@Fdawgs added the test and fixed lint :)

Note sure if it is a good idea, when encapsulation is exist, you can have same $id with different content.

would you provide an example please? when you add a schema to the ajv instance with an existing id it throws an error

when you add a schema to the ajv instance with an existing id it throws an error

That is the exact problem I mention, fastify allows the same $id of schema exists across different encapsulated context.
You can see the below foo schema shared the same name but different properties.

import fastify from "fastify";
const app = fastify();
app.register(async (instance) => {
 instance.addSchema({
 $id: 'foo',
 type: 'object',
 properties: {
 foo: { type: 'string' }
 }
 })
})
app.register(async (instance) => {
 instance.addSchema({
 $id: 'foo',
 type: 'object',
 properties: {
 foo: { type: 'string' },
 bar: { type: 'string' }
 }
 })
})
await app.ready()

Isn't addSchema encapsulation aware?

The ajv instance is encapsulated too.

The problem is similar to @fastify/swagger. The instance of ajv is global in the root context of registered level.
So, the problem still persist, unless it creates a new instance of ajv on each new context creation.