diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 415889c83..d374fbd9e 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -15,9 +15,9 @@ jobs:
         node-version: [16.x, 18.x, 20.x, 21.x]
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: use Node.js ${{ matrix.node-version }}
-        uses: actions/setup-node@v2
+        uses: actions/setup-node@v4
         with:
           node-version: ${{ matrix.node-version }}
       - run: npm install
@@ -32,6 +32,6 @@ jobs:
       - run: npm run build
       - run: npm run test-ci
       - name: coveralls
-        uses: coverallsapp/github-action@master
+        uses: coverallsapp/github-action@v2
         with:
           github-token: ${{ secrets.GITHUB_TOKEN }}
diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index 2da03454b..35bf8ed67 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -8,21 +8,21 @@ jobs:
   publish-npm:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
-      - uses: actions/setup-node@v2
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
         with:
-          node-version: 14
+          node-version: 18
           registry-url: https://registry.npmjs.org/
       - run: npm install
       - run: git submodule update --init
       - run: npm run test-ci
       - name: Publish beta version to npm
-        if: "github.event.release.prerelease"
+        if: ${{ github.event.release.prerelease }}
         run: npm publish --tag beta
         env:
           NODE_AUTH_TOKEN: ${{ secrets.NPM_TOKEN }}
       - name: Publish to npm
-        if: "!github.event.release.prerelease"
+        if: ${{ !github.event.release.prerelease }}
         run: npm publish
         env:
           NODE_AUTH_TOKEN: ${{ secrets.NPM_TOKEN }}
diff --git a/README.md b/README.md
index c10422e63..d8ee276ce 100644
--- a/README.md
+++ b/README.md
@@ -8,7 +8,7 @@ The fastest JSON validator for Node.js and browser.
 
 Supports JSON Schema draft-04/06/07/2019-09/2020-12 ([draft-04 support](https://ajv.js.org/json-schema.html#draft-04) requires ajv-draft-04 package) and JSON Type Definition [RFC8927](https://datatracker.ietf.org/doc/rfc8927/).
 
-[![build](https://github.com/ajv-validator/ajv/workflows/build/badge.svg)](https://github.com/ajv-validator/ajv/actions?query=workflow%3Abuild)
+[![build](https://github.com/ajv-validator/ajv/actions/workflows/build.yml/badge.svg)](https://github.com/ajv-validator/ajv/actions?query=workflow%3Abuild)
 [![npm](https://img.shields.io/npm/v/ajv.svg)](https://www.npmjs.com/package/ajv)
 [![npm downloads](https://img.shields.io/npm/dm/ajv.svg)](https://www.npmjs.com/package/ajv)
 [![Coverage Status](https://coveralls.io/repos/github/ajv-validator/ajv/badge.svg?branch=master)](https://coveralls.io/github/ajv-validator/ajv?branch=master)
diff --git a/docs/json-schema.md b/docs/json-schema.md
index ac2d82d84..c888c636d 100644
--- a/docs/json-schema.md
+++ b/docs/json-schema.md
@@ -280,6 +280,10 @@ The value of the keyword should be a number. The data to be valid should be a mu
 
 ### `maxLength` / `minLength`
 
+::: warning Grapheme clusters will count as multiple characters
+Certain charsets have characters that are made up of multiple Unicode code points. These [grapheme clusters](https://www.unicode.org/reports/tr29/tr29-17.html#Grapheme_Cluster_Boundaries) are counted as multiple in length calculations.
+:::
+
 The value of the keywords should be a number. The data to be valid should have length satisfying this rule. Unicode pairs are counted as a single character.
 
 **Examples**
diff --git a/docs/options.md b/docs/options.md
index c4a303680..6f74b02b3 100644
--- a/docs/options.md
+++ b/docs/options.md
@@ -240,7 +240,7 @@ An array or object of schemas that will be added to the instance. In case you pa
 
 ### logger
 
-Sets the logging method. Default is the global `console` object that should have methods `log`, `warn` and `error`. See [Error logging](#error-logging).
+Sets the logging method. Default is the global `console` object that should have methods `log`, `warn` and `error`. See [Error logging](./api.md#error-logging).
 
 Option values:
 
diff --git a/docs/packages/README.md b/docs/packages/README.md
index e16bd3483..1d6bf48db 100644
--- a/docs/packages/README.md
+++ b/docs/packages/README.md
@@ -14,11 +14,11 @@ If you have published a useful plugin please submit a PR to add it to the next s
 
 ## Related packages
 
-- [ajv-formats](./ajv-formats.md) - formats defined in JSON Schema specification
-- [ajv-keywords](./ajv-keywords) - additional validation keywords (select, typeof, etc.)
-- [ajv-errors](./ajv-errors.md) - defining error messages in the schema
-- [ajv-i18n](./ajv-i18n) - internationalised error messages
-- [ajv-cli](./ajv-cli.md) - command line interface
+- [ajv-formats](https://github.com/ajv-validator/ajv-formats) - formats defined in JSON Schema specification
+- [ajv-keywords](https://github.com/ajv-validator/ajv-keywords) - additional validation keywords (select, typeof, etc.)
+- [ajv-errors](https://github.com/ajv-validator/ajv-errors) - defining error messages in the schema
+- [ajv-i18n](https://github.com/ajv-validator/ajv-i18n) - internationalised error messages
+- [ajv-cli](https://github.com/ajv-validator/ajv-cli) - command line interface
 - [ajv-bsontype](https://github.com/BoLaMN/ajv-bsontype) - MongoDB's bsonType formats
 - [ajv-formats-draft2019](https://github.com/luzlab/ajv-formats-draft2019) - formats for draft-2019-09 that are not included in [ajv-formats](./ajv-formats.md) (`idn-hostname`, `idn-email`, `iri` and `iri-reference`)
 - [ajv-merge-patch](https://github.com/ajv-validator/ajv-merge-patch) - keywords $merge and $patch
diff --git a/docs/security.md b/docs/security.md
index 7ecf8b031..da851ce5e 100644
--- a/docs/security.md
+++ b/docs/security.md
@@ -93,7 +93,7 @@ If you use Ajv to validate data from untrusted sources **it is strongly recommen
 
 - making assessment of "format" implementations in [ajv-formats](https://github.com/ajv-validator/ajv-formats).
 - passing `"fast"` option to ajv-formats plugin (see its docs) that simplifies some of the regular expressions (although it does not guarantee that they are safe).
-- replacing format implementations provided by ajv-formats with your own implementations of "format" keyword that either use different regular expressions or another approach to format validation. Please see [addFormat](#api-addformat) method.
+- replacing format implementations provided by ajv-formats with your own implementations of "format" keyword that either use different regular expressions or another approach to format validation. Please see [addFormat](https://github.com/ajv-validator/ajv/blob/master/docs/api.md#api-addformat) method.
 - disabling format validation by ignoring "format" keyword with option `format: false`
 
 Whatever mitigation you choose, please assume all formats provided by ajv-formats as potentially unsafe and make your own assessment of their suitability for your validation scenarios.
diff --git a/lib/2019.ts b/lib/2019.ts
index 45a3fa535..3f7194f1b 100644
--- a/lib/2019.ts
+++ b/lib/2019.ts
@@ -10,7 +10,7 @@ import addMetaSchema2019 from "./refs/json-schema-2019-09"
 
 const META_SCHEMA_ID = "https://json-schema.org/draft/2019-09/schema"
 
-class Ajv2019 extends AjvCore {
+export class Ajv2019 extends AjvCore {
   constructor(opts: Options = {}) {
     super({
       ...opts,
@@ -44,6 +44,7 @@ class Ajv2019 extends AjvCore {
 }
 
 module.exports = exports = Ajv2019
+module.exports.Ajv2019 = Ajv2019
 Object.defineProperty(exports, "__esModule", {value: true})
 
 export default Ajv2019
diff --git a/lib/2020.ts b/lib/2020.ts
index afbdda200..cfb36af9d 100644
--- a/lib/2020.ts
+++ b/lib/2020.ts
@@ -7,7 +7,7 @@ import addMetaSchema2020 from "./refs/json-schema-2020-12"
 
 const META_SCHEMA_ID = "https://json-schema.org/draft/2020-12/schema"
 
-class Ajv2020 extends AjvCore {
+export class Ajv2020 extends AjvCore {
   constructor(opts: Options = {}) {
     super({
       ...opts,
@@ -38,6 +38,7 @@ class Ajv2020 extends AjvCore {
 }
 
 module.exports = exports = Ajv2020
+module.exports.Ajv2020 = Ajv2020
 Object.defineProperty(exports, "__esModule", {value: true})
 
 export default Ajv2020
diff --git a/lib/ajv.ts b/lib/ajv.ts
index 7f87c8aea..8275b93a8 100644
--- a/lib/ajv.ts
+++ b/lib/ajv.ts
@@ -8,7 +8,7 @@ const META_SUPPORT_DATA = ["/properties"]
 
 const META_SCHEMA_ID = "http://json-schema.org/draft-07/schema"
 
-class Ajv extends AjvCore {
+export class Ajv extends AjvCore {
   _addVocabularies(): void {
     super._addVocabularies()
     draft7Vocabularies.forEach((v) => this.addVocabulary(v))
@@ -32,6 +32,7 @@ class Ajv extends AjvCore {
 }
 
 module.exports = exports = Ajv
+module.exports.Ajv = Ajv
 Object.defineProperty(exports, "__esModule", {value: true})
 
 export default Ajv
diff --git a/lib/compile/codegen/index.ts b/lib/compile/codegen/index.ts
index 9d29055dc..5a6d1ee58 100644
--- a/lib/compile/codegen/index.ts
+++ b/lib/compile/codegen/index.ts
@@ -41,7 +41,11 @@ abstract class Node {
 }
 
 class Def extends Node {
-  constructor(private readonly varKind: Name, private readonly name: Name, private rhs?: SafeExpr) {
+  constructor(
+    private readonly varKind: Name,
+    private readonly name: Name,
+    private rhs?: SafeExpr
+  ) {
     super()
   }
 
@@ -63,7 +67,11 @@ class Def extends Node {
 }
 
 class Assign extends Node {
-  constructor(readonly lhs: Code, public rhs: SafeExpr, private readonly sideEffects?: boolean) {
+  constructor(
+    readonly lhs: Code,
+    public rhs: SafeExpr,
+    private readonly sideEffects?: boolean
+  ) {
     super()
   }
 
@@ -84,7 +92,12 @@ class Assign extends Node {
 }
 
 class AssignOp extends Assign {
-  constructor(lhs: Code, private readonly op: Code, rhs: SafeExpr, sideEffects?: boolean) {
+  constructor(
+    lhs: Code,
+    private readonly op: Code,
+    rhs: SafeExpr,
+    sideEffects?: boolean
+  ) {
     super(lhs, rhs, sideEffects)
   }
 
@@ -211,7 +224,10 @@ class Else extends BlockNode {
 class If extends BlockNode {
   static readonly kind = "if"
   else?: If | Else
-  constructor(private condition: Code | boolean, nodes?: ChildNode[]) {
+  constructor(
+    private condition: Code | boolean,
+    nodes?: ChildNode[]
+  ) {
     super(nodes)
   }
 
@@ -331,7 +347,11 @@ class ForIter extends For {
 
 class Func extends BlockNode {
   static readonly kind = "func"
-  constructor(public name: Name, public args: Code, public async?: boolean) {
+  constructor(
+    public name: Name,
+    public args: Code,
+    public async?: boolean
+  ) {
     super()
   }
 
diff --git a/lib/compile/validate/dataType.ts b/lib/compile/validate/dataType.ts
index b315c2ce9..d8142b3e1 100644
--- a/lib/compile/validate/dataType.ts
+++ b/lib/compile/validate/dataType.ts
@@ -30,6 +30,7 @@ export function getSchemaTypes(schema: AnySchemaObject): JSONType[] {
   return types
 }
 
+// eslint-disable-next-line @typescript-eslint/no-redundant-type-constituents
 export function getJSONTypes(ts: unknown | unknown[]): JSONType[] {
   const types: unknown[] = Array.isArray(ts) ? ts : ts ? [ts] : []
   if (types.every(isJSONType)) return types
diff --git a/lib/core.ts b/lib/core.ts
index 3686ffe76..e41ca3e2a 100644
--- a/lib/core.ts
+++ b/lib/core.ts
@@ -343,14 +343,17 @@ export default class Ajv {
   validate<T>(schema: JTDSchemaType<T>, data: unknown): data is T
   // This overload is only intended for typescript inference, the first
   // argument prevents manual type annotation from matching this overload
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
   validate<N extends never, T extends SomeJTDSchemaType>(
     schema: T,
     data: unknown
   ): data is JTDDataType<T>
+  // eslint-disable-next-line @typescript-eslint/no-redundant-type-constituents
   validate<T>(schema: AsyncSchema, data: unknown | T): Promise<T>
   validate<T>(schemaKeyRef: AnySchema | string, data: unknown): data is T | Promise<T>
   validate<T>(
     schemaKeyRef: AnySchema | string, // key, ref or schema object
+    // eslint-disable-next-line @typescript-eslint/no-redundant-type-constituents
     data: unknown | T // to be validated
   ): boolean | Promise<T> {
     let v: AnyValidateFunction | undefined
@@ -374,6 +377,7 @@ export default class Ajv {
   compile<T = unknown>(schema: JTDSchemaType<T>, _meta?: boolean): ValidateFunction<T>
   // This overload is only intended for typescript inference, the first
   // argument prevents manual type annotation from matching this overload
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
   compile<N extends never, T extends SomeJTDSchemaType>(
     schema: T,
     _meta?: boolean
diff --git a/lib/jtd.ts b/lib/jtd.ts
index 96eb7b9dc..a7e7bce3b 100644
--- a/lib/jtd.ts
+++ b/lib/jtd.ts
@@ -35,7 +35,7 @@ type JTDOptions = CurrentOptions & {
   multipleOfPrecision?: never
 }
 
-class Ajv extends AjvCore {
+export class Ajv extends AjvCore {
   constructor(opts: JTDOptions = {}) {
     super({
       ...opts,
@@ -93,6 +93,7 @@ class Ajv extends AjvCore {
 }
 
 module.exports = exports = Ajv
+module.exports.Ajv = Ajv
 Object.defineProperty(exports, "__esModule", {value: true})
 
 export default Ajv
diff --git a/lib/types/index.ts b/lib/types/index.ts
index 123d9df16..b5ef53eeb 100644
--- a/lib/types/index.ts
+++ b/lib/types/index.ts
@@ -48,6 +48,7 @@ export interface DataValidationCxt<T extends string | number = string | number>
 }
 
 export interface ValidateFunction<T = unknown> {
+  // eslint-disable-next-line @typescript-eslint/no-redundant-type-constituents
   (this: Ajv | any, data: any, dataCxt?: DataValidationCxt): data is T
   errors?: null | ErrorObject[]
   evaluated?: Evaluated
@@ -138,9 +139,12 @@ export interface DataValidateFunction {
 }
 
 export interface SchemaValidateFunction {
-  (schema: any, data: any, parentSchema?: AnySchemaObject, dataCxt?: DataValidationCxt):
-    | boolean
-    | Promise<any>
+  (
+    schema: any,
+    data: any,
+    parentSchema?: AnySchemaObject,
+    dataCxt?: DataValidationCxt
+  ): boolean | Promise<any>
   errors?: Partial<ErrorObject>[]
 }
 
diff --git a/lib/vocabularies/discriminator/index.ts b/lib/vocabularies/discriminator/index.ts
index 98f0f8cfb..19ae6049f 100644
--- a/lib/vocabularies/discriminator/index.ts
+++ b/lib/vocabularies/discriminator/index.ts
@@ -3,6 +3,7 @@ import type {KeywordCxt} from "../../compile/validate"
 import {_, getProperty, Name} from "../../compile/codegen"
 import {DiscrError, DiscrErrorObj} from "../discriminator/types"
 import {resolveRef, SchemaEnv} from "../../compile"
+import MissingRefError from "../../compile/ref_error"
 import {schemaHasRulesButRef} from "../../compile/util"
 
 export type DiscriminatorError = DiscrErrorObj<DiscrError.Tag> | DiscrErrorObj<DiscrError.Mapping>
@@ -66,8 +67,10 @@ const def: CodeKeywordDefinition = {
       for (let i = 0; i < oneOf.length; i++) {
         let sch = oneOf[i]
         if (sch?.$ref && !schemaHasRulesButRef(sch, it.self.RULES)) {
-          sch = resolveRef.call(it.self, it.schemaEnv.root, it.baseId, sch?.$ref)
+          const ref = sch.$ref
+          sch = resolveRef.call(it.self, it.schemaEnv.root, it.baseId, ref)
           if (sch instanceof SchemaEnv) sch = sch.schema
+          if (sch === undefined) throw new MissingRefError(it.opts.uriResolver, it.baseId, ref)
         }
         const propSch = sch?.properties?.[tagName]
         if (typeof propSch != "object") {
diff --git a/package.json b/package.json
index 8ddabaa04..ca56231a8 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "ajv",
-  "version": "8.12.0",
+  "version": "8.16.0",
   "description": "Another JSON Schema Validator",
   "main": "dist/ajv.js",
   "types": "dist/ajv.d.ts",
@@ -64,25 +64,25 @@
     "uri-js": "^4.4.1"
   },
   "devDependencies": {
-    "@ajv-validator/config": "^0.3.0",
+    "@ajv-validator/config": "^0.5.0",
     "@rollup/plugin-commonjs": "^25.0.7",
     "@rollup/plugin-json": "^6.1.0",
     "@rollup/plugin-node-resolve": "^15.2.3",
     "@rollup/plugin-typescript": "^11.1.6",
     "@types/chai": "^4.3.11",
     "@types/mocha": "^10.0.6",
-    "@types/node": "^20.11.19",
+    "@types/node": "^20.11.30",
     "@types/require-from-string": "^1.2.3",
-    "@typescript-eslint/eslint-plugin": "^3.10.1",
-    "@typescript-eslint/parser": "^3.10.1",
-    "ajv-formats": "^3.0.0-rc.0",
+    "@typescript-eslint/eslint-plugin": "^7.3.1",
+    "@typescript-eslint/parser": "^7.3.1",
+    "ajv-formats": "^3.0.1",
     "browserify": "^17.0.0",
     "chai": "^4.4.1",
     "cross-env": "^7.0.3",
     "dayjs": "^1.11.10",
     "dayjs-plugin-utc": "^0.1.2",
-    "eslint": "^7.32.0",
-    "eslint-config-prettier": "^7.2.0",
+    "eslint": "^8.57.0",
+    "eslint-config-prettier": "^9.1.0",
     "fast-uri": "^2.3.0",
     "glob": "^10.3.10",
     "husky": "^9.0.11",
@@ -98,13 +98,13 @@
     "module-from-string": "^3.3.0",
     "node-fetch": "^3.3.2",
     "nyc": "^15.1.0",
-    "prettier": "^2.8.8",
+    "prettier": "3.0.3",
     "re2": "^1.20.9",
     "rollup": "^2.79.1",
     "rollup-plugin-terser": "^7.0.2",
     "ts-node": "^10.9.2",
     "tsify": "^5.0.4",
-    "typescript": "^4.9.5"
+    "typescript": "5.3.3"
   },
   "collective": {
     "type": "opencollective",
diff --git a/spec/discriminator.spec.ts b/spec/discriminator.spec.ts
index 28ff12146..74ba33ef0 100644
--- a/spec/discriminator.spec.ts
+++ b/spec/discriminator.spec.ts
@@ -159,6 +159,67 @@ describe("discriminator keyword", function () {
     })
   })
 
+  describe("schema with external $refs", () => {
+    const schemas = {
+      main: {
+        type: "object",
+        discriminator: {propertyName: "foo"},
+        required: ["foo"],
+        oneOf: [
+          {
+            $ref: "schema1",
+          },
+          {
+            $ref: "schema2",
+          },
+        ],
+      },
+      schema1: {
+        type: "object",
+        properties: {
+          foo: {const: "x"},
+        },
+      },
+      schema2: {
+        type: "object",
+        properties: {
+          foo: {enum: ["y", "z"]},
+        },
+      },
+    }
+
+    const data = {foo: "x"}
+    const badData = {foo: "w"}
+
+    it("compile should resolve each $ref to a schema that was added with addSchema", () => {
+      const opts = {
+        discriminator: true,
+      }
+      const ajv = new _Ajv(opts)
+      ajv.addSchema(schemas.main, "https://host/main")
+      ajv.addSchema(schemas.schema1, "https://host/schema1")
+      ajv.addSchema(schemas.schema2, "https://host/schema2")
+
+      const validate = ajv.compile({$ref: "https://host/main"})
+      assert.strictEqual(validate(data), true)
+      assert.strictEqual(validate(badData), false)
+    })
+    it("compileAsync should loadSchema each $ref", async () => {
+      const opts = {
+        discriminator: true,
+        loadSchema(url) {
+          if (!url.startsWith("https://host/")) return undefined
+          const name = url.substring("https://host/".length)
+          return schemas[name]
+        },
+      }
+      const ajv = new _Ajv(opts)
+      const validate = await ajv.compileAsync({$ref: "https://host/main"})
+      assert.strictEqual(validate(data), true)
+      assert.strictEqual(validate(badData), false)
+    })
+  })
+
   describe("validation with deeply referenced schemas", () => {
     const schema = [
       {
diff --git a/spec/types/async-validate.spec.ts b/spec/types/async-validate.spec.ts
index 8e3f0588c..83a61417a 100644
--- a/spec/types/async-validate.spec.ts
+++ b/spec/types/async-validate.spec.ts
@@ -109,10 +109,10 @@ describe("$async validation and type guards", () => {
       const data: unknown = {foo: 1}
       let result: boolean | Promise<Foo>
       if ((result = validate(data))) {
-        if (typeof result == "boolean") {
-          data.foo.should.equal(1)
-        } else {
+        if (result instanceof Promise) {
           await result.then((_data) => _data.foo.should.equal(1))
+        } else {
+          should.fail()
         }
       } else {
         should.fail()