fix(naming): do not force clusterName to DEFAULT in v3 HTTP API

naming/src/main/java/com/alibaba/nacos/naming/core/CatalogServiceV2Impl.java

@@ -113,7 +113,7 @@ public List<? extends Instance> listInstances(String namespaceId, String groupNa
             throw new NacosException(NacosException.NOT_FOUND,
                     String.format("service %s@@%s is not found!", groupName, serviceName));
         }
-        if (!serviceStorage.getClusters(service).contains(clusterName)) {
+        if (StringUtils.isNotBlank(clusterName) && !serviceStorage.getClusters(service).contains(clusterName)) {
             throw new NacosException(NacosException.NOT_FOUND, "cluster " + clusterName + " is not found!");
         }
         ServiceInfo serviceInfo = serviceStorage.getData(service);

naming/src/main/java/com/alibaba/nacos/naming/model/form/InstanceListForm.java

@@ -21,7 +21,6 @@
 import com.alibaba.nacos.api.model.v2.ErrorCode;
 import com.alibaba.nacos.common.utils.StringUtils;
 import com.alibaba.nacos.api.model.NacosForm;
-import com.alibaba.nacos.naming.misc.UtilsAndCommons;
 import org.springframework.http.HttpStatus;
 
 import java.util.Objects;
@@ -69,7 +68,7 @@ public void fillDefaultValue() {
             groupName = Constants.DEFAULT_GROUP;
         }
         if (StringUtils.isBlank(clusterName)) {
-            clusterName = UtilsAndCommons.DEFAULT_CLUSTER_NAME;
+            clusterName = StringUtils.EMPTY;
         }
         if (null == healthyOnly) {
             healthyOnly = false;

naming/src/test/java/com/alibaba/nacos/naming/controllers/v3/InstanceOpenApiControllerTest.java

@@ -18,6 +18,7 @@
 
 import com.alibaba.nacos.api.common.Constants;
 import com.alibaba.nacos.api.exception.NacosException;
+import com.alibaba.nacos.common.utils.StringUtils;
 import com.alibaba.nacos.api.model.v2.ErrorCode;
 import com.alibaba.nacos.api.model.v2.Result;
 import com.alibaba.nacos.api.naming.NamingResponseCode;
@@ -119,7 +120,7 @@ void testList() throws Exception {
         serviceInfo.setName("test");
         serviceInfo.setHosts(Collections.singletonList(instance));
         when(instanceOperator.listInstance(Constants.DEFAULT_NAMESPACE_ID, Constants.DEFAULT_GROUP, "test", null,
-                Constants.DEFAULT_CLUSTER_NAME, false)).thenReturn(serviceInfo);
+                StringUtils.EMPTY, false)).thenReturn(serviceInfo);
         Result<List<Instance>> actual = instanceOpenApiController.list(instanceForm);
         assertEquals(ErrorCode.SUCCESS.getCode(), actual.getCode());
         assertEquals(ErrorCode.SUCCESS.getMsg(), actual.getMessage());

naming/src/test/java/com/alibaba/nacos/naming/core/CatalogServiceV2ImplTest.java

@@ -131,7 +131,38 @@ void testListInstancesNonExistCluster() throws NacosException {
             catalogServiceV2Impl.listInstances("A", "B", "C", "DD");
         });
     }
-
+
+    @Test
+    void testListInstancesWithBlankClusterNameReturnsAll() throws NacosException {
+        ServiceInfo serviceInfo = new ServiceInfo();
+        serviceInfo.setGroupName("B");
+        serviceInfo.setName("C");
+        Instance instance1 = new Instance();
+        instance1.setClusterName("cluster1");
+        instance1.setIp("1.1.1.1");
+        Instance instance2 = new Instance();
+        instance2.setClusterName("cluster2");
+        instance2.setIp("2.2.2.2");
+        serviceInfo.setHosts(List.of(instance1, instance2));
+        Mockito.when(serviceStorage.getData(Mockito.any())).thenReturn(serviceInfo);
+        List<? extends Instance> instances = catalogServiceV2Impl.listInstances("A", "B", "C", "");
+        assertEquals(2, instances.size());
+    }
+
+    @Test
+    void testListInstancesWithNullClusterNameReturnsAll() throws NacosException {
+        ServiceInfo serviceInfo = new ServiceInfo();
+        serviceInfo.setGroupName("B");
+        serviceInfo.setName("C");
+        Instance instance = new Instance();
+        instance.setClusterName("anyCluster");
+        instance.setIp("1.1.1.1");
+        serviceInfo.setHosts(Collections.singletonList(instance));
+        Mockito.when(serviceStorage.getData(Mockito.any())).thenReturn(serviceInfo);
+        List<? extends Instance> instances = catalogServiceV2Impl.listInstances("A", "B", "C", null);
+        assertEquals(1, instances.size());
+    }
+
     @Test
     void testPageListService() throws NacosException {
         ServiceInfo serviceInfo = new ServiceInfo();

naming/src/test/java/com/alibaba/nacos/naming/model/form/InstanceListFormTest.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright 1999-2025 Alibaba Group Holding Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.alibaba.nacos.naming.model.form;
+
+import com.alibaba.nacos.api.common.Constants;
+import com.alibaba.nacos.api.exception.api.NacosApiException;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+class InstanceListFormTest {
+
+    @Test
+    void testFillDefaultValueWhenClusterNameIsBlank() throws NacosApiException {
+        InstanceListForm form = new InstanceListForm();
+        form.setServiceName("testService");
+        form.validate();
+        assertEquals("", form.getClusterName());
+    }
+
+    @Test
+    void testFillDefaultValueWhenClusterNameIsProvided() throws NacosApiException {
+        InstanceListForm form = new InstanceListForm();
+        form.setServiceName("testService");
+        form.setClusterName("myCluster");
+        form.validate();
+        assertEquals("myCluster", form.getClusterName());
+    }
+
+    @Test
+    void testFillDefaultValueForNamespaceAndGroup() throws NacosApiException {
+        InstanceListForm form = new InstanceListForm();
+        form.setServiceName("testService");
+        form.validate();
+        assertEquals(Constants.DEFAULT_NAMESPACE_ID, form.getNamespaceId());
+        assertEquals(Constants.DEFAULT_GROUP, form.getGroupName());
+    }
+
+    @Test
+    void testValidateThrowsExceptionWhenServiceNameIsBlank() {
+        InstanceListForm form = new InstanceListForm();
+        assertThrows(NacosApiException.class, () -> form.validate());
+    }
+}