappedModel)
 		// 应用渠道模型映射到请求体
-		forwardBody := body
-		if channelMappingMsg.Mapped {
-			forwardBody = h.gatewayService.ReplaceModelInBody(body, channelMappingMsg.MappedModel)
-		}
+		forwardBody := mappedBodyForMessages(channelMappingMsg.Mapped, channelMappingMsg.MappedModel)
 		writerSizeBeforeForward := c.Writer.Size()
 		result, err := func() (*service.OpenAIForwardResult, error) {
 			defer func() {
diff --git a/backend/internal/handler/openai_gateway_handler_test.go b/backend/internal/handler/openai_gateway_handler_test.go
index e7605e35e8..7743d6ab62 100644
--- a/backend/internal/handler/openai_gateway_handler_test.go
+++ b/backend/internal/handler/openai_gateway_handler_test.go
@@ -445,6 +445,41 @@ func TestResolveOpenAIMessagesDispatchMappedModel(t *testing.T) {
 	})
 }
 
+func TestOpenAIModelMappedBody(t *testing.T) {
+	body := []byte(`{"model":"alias","input":"hello"}`)
+	calls := 0
+
+	forwardBody := openAIModelMappedBody(body, true, "gpt-5.4", func(body []byte, newModel string) []byte {
+		calls++
+		return service.ReplaceModelInBody(body, newModel)
+	})
+
+	require.Equal(t, 1, calls)
+	require.Equal(t, "gpt-5.4", gjson.GetBytes(forwardBody, "model").String())
+	require.Equal(t, "alias", gjson.GetBytes(body, "model").String())
+}
+
+func TestOpenAIModelMappedBodyCache(t *testing.T) {
+	body := []byte(`{"model":"alias","input":"hello"}`)
+	calls := 0
+	mappedBody := newOpenAIModelMappedBodyCache(body, func(body []byte, newModel string) []byte {
+		calls++
+		return service.ReplaceModelInBody(body, newModel)
+	})
+
+	first := mappedBody(true, "gpt-5.4")
+	second := mappedBody(true, "gpt-5.4")
+	third := mappedBody(true, "gpt-5.3-codex")
+	unmapped := mappedBody(false, "ignored")
+
+	require.Equal(t, 2, calls)
+	require.Equal(t, "gpt-5.4", gjson.GetBytes(first, "model").String())
+	require.Equal(t, "gpt-5.4", gjson.GetBytes(second, "model").String())
+	require.Equal(t, "gpt-5.3-codex", gjson.GetBytes(third, "model").String())
+	require.Equal(t, body, unmapped)
+	require.Same(t, &first[0], &second[0])
+}
+
 func TestOpenAIResponses_MissingDependencies_ReturnsServiceUnavailable(t *testing.T) {
 	gin.SetMode(gin.TestMode)