diff --git a/frontend/src/components/ConnectionModal.edit-password.test.tsx b/frontend/src/components/ConnectionModal.edit-password.test.tsx
index 735b666..112d093 100644
--- a/frontend/src/components/ConnectionModal.edit-password.test.tsx
+++ b/frontend/src/components/ConnectionModal.edit-password.test.tsx
@@ -32,15 +32,22 @@ describe('ConnectionModal data source registry', () => {
     expect(source).toContain('icon: getDbIcon(item.key, undefined, 36)');
     expect(source).toContain('type === "elasticsearch"');
     expect(source).toContain("return '支持索引浏览、Mapping 检查、JSON DSL 和 query_string 查询';");
+    expect(source).toContain('const PRIMARY_USERNAME_OPTIONAL_TYPES = new Set([');
+    expect(source).toContain('"mqtt",');
     expect(source).toContain(
       'type === "clickhouse" ? "default" : (type === "redis" || type === "elasticsearch" || type === "chroma" || type === "qdrant" || type === "rocketmq" || type === "mqtt" || type === "kafka" || type === "rabbitmq") ? "" : "root";',
     );
-    expect(source).toContain(
-      'placeholder={(dbType === "elasticsearch" || dbType === "chroma" || dbType === "qdrant" || dbType === "rocketmq" || dbType === "mqtt" || dbType === "kafka" || dbType === "rabbitmq") ? "未开启认证可留空" : undefined}',
-    );
+    expect(source).toContain('PRIMARY_USERNAME_OPTIONAL_TYPES.has(dbType)');
     expect(source).toContain('label="显示数据库 (留空显示全部)"');
   });
 
+  it('keeps MQTT username optional during test-connection validation', () => {
+    expect(source).toContain('"mqtt",');
+    expect(source).toContain('PRIMARY_USERNAME_OPTIONAL_TYPES.has(dbType)');
+    expect(source).toContain(': [createUriAwareRequiredRule("请输入用户名")]');
+    expect(source).toContain('? "未开启认证可留空"');
+  });
+
   it('exposes Chroma in the create-connection picker with vector defaults', () => {
     expect(source).toContain("case 'chroma':");
     expect(source).toContain('return 8000;');
diff --git a/frontend/src/components/ConnectionModal.tsx b/frontend/src/components/ConnectionModal.tsx
index 0a65376..ee4cb48 100644
--- a/frontend/src/components/ConnectionModal.tsx
+++ b/frontend/src/components/ConnectionModal.tsx
@@ -140,6 +140,16 @@ const MAX_URI_LENGTH = 4096;
 const MAX_CONNECTION_PARAMS_LENGTH = 4096;
 const MAX_URI_HOSTS = 32;
 const MAX_TIMEOUT_SECONDS = 3600;
+const PRIMARY_USERNAME_OPTIONAL_TYPES = new Set([
+  "mongodb",
+  "elasticsearch",
+  "chroma",
+  "qdrant",
+  "rocketmq",
+  "mqtt",
+  "kafka",
+  "rabbitmq",
+]);
 const CONNECTION_MODAL_WIDTH = 960;
 const CONNECTION_MODAL_BODY_HEIGHT = 620;
 const REDIS_DEFAULT_DATABASE_COUNT = 16;
@@ -5634,13 +5644,20 @@ const ConnectionModal: React.FC<{
                           name="user"
                           label={dbType === "rocketmq" ? "Access Key" : "用户名"}
                           rules={
-                            (dbType === "mongodb" || dbType === "elasticsearch" || dbType === "chroma" || dbType === "qdrant" || dbType === "rocketmq" || dbType === "kafka" || dbType === "rabbitmq")
+                            PRIMARY_USERNAME_OPTIONAL_TYPES.has(dbType)
                               ? []
                               : [createUriAwareRequiredRule("请输入用户名")]
                           }
                           style={{ marginBottom: 0 }}
                         >
-                          <Input {...noAutoCapInputProps} placeholder={(dbType === "elasticsearch" || dbType === "chroma" || dbType === "qdrant" || dbType === "rocketmq" || dbType === "mqtt" || dbType === "kafka" || dbType === "rabbitmq") ? "未开启认证可留空" : undefined} />
+                          <Input
+                            {...noAutoCapInputProps}
+                            placeholder={
+                              PRIMARY_USERNAME_OPTIONAL_TYPES.has(dbType)
+                                ? "未开启认证可留空"
+                                : undefined
+                            }
+                          />
                         </Form.Item>
                         <Form.Item
                           name="password"