[DenisCDev]

responseSchemaCompiler drops the schema being compiled from the external schemas when it has the same $id (#14). It does that by replacing fjsOpts.schema, but fjsOpts is bound once in buildSerializerFactory and shared by every later call. After the first compile the schema is gone for good, and a later { $ref: '<id>#' } fails with Cannot resolve ref.

In Fastify this shows up inside a plugin that has no addSchema of its own, because that context reuses one serializer compiler for all its routes:

// repro.mjs
import Fastify from 'fastify'

const app = Fastify()
app.addSchema({ $id: 'user', type: 'object', properties: { name: { type: 'string' } } })
app.register(async (child) => {
  child.get('/a', { schema: { response: { 200: child.getSchema('user') } } }, async () => ({ name: 'a' }))
  child.get('/b', { schema: { response: { 200: { $ref: 'user#' } } } }, async () => ({ name: 'b' }))
})
await app.ready()
// FST_ERR_SCH_SERIALIZATION_BUILD: Failed building the serialization schema for GET: /b,
// due to error Cannot resolve ref "user#". Schema with id "user" is not found.

It works if the two routes are swapped, or if they are registered on the root instance (the root has its own addSchema, so it builds a new compiler per route). Reproduced on fastify 5.12.7 and on fastify main.

The fix removes the $id from a per-call copy of the options, so the behavior from #14 is kept for each compile and the shared options are left untouched.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

The new test in test/duplicate-schema.test.js fails on main with Schema with id "schema1" is not found and passes here. npm test (11/11, 100% coverage, tstyche) and npm run lint pass. I also ran fastify's schema and internals tests (test/schema-*.test.js, test/output-validation.test.js and test/internals/*.test.js, 25 files) against fastify main with this compiler in place: 2572 passed, 0 failed, 6 skipped.

[SameRainbows]

Validated this locally. Reassigning fjsOpts to a shallow copy with a cloned schema map cleanly prevents the delete mutation from mutating the bound options across subsequent route compilations.

The reproduction scenario with plugin contexts (child.get('/a', ...) followed by child.get('/b', ...) with $ref) is verified, and the test in test/duplicate-schema.test.js exercises the exact sequence without adding overhead.

Ready to merge from my testing.