agent-skills/skills/vue-best-practices/reference/v-model-vue3-breaking-changes.md

title, impact, impactDescription, type, tags

title	impact	impactDescription	type	tags
v-model Breaking Changes from Vue 2 to Vue 3	HIGH	Vue 3 changed v-model prop/event names and removed .sync modifier - migration required	migration	vue3 vue2 v-model migration breaking-changes sync

v-model Breaking Changes from Vue 2 to Vue 3

Impact: HIGH - Vue 3 fundamentally changed how v-model works on custom components. Code using the Vue 2 pattern will silently fail - the component won't receive the prop value and changes won't propagate to the parent.

Task Checklist

• Change prop name from value to modelValue
• Change event name from input to update:modelValue
• Replace .sync modifier with v-model:propName
• Remove model component option (use defineModel or named v-model)
• Update any v-bind.sync to v-model with named argument

Key Breaking Changes

Feature	Vue 2	Vue 3
Default prop	value	modelValue
Default event	input	update:modelValue
Custom name	model: { prop, event }	v-model:customName
Sync modifier	v-bind:prop.sync	v-model:prop
Multiple models	Not supported	Fully supported

Vue 2 Pattern (No longer works in Vue 3):

<!-- Vue 2 Child Component -->
<script>
export default {
  props: ['value'],  // WRONG in Vue 3
  methods: {
    update(val) {
      this.$emit('input', val)  // WRONG in Vue 3
    }
  }
}
</script>

<template>
  <input :value="value" @input="update($event.target.value)">
</template>

Vue 3 Pattern (Options API):

<!-- Vue 3 Child Component -->
<script>
export default {
  props: ['modelValue'],  // Changed from 'value'
  emits: ['update:modelValue'],  // Declare emits
  methods: {
    update(val) {
      this.$emit('update:modelValue', val)  // Changed from 'input'
    }
  }
}
</script>

<template>
  <input :value="modelValue" @input="update($event.target.value)">
</template>

Vue 3 Pattern (Composition API with defineModel):

<!-- Vue 3 Child Component - Recommended -->
<script setup>
const model = defineModel()  // Handles prop and emit automatically
</script>

<template>
  <input v-model="model">
</template>

Migrating the .sync Modifier

Vue 2's .sync modifier is removed. Use named v-model instead.

Vue 2:

<!-- Parent -->
<MyComponent :title.sync="pageTitle" />

<!-- Child -->
<script>
export default {
  props: ['title'],
  methods: {
    updateTitle(val) {
      this.$emit('update:title', val)  // .sync pattern
    }
  }
}
</script>

Vue 3:

<!-- Parent -->
<MyComponent v-model:title="pageTitle" />

<!-- Child with defineModel -->
<script setup>
const title = defineModel('title')
</script>

<template>
  <input v-model="title">
</template>

<!-- Child with manual props/emits -->
<script setup>
const props = defineProps(['title'])
const emit = defineEmits(['update:title'])
</script>

<template>
  <input
    :value="props.title"
    @input="emit('update:title', $event.target.value)"
  >
</template>

Migrating Custom model Option

Vue 2's model component option is removed.

Vue 2:

<script>
export default {
  model: {
    prop: 'checked',
    event: 'change'
  },
  props: ['checked']
}
</script>

Vue 3:

<!-- Use named v-model argument instead -->
<!-- Parent -->
<MyCheckbox v-model:checked="isChecked" />

<!-- Child -->
<script setup>
const checked = defineModel('checked')
</script>

Multiple v-model Bindings (New in Vue 3)

Vue 3 allows multiple v-model directives on a single component:

<!-- Parent -->
<UserForm
  v-model:firstName="first"
  v-model:lastName="last"
  v-model:email="email"
/>

<!-- Child -->
<script setup>
const firstName = defineModel('firstName')
const lastName = defineModel('lastName')
const email = defineModel('email')
</script>

<template>
  <input v-model="firstName" placeholder="First Name">
  <input v-model="lastName" placeholder="Last Name">
  <input v-model="email" type="email" placeholder="Email">
</template>

Reference

• Vue 3 Migration Guide - v-model
• Vue.js Component v-model