diff --git a/benchmarks/typescript/simple/package.json b/benchmarks/typescript/simple/package.json
index 6571d866d06..b936a44a756 100644
--- a/benchmarks/typescript/simple/package.json
+++ b/benchmarks/typescript/simple/package.json
@@ -1,7 +1,7 @@
 {
   "dependencies": {
     "mongoose": "file:../../../mongoose.tgz",
-    "typescript": "5.5.x"
+    "typescript": "5.8.x"
   },
   "scripts": {
     "benchmark": "tsc --extendedDiagnostics"
diff --git a/test/types/discriminator.test.ts b/test/types/discriminator.test.ts
index fa8e9a3a1b0..87b36d94944 100644
--- a/test/types/discriminator.test.ts
+++ b/test/types/discriminator.test.ts
@@ -1,4 +1,5 @@
 import mongoose, { Document, Model, Schema, SchemaDefinition, SchemaOptions, Types, model } from 'mongoose';
+import { expectType } from 'tsd';
 
 const schema: Schema = new Schema({ name: { type: 'String' } });
 
@@ -76,3 +77,42 @@ function test(): void {
 
   const sampleCardDb: CardDb = sampleLandDb;
 }
+
+function gh15535() {
+  const ParentSchema = new Schema({
+    field1: {
+      type: String,
+      required: true
+    },
+    field2: Number
+  }, {
+    discriminatorKey: 'field1',
+    methods: {
+      getField2() {
+        return this.field2;
+      }
+    }
+  });
+
+  const ParentModel = mongoose.model('Parent', ParentSchema);
+
+  const ChildSchema = new Schema({
+    field3: String
+  }, {
+    methods: {
+      getField3() {
+        return this.field3;
+      }
+    }
+  });
+
+
+  const ChildModel = ParentModel.discriminator('child', ChildSchema);
+
+  const doc = new ChildModel({});
+  expectType<string>(doc.field1);
+  expectType<number | null | undefined>(doc.field2);
+  expectType<number | null | undefined>(doc.getField2());
+  expectType<string | null | undefined>(doc.field3);
+  expectType<string | null | undefined>(doc.getField3());
+}
diff --git a/test/types/schema.test.ts b/test/types/schema.test.ts
index a19a910612b..e798d4bb50a 100644
--- a/test/types/schema.test.ts
+++ b/test/types/schema.test.ts
@@ -532,7 +532,7 @@ export function autoTypedSchema() {
   }, {
     statics: {
       staticFn() {
-        expectType<Model<InferSchemaType<typeof AutoTypedSchema>>>(this);
+        expectAssignable<Model<InferSchemaType<typeof AutoTypedSchema>>>(this);
         return 'Returned from staticFn' as const;
       }
     },
diff --git a/types/index.d.ts b/types/index.d.ts
index effc39c8203..a6a1bb71ac0 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -607,7 +607,7 @@ declare module 'mongoose' {
     | typeof Schema.Types.UUID;
 
 
-  export type InferId<T> = T extends { _id?: any } ? T['_id'] : Types.ObjectId;
+  export type InferId<T> = mongodb.InferIdType<T>;
 
   export interface VirtualTypeOptions<HydratedDocType = Document, DocType = unknown> {
     /** If `ref` is not nullish, this becomes a populated virtual. */
@@ -902,11 +902,6 @@ declare module 'mongoose' {
 
   export type SchemaDefinitionType<T> = T extends Document ? Omit<T, Exclude<keyof Document, '_id' | 'id' | '__v'>> : T;
 
-  /**
-   * Helper to choose the best option between two type helpers
-   */
-  export type _pickObject<T1, T2, Fallback> = T1 extends false ? T2 extends false ? Fallback : T2 : T1;
-
   /* for ts-mongoose */
   export class mquery { }
 
diff --git a/types/models.d.ts b/types/models.d.ts
index 8b98d38563e..6dbc8952eef 100644
--- a/types/models.d.ts
+++ b/types/models.d.ts
@@ -273,7 +273,6 @@ declare module 'mongoose' {
     THydratedDocumentType = HydratedDocument<TRawDocType, TVirtuals & TInstanceMethods, TQueryHelpers, TVirtuals>,
     TSchema = any> extends
     NodeJS.EventEmitter,
-    AcceptsDiscriminator,
     IndexManager,
     SessionStarter {
     new <DocType = Partial<TRawDocType>>(doc?: DocType, fields?: any | null, options?: boolean | AnyObject): THydratedDocumentType;
@@ -313,11 +312,11 @@ declare module 'mongoose' {
      * round trip to the MongoDB server.
      */
     bulkWrite<DocContents = TRawDocType>(
-      writes: Array<AnyBulkWriteOperation<DocContents extends Document ? any : (DocContents extends {} ? DocContents : any)>>,
+      writes: Array<AnyBulkWriteOperation<DocContents>>,
       options: MongooseBulkWriteOptions & { ordered: false }
     ): Promise<MongooseBulkWriteResult>;
     bulkWrite<DocContents = TRawDocType>(
-      writes: Array<AnyBulkWriteOperation<DocContents extends Document ? any : (DocContents extends {} ? DocContents : any)>>,
+      writes: Array<AnyBulkWriteOperation<DocContents>>,
       options?: MongooseBulkWriteOptions
     ): Promise<MongooseBulkWriteResult>;
 
@@ -426,6 +425,28 @@ declare module 'mongoose' {
       TInstanceMethods & TVirtuals
     >;
 
+    /** Adds a discriminator type. */
+    discriminator<TDiscriminatorSchema extends Schema<any, any>>(
+      name: string | number,
+      schema: TDiscriminatorSchema,
+      value?: string | number | ObjectId | DiscriminatorOptions
+    ): Model<
+      TRawDocType & InferSchemaType<TDiscriminatorSchema>,
+      TQueryHelpers & ObtainSchemaGeneric<TDiscriminatorSchema, 'TQueryHelpers'>,
+      TInstanceMethods & ObtainSchemaGeneric<TDiscriminatorSchema, 'TInstanceMethods'>,
+      TVirtuals & ObtainSchemaGeneric<TDiscriminatorSchema, 'TVirtuals'>
+    > & ObtainSchemaGeneric<TDiscriminatorSchema, 'TStaticMethods'>;
+    discriminator<D>(
+      name: string | number,
+      schema: Schema,
+      value?: string | number | ObjectId | DiscriminatorOptions
+    ): Model<D>;
+    discriminator<T, U>(
+      name: string | number,
+      schema: Schema<T, U>,
+      value?: string | number | ObjectId | DiscriminatorOptions
+    ): U;
+
     /**
      * Delete an existing [Atlas search index](https://www.mongodb.com/docs/atlas/atlas-search/create-index/) by name.
      * This function only works when connected to MongoDB Atlas.
@@ -885,7 +906,7 @@ declare module 'mongoose' {
     replaceOne<ResultDoc = THydratedDocumentType>(
       filter?: RootFilterQuery<TRawDocType>,
       replacement?: TRawDocType | AnyObject,
-      options?: (mongodb.ReplaceOptions & MongooseQueryOptions<TRawDocType>) | null
+      options?: (mongodb.ReplaceOptions & QueryOptions<TRawDocType>) | null
     ): QueryWithHelpers<UpdateWriteOpResult, ResultDoc, TQueryHelpers, TRawDocType, 'replaceOne', TInstanceMethods & TVirtuals>;
 
     /** Apply changes made to this model's schema after this model was compiled. */
diff --git a/types/query.d.ts b/types/query.d.ts
index e625ec56635..5bfcd63fdcf 100644
--- a/types/query.d.ts
+++ b/types/query.d.ts
@@ -31,19 +31,9 @@ declare module 'mongoose' {
     | 'strictQuery'
     | 'translateAliases';
 
-  type MongooseQueryOptions<
-    DocType = unknown,
-    Keys extends keyof QueryOptions<DocType> = MongooseBaseQueryOptionKeys | 'timestamps' | 'lean'
-  > = Pick<QueryOptions<DocType>, Keys> & {
-    [other: string]: any;
-  };
+  type MongooseBaseQueryOptions<DocType = unknown> = Pick<QueryOptions<DocType>, MongooseBaseQueryOptionKeys>;
 
-  type MongooseBaseQueryOptions<DocType = unknown> = MongooseQueryOptions<DocType, MongooseBaseQueryOptionKeys>;
-
-  type MongooseUpdateQueryOptions<DocType = unknown> = MongooseQueryOptions<
-    DocType,
-    MongooseBaseQueryOptionKeys | 'timestamps'
-  >;
+  type MongooseUpdateQueryOptions<DocType = unknown> = Pick<QueryOptions<DocType>, MongooseBaseQueryOptionKeys | 'timestamps'>;
 
   type ProjectionFields<DocType> = { [Key in keyof DocType]?: any } & Record<string, any>;
 
@@ -239,7 +229,7 @@ declare module 'mongoose' {
     : MergeType<ResultType, Paths>;
 
   class Query<ResultType, DocType, THelpers = {}, RawDocType = unknown, QueryOp = 'find', TDocOverrides = Record<string, never>> implements SessionOperation {
-    _mongooseOptions: MongooseQueryOptions<DocType>;
+    _mongooseOptions: QueryOptions<DocType>;
 
     /**
      * Returns a wrapper around a [mongodb driver cursor](https://mongodb.github.io/node-mongodb-native/4.9/classes/FindCursor.html).
@@ -634,7 +624,7 @@ declare module 'mongoose' {
      * Getter/setter around the current mongoose-specific options for this query
      * Below are the current Mongoose-specific options.
      */
-    mongooseOptions(val?: MongooseQueryOptions): MongooseQueryOptions;
+    mongooseOptions(val?: QueryOptions<DocType>): QueryOptions<DocType>;
 
     /** Specifies a `$ne` query condition. When called with one argument, the most recent path passed to `where()` is used. */
     ne<K = string>(path: K, val: any): this;