Vue 3 v-model on custom component silently breaks when using legacy prop names

Problem

A Vue 3 component that still exposes a value prop and emits an input event will not react to v-model bindings in the parent. The directive silently ignores the binding, leaving the parent state unchanged.

Goal

Determine the exact conditions that cause v-model to fail on a custom component and establish a reliable migration pattern from Vue 2’s implicit value/input pair to Vue 3’s explicit modelValue/update:modelValue contract.

Constraints & Uncertainty

• Component may or may not declare modelValue in props.
• Emits list may omit update:modelValue or retain legacy input.
• Parent uses v-model without specifying a modifier or custom event name.

Questions

1. When a Vue 3 component declares only a value prop and emits input, does the framework provide any warning or diagnostic about the missing modelValue contract?
2. What is the precise sequence of prop and event declarations required for v-model to function correctly on a custom component in Vue 3?
3. Is there a backward‑compatibility mode or a migration helper that automatically maps value/input to modelValue/update:modelValue during the upgrade?