Local references in external schemas are not resolved.

[csdebruin]

    const resolvedSchema = resolver.resolve(
        {
            "$id": "test",
            "title": "Test",
            "type": "object",
            "properties": {
                "TestString": { "$ref": "DefSchema#/$defs/Username" }
            }                      
        }, 
        {
            externalSchemas: [
                {
                    "$id": "DefSchema",
                    "$defs": {
                        "Username": { "$ref": "#/$defs/string:10" },
                        "string:10": {
                            "type": "string",
                            "maxLength": 10
                        }
                    }
                }
            ]
        }
    )

Produces the following incorrect reference:

{
  "$id": "test",
  "definitions": {
     "def-0": {
      "$defs": {
        "Username": { "$ref": "#/$defs/string:10" }
      }
    }
  }
}

As opposed to the expected:

{
  "$id": "test",
  "definitions": {
     "def-0": {
      "$defs": {
        "Username": { "$ref": "#/definitions/def-0/$defs/string:10" }
      }
    }
  }
}

[Eomm]

Finally I was able to replicate it:

const RefResolver = require('json-schema-resolver')

const ref = RefResolver({
  clone: true, // Clone the input schema without changing it. Default: false,
  buildLocalReference (json, baseUri, fragment, i) {
    return `def-${i}` // default value
  }
})

const inputSchema = {
  "$id": "http://example.com/root.json",
  "definitions": {
    "A": { "$id": "#foo" },
    "B": {
      "$id": "other.json",
      "definitions": {
        "X": { "$id": "#bar", type: 'string' },
        "Y": { "$id": "t/inner.json", type: 'boolean' }
      }
    },
    "C": {
      "$id": "urn:uuid:ee564b8a-7a87-4125-8c96-e9f123d6766f",
      type: 'object',
    }
  }
}

const addresSchema = {
  $id: 'relativeAddress', // Note: prefer always absolute URI like: http://mysite.com
  type: 'object',
  properties: {
    zip: { $ref: 'urn:uuid:ee564b8a-7a87-4125-8c96-e9f123d6766f' },
    zip2: { $ref: 'http://example.com/t/inner.json' },
    city: { $ref: 'http://example.com/root.json#foo' }
  }
}

const singleSchema = ref.resolve(addresSchema, { externalSchemas: [inputSchema] })
console.log(singleSchema)

{
  '$id': 'relativeAddress',
  type: 'object',
  properties: {
    zip: { '$ref': '#/definitions/def-5' },
    zip2: { '$ref': '#/definitions/def-4' },
    city: { '$ref': '#/definitions/def-0foo' } ❗️ the $ref has the additional `foo` 
  },
  definitions: {
    'def-0': {
      '$id': 'http://example.com/root.json',
      definitions: [Object],
      [Symbol(json-schema-resolver.refToDef)]: 'def-0',
      [Symbol(json-schema-resolver.consumed)]: true
    },
    'def-4': {
      '$id': 't/inner.json',
      type: 'boolean',
      [Symbol(json-schema-resolver.refToDef)]: 'def-4',
      [Symbol(json-schema-resolver.consumed)]: true
    },
    'def-5': {
      '$id': 'urn:uuid:ee564b8a-7a87-4125-8c96-e9f123d6766f',
      type: 'object',
      [Symbol(json-schema-resolver.refToDef)]: 'def-5',
      [Symbol(json-schema-resolver.consumed)]: true
    }
  },
  [Symbol(json-schema-resolver.ignore)]: true
}