diff --git a/packages/form/package.json b/packages/form/package.json
index eb2f254..7929a09 100644
--- a/packages/form/package.json
+++ b/packages/form/package.json
@@ -1,7 +1,7 @@
 {
   "name": "schemastery-vue",
   "description": "Type driven schema validator",
-  "version": "4.0.0-alpha.4",
+  "version": "4.0.0-alpha.5",
   "main": "src/index.ts",
   "repository": {
     "type": "git",
diff --git a/packages/form/src/base.vue b/packages/form/src/base.vue
index 0ad5d2a..d7c6435 100644
--- a/packages/form/src/base.vue
+++ b/packages/form/src/base.vue
@@ -42,7 +42,7 @@ defineProps({
   disabled: {} as PropType<boolean>,
   prefix: {} as PropType<string>,
   initial: {} as PropType<{}>,
-  foldable: Boolean,
+  extra: {} as PropType<any>,
 })
 
 defineEmits(['update:modelValue', 'visible-change'])
@@ -69,10 +69,6 @@ defineEmits(['update:modelValue', 'visible-change'])
     }
   }
 
-  .el-button.ellipsis {
-    padding: 8px 10px;
-  }
-
   .el-button + .el-button {
     margin-left: 0;
   }
diff --git a/packages/form/src/extensions/bitset.vue b/packages/form/src/extensions/bitset.vue
index 2ef8979..75a4124 100644
--- a/packages/form/src/extensions/bitset.vue
+++ b/packages/form/src/extensions/bitset.vue
@@ -29,7 +29,6 @@ defineProps({
   disabled: {} as PropType<boolean>,
   prefix: {} as PropType<string>,
   initial: {} as PropType<{}>,
-  foldable: Boolean,
 })
 
 defineEmits(['update:modelValue'])
diff --git a/packages/form/src/extensions/group.vue b/packages/form/src/extensions/group.vue
index 1b2f749..9714b8a 100644
--- a/packages/form/src/extensions/group.vue
+++ b/packages/form/src/extensions/group.vue
@@ -6,7 +6,7 @@
     <template #prefix><slot name="prefix"></slot></template>
     <template #suffix><slot name="suffix"></slot></template>
     <template #control>
-      <el-button type="primary" @click="add()" :disabled="disabled">添加项</el-button>
+      <el-button type="primary" @click="add()" :disabled="disabled">添加项目</el-button>
     </template>
   </schema-base>
 
@@ -15,17 +15,20 @@
       v-for="([key, _], index) in entries"
       :key="index"
       v-model="entries[index][1]"
-      :invalid="entries.filter(e => e[0] === key).length > 1"
       :initial="(initial ?? schema.meta.default)[key]"
       :schema="schema.inner"
       :disabled="disabled"
       :prefix="schema.type === 'array' ? `${prefix.slice(0, -1)}[${key}].` : prefix + key + '.'"
-      foldable
+      :extra="{
+        foldable: true,
+        changed: key in (initial ?? schema.meta.default) ? undefined : true,
+        invalid: entries.filter(e => e[0] === key).length > 1,
+      }"
     >
       <template #menu>
-        <el-dropdown-item divided :disabled="!index" @click="up(index)">上移</el-dropdown-item>
-        <el-dropdown-item :disabled="index === entries.length - 1" @click="down(index)">下移</el-dropdown-item>
-        <el-dropdown-item @click="del(index)">删除</el-dropdown-item>
+        <el-dropdown-item divided :disabled="!index" @click="up(index)">上移项目</el-dropdown-item>
+        <el-dropdown-item :disabled="index === entries.length - 1" @click="down(index)">下移项目</el-dropdown-item>
+        <el-dropdown-item @click="del(index)">删除项目</el-dropdown-item>
       </template>
       <template #title>
         <span class="prefix">{{ prefix.slice(0, -1) }}</span>
@@ -56,7 +59,7 @@ defineProps({
   disabled: {} as PropType<boolean>,
   prefix: {} as PropType<string>,
   initial: {} as PropType<any>,
-  foldable: Boolean,
+  extra: {} as PropType<any>,
 })
 
 defineEmits(['update:modelValue'])
diff --git a/packages/form/src/extensions/intersect.vue b/packages/form/src/extensions/intersect.vue
index 26c660c..c8c6fec 100644
--- a/packages/form/src/extensions/intersect.vue
+++ b/packages/form/src/extensions/intersect.vue
@@ -15,6 +15,14 @@
   </k-schema>
 </template>
 
+<script lang="ts">
+
+export default {
+  inheritAttrs: false,
+}
+
+</script>
+
 <script lang="ts" setup>
 
 import { PropType } from 'vue'
@@ -26,8 +34,7 @@ defineProps({
   disabled: {} as PropType<boolean>,
   prefix: {} as PropType<string>,
   initial: {} as PropType<any>,
-  class: {} as PropType<any>,
-  foldable: Boolean,
+  extra: {} as PropType<any>,
 })
 
 defineEmits(['update:modelValue'])
diff --git a/packages/form/src/extensions/object.vue b/packages/form/src/extensions/object.vue
index 7669f6c..4526311 100644
--- a/packages/form/src/extensions/object.vue
+++ b/packages/form/src/extensions/object.vue
@@ -1,10 +1,14 @@
 <template>
-  <schema-base v-if="foldable">
+  <schema-base v-if="extra?.foldable" v-bind="$attrs">
     <template #title><slot name="title"></slot></template>
     <template #desc>
-      <k-markdown :source="schema.meta.description"></k-markdown>
+      <slot name="desc">
+        <k-markdown :source="schema?.meta.description"></k-markdown>
+      </slot>
     </template>
     <template #menu><slot name="menu"></slot></template>
+    <template #prefix><slot name="prefix"></slot></template>
+    <template #suffix><slot name="suffix"></slot></template>
   </schema-base>
   <h2 class="k-schema-header" v-else-if="schema.meta.description">
     {{ schema.meta.description }}
@@ -25,6 +29,14 @@
   </k-schema>
 </template>
 
+<script lang="ts">
+
+export default {
+  inheritAttrs: false,
+}
+
+</script>
+
 <script lang="ts" setup>
 
 import { PropType } from 'vue'
@@ -37,8 +49,7 @@ defineProps({
   disabled: {} as PropType<boolean>,
   prefix: {} as PropType<string>,
   initial: {} as PropType<any>,
-  foldable: Boolean,
-  class: {} as PropType<any>,
+  extra: {} as PropType<any>,
 })
 
 defineEmits(['update:modelValue'])
diff --git a/packages/form/src/extensions/radio.vue b/packages/form/src/extensions/radio.vue
index a219c4a..3fc008e 100644
--- a/packages/form/src/extensions/radio.vue
+++ b/packages/form/src/extensions/radio.vue
@@ -29,7 +29,6 @@ defineProps({
   disabled: {} as PropType<boolean>,
   prefix: {} as PropType<string>,
   initial: {} as PropType<{}>,
-  foldable: Boolean,
 })
 
 defineEmits(['update:modelValue'])
diff --git a/packages/form/src/extensions/table.vue b/packages/form/src/extensions/table.vue
index ba976c2..fe6147d 100644
--- a/packages/form/src/extensions/table.vue
+++ b/packages/form/src/extensions/table.vue
@@ -50,7 +50,6 @@ defineProps({
   disabled: {} as PropType<boolean>,
   prefix: {} as PropType<string>,
   initial: {} as PropType<{}>,
-  foldable: Boolean,
 })
 
 defineEmits(['update:modelValue'])
diff --git a/packages/form/src/extensions/textarea.vue b/packages/form/src/extensions/textarea.vue
index 4b64770..60594ce 100644
--- a/packages/form/src/extensions/textarea.vue
+++ b/packages/form/src/extensions/textarea.vue
@@ -29,7 +29,6 @@ const props = defineProps({
   disabled: {} as PropType<boolean>,
   prefix: {} as PropType<string>,
   initial: {} as PropType<{}>,
-  foldable: Boolean,
 })
 
 defineEmits(['update:modelValue'])
diff --git a/packages/form/src/extensions/tuple.vue b/packages/form/src/extensions/tuple.vue
index 27a67f0..3a8fdea 100644
--- a/packages/form/src/extensions/tuple.vue
+++ b/packages/form/src/extensions/tuple.vue
@@ -30,7 +30,6 @@ const props = defineProps({
   disabled: {} as PropType<boolean>,
   prefix: {} as PropType<string>,
   initial: {} as PropType<{}>,
-  foldable: Boolean,
 })
 
 const emit = defineEmits(['update:modelValue'])
diff --git a/packages/form/src/extensions/union.vue b/packages/form/src/extensions/union.vue
index 11d37fb..5edeb5e 100644
--- a/packages/form/src/extensions/union.vue
+++ b/packages/form/src/extensions/union.vue
@@ -6,6 +6,7 @@
     :disabled="disabled"
     :prefix="prefix"
     :extra="{
+      foldable: !!prefix,
       required: !!schema.meta.required && isNullable(schema.meta.default) && isNullable(modelValue),
     }"
   >
@@ -15,6 +16,7 @@
     </template>
     <template #prefix>
       <el-select
+        v-if="choices.length > 1"
         v-model="selectModel"
         filterable
         :disabled="disabled"
@@ -42,8 +44,7 @@ const props = defineProps({
   disabled: {} as PropType<boolean>,
   prefix: {} as PropType<string>,
   initial: {} as PropType<any>,
-  class: {} as PropType<any>,
-  foldable: Boolean,
+  extra: {} as PropType<any>,
 })
 
 const emit = defineEmits(['update:modelValue'])
@@ -57,7 +58,7 @@ let stop: WatchStopHandle
 const doWatch = () => watch(config, (value) => {
   const index = choices.value.indexOf(active.value)
   if (index >= 0) cache.value[index] = value
-  emit('update:modelValue', deepEqual(value, props.schema.meta.default) ? undefined : value)
+  emit('update:modelValue', deepEqual(value, props.schema.meta.default) ? null : value)
 }, { deep: true })
 
 watch(() => props.schema, (value) => {
diff --git a/packages/form/src/index.ts b/packages/form/src/index.ts
index 93301af..3d3bd07 100644
--- a/packages/form/src/index.ts
+++ b/packages/form/src/index.ts
@@ -21,6 +21,7 @@ namespace form {
   export interface Extension {
     type: string
     role?: string
+    validate?: (value: any, schema: any) => boolean
     component: Component
   }
 
@@ -29,26 +30,31 @@ namespace form {
   extensions.add({
     type: 'bitset',
     component: Bitset,
+    validate: value => typeof value === 'number',
   })
 
   extensions.add({
     type: 'array',
     component: Group,
+    validate: value => Array.isArray(value),
   })
 
   extensions.add({
     type: 'dict',
     component: Group,
+    validate: value => typeof value === 'object',
   })
 
   extensions.add({
     type: 'object',
     component: Object,
+    validate: value => typeof value === 'object',
   })
 
   extensions.add({
     type: 'intersect',
     component: Intersect,
+    validate: value => typeof value === 'object',
   })
 
   extensions.add({
@@ -61,23 +67,27 @@ namespace form {
     type: 'array',
     role: 'table',
     component: Table,
+    validate: value => Array.isArray(value),
   })
 
   extensions.add({
     type: 'dict',
     role: 'table',
     component: Table,
+    validate: value => typeof value === 'object',
   })
 
   extensions.add({
     type: 'string',
     role: 'textarea',
     component: Textarea,
+    validate: value => typeof value === 'string',
   })
 
   extensions.add({
     type: 'tuple',
     component: Tuple,
+    validate: value => Array.isArray(value),
   })
 
   extensions.add({
diff --git a/packages/form/src/schema.vue b/packages/form/src/schema.vue
index 18552a3..8cc4c5c 100644
--- a/packages/form/src/schema.vue
+++ b/packages/form/src/schema.vue
@@ -1,17 +1,17 @@
 <template>
   <schema-component
-    v-if="!schema?.meta.hidden"
+    v-if="!schema?.meta.hidden && (extra?.foldable || (schema && schema.type !== 'const'))"
     :schema="schema"
     :prefix="prefix"
     :initial="initial"
     :disabled="disabled"
-    :foldable="foldable"
+    :extra="extra"
     :modelValue="modelValue"
     @update:modelValue="$emit('update:modelValue', $event)"
     :class="{
-      changed: !deepEqual(initial, modelValue),
+      changed: extra?.changed ?? !deepEqual(initial, modelValue),
       required: extra?.required ?? (schema?.meta.required && isNullable(schema?.meta.default) && isNullable(modelValue)),
-      invalid,
+      invalid: extra?.invalid,
     }"
   >
     <template #title><slot name="title"></slot></template>
@@ -28,7 +28,7 @@
     <template #prefix><slot name="prefix"></slot></template>
     <template #suffix><slot name="suffix"></slot></template>
     <template #control>
-      <schema-primitive v-if="['string', 'number', 'boolean'].includes(schema?.type)"
+      <schema-primitive v-if="isPrimitive"
         :schema="schema"
         :disabled="disabled"
         :modelValue="modelValue"
@@ -49,21 +49,28 @@ import SchemaBase from './base.vue'
 const props = defineProps({
   schema: {} as PropType<Schema>,
   initial: {} as PropType<any>,
-  modelValue: {},
-  invalid: Boolean,
+  modelValue: {} as PropType<any>,
   extra: {} as PropType<any>,
   disabled: Boolean,
-  foldable: Boolean,
   branch: Boolean,
   prefix: { type: String, default: '' },
 })
 
 defineEmits(['update:modelValue'])
 
+const isPrimitive = computed(() => {
+  return ['string', 'number', 'boolean'].includes(props.schema?.type)
+    && (isNullable(props.modelValue) || typeof props.modelValue === props.schema.type)
+})
+
 const SchemaComponent = computed(() => {
   const candidates = [...form.extensions].map((ext) => {
     if (ext.type && props.schema?.type !== ext.type) return
     if (ext.role && props.schema?.meta.role !== ext.role) return
+    if (ext.validate) {
+      const valid = isNullable(props.modelValue) || ext.validate(props.modelValue, props.schema)
+      if (!valid) return
+    }
     return [ext.component, +!!ext.type + +!!ext.role] as const
   }).filter(Boolean).sort((a, b) => b[1] - a[1])
   candidates.push([SchemaBase, 0])