diff --git a/libs/admin-console/src/common/collections/services/vnext-collection.state.ts b/libs/admin-console/src/common/collections/services/vnext-collection.state.ts
index 331c80436f7..9d0ded718b5 100644
--- a/libs/admin-console/src/common/collections/services/vnext-collection.state.ts
+++ b/libs/admin-console/src/common/collections/services/vnext-collection.state.ts
@@ -1,5 +1,3 @@
-import { Jsonify } from "type-fest";
-
 import {
   COLLECTION_DATA,
   DeriveDefinition,
@@ -15,7 +13,7 @@ export const ENCRYPTED_COLLECTION_DATA_KEY = UserKeyDefinition.record<Collection
   COLLECTION_DATA,
   "collections",
   {
-    deserializer: (jsonData: Jsonify<CollectionData>) => CollectionData.fromJSON(jsonData),
+    deserializer: (jsonData) => CollectionData.fromJSON(jsonData!),
     clearOn: ["logout"],
   },
 );
diff --git a/libs/auth/src/common/services/login-email/login-email.service.ts b/libs/auth/src/common/services/login-email/login-email.service.ts
index 6ca817772b0..340b3cbce3b 100644
--- a/libs/auth/src/common/services/login-email/login-email.service.ts
+++ b/libs/auth/src/common/services/login-email/login-email.service.ts
@@ -16,11 +16,11 @@ import {
 import { LoginEmailServiceAbstraction } from "../../abstractions/login-email.service";
 
 export const LOGIN_EMAIL = new KeyDefinition<string>(LOGIN_EMAIL_MEMORY, "loginEmail", {
-  deserializer: (value: string) => value,
+  deserializer: (value: string | null) => value,
 });
 
 export const STORED_EMAIL = new KeyDefinition<string>(LOGIN_EMAIL_DISK, "storedEmail", {
-  deserializer: (value: string) => value,
+  deserializer: (value: string | null) => value,
 });
 
 export class LoginEmailService implements LoginEmailServiceAbstraction {
diff --git a/libs/common/src/admin-console/services/organization-management-preferences/default-organization-management-preferences.service.ts b/libs/common/src/admin-console/services/organization-management-preferences/default-organization-management-preferences.service.ts
index e257b691638..ba5f810778e 100644
--- a/libs/common/src/admin-console/services/organization-management-preferences/default-organization-management-preferences.service.ts
+++ b/libs/common/src/admin-console/services/organization-management-preferences/default-organization-management-preferences.service.ts
@@ -1,5 +1,4 @@
 import { map } from "rxjs";
-import { Jsonify } from "type-fest";
 
 import {
   ORGANIZATION_MANAGEMENT_PREFERENCES_DISK,
@@ -20,7 +19,7 @@ import {
  */
 function buildKeyDefinition<T>(key: string): UserKeyDefinition<T> {
   return new UserKeyDefinition<T>(ORGANIZATION_MANAGEMENT_PREFERENCES_DISK, key, {
-    deserializer: (obj: Jsonify<T>) => obj as T,
+    deserializer: (obj) => obj! as T,
     clearOn: ["logout"],
   });
 }
diff --git a/libs/common/src/admin-console/services/organization/organization.state.ts b/libs/common/src/admin-console/services/organization/organization.state.ts
index fea0423f389..523e0d21a31 100644
--- a/libs/common/src/admin-console/services/organization/organization.state.ts
+++ b/libs/common/src/admin-console/services/organization/organization.state.ts
@@ -1,5 +1,3 @@
-import { Jsonify } from "type-fest";
-
 import { ORGANIZATIONS_DISK, UserKeyDefinition } from "../../../platform/state";
 import { OrganizationData } from "../../models/data/organization.data";
 
@@ -13,7 +11,7 @@ export const ORGANIZATIONS = UserKeyDefinition.record<OrganizationData>(
   ORGANIZATIONS_DISK,
   "organizations",
   {
-    deserializer: (obj: Jsonify<OrganizationData>) => OrganizationData.fromJSON(obj),
+    deserializer: (obj) => OrganizationData.fromJSON(obj!),
     clearOn: ["logout"],
   },
 );
diff --git a/libs/common/src/platform/models/domain/symmetric-crypto-key.ts b/libs/common/src/platform/models/domain/symmetric-crypto-key.ts
index 1fdca04aceb..b41903100f5 100644
--- a/libs/common/src/platform/models/domain/symmetric-crypto-key.ts
+++ b/libs/common/src/platform/models/domain/symmetric-crypto-key.ts
@@ -122,7 +122,7 @@ export class SymmetricCryptoKey {
     return new SymmetricCryptoKey(arrayBuffer);
   }
 
-  static fromJSON(obj: Jsonify<SymmetricCryptoKey>): SymmetricCryptoKey {
+  static fromJSON(obj: Jsonify<SymmetricCryptoKey> | null): SymmetricCryptoKey {
     return SymmetricCryptoKey.fromString(obj?.keyB64);
   }
 }
diff --git a/libs/common/src/platform/state/deserialization-helpers.ts b/libs/common/src/platform/state/deserialization-helpers.ts
index cb7d393a83a..f25edc2361c 100644
--- a/libs/common/src/platform/state/deserialization-helpers.ts
+++ b/libs/common/src/platform/state/deserialization-helpers.ts
@@ -8,8 +8,8 @@ import { Jsonify } from "type-fest";
  * @returns
  */
 export function array<T>(
-  elementDeserializer: (element: Jsonify<T>) => T,
-): (array: Jsonify<T[]>) => T[] {
+  elementDeserializer: (element: Jsonify<T> | null) => T | null,
+): (array: Jsonify<T[]> | null) => T[] | null {
   return (array) => {
     if (array == null) {
       return null;
@@ -24,9 +24,9 @@ export function array<T>(
  * @param valueDeserializer
  */
 export function record<T, TKey extends string | number = string>(
-  valueDeserializer: (value: Jsonify<T>) => T,
-): (record: Jsonify<Record<TKey, T>>) => Record<TKey, T> {
-  return (jsonValue: Jsonify<Record<TKey, T> | null>) => {
+  valueDeserializer: (value: Jsonify<T> | null) => T | null,
+): (record: Jsonify<Record<TKey, T>> | null) => Record<TKey, T> | null {
+  return (jsonValue: Jsonify<Record<TKey, T>> | null) => {
     if (jsonValue == null) {
       return null;
     }
diff --git a/libs/common/src/platform/state/key-definition.ts b/libs/common/src/platform/state/key-definition.ts
index 519e98ef52d..9888b111b61 100644
--- a/libs/common/src/platform/state/key-definition.ts
+++ b/libs/common/src/platform/state/key-definition.ts
@@ -1,7 +1,6 @@
-// FIXME: Update this file to be type safe and remove this and next line
-// @ts-strict-ignore
 import { Jsonify } from "type-fest";
 
+import { mergeOptions, OptionsWithDefaultsDeep } from "../../types/options";
 import { StorageKey } from "../../types/state";
 
 import { array, record } from "./deserialization-helpers";
@@ -42,7 +41,7 @@ export type KeyDefinitionOptions<T> = {
    * @param jsonValue The JSON object representation of your state.
    * @returns The fully typed version of your state.
    */
-  readonly deserializer: (jsonValue: Jsonify<T>) => T | null;
+  readonly deserializer: (jsonValue: Jsonify<T> | null) => T | null;
   /**
    * The number of milliseconds to wait before cleaning up the state after the last subscriber has unsubscribed.
    * Defaults to 1000ms.
@@ -55,13 +54,27 @@ export type KeyDefinitionOptions<T> = {
   readonly debug?: DebugOptions;
 };
 
+const DEFAULT_KEY_DEFINITION_OPTIONS = Object.freeze({
+  cleanupDelayMs: 1000 as number,
+  debug: {
+    enableUpdateLogging: false,
+    enableRetrievalLogging: false,
+  },
+});
+
 /**
  * KeyDefinitions describe the precise location to store data for a given piece of state.
  * The StateDefinition is used to describe the domain of the state, and the KeyDefinition
  * sub-divides that domain into specific keys.
  */
 export class KeyDefinition<T> {
-  readonly debug: Required<DebugOptions>;
+  private readonly options: OptionsWithDefaultsDeep<
+    KeyDefinitionOptions<T>,
+    typeof DEFAULT_KEY_DEFINITION_OPTIONS
+  >;
+  get debug() {
+    return Object.freeze({ ...this.options.debug });
+  }
 
   /**
    * Creates a new instance of a KeyDefinition
@@ -75,24 +88,19 @@ export class KeyDefinition<T> {
   constructor(
     readonly stateDefinition: StateDefinition,
     readonly key: string,
-    private readonly options: KeyDefinitionOptions<T>,
+    options: KeyDefinitionOptions<T>,
   ) {
     if (options.deserializer == null) {
       throw new Error(`'deserializer' is a required property on key ${this.errorKeyName}`);
     }
 
-    if (options.cleanupDelayMs < 0) {
+    this.options = mergeOptions(options, DEFAULT_KEY_DEFINITION_OPTIONS);
+
+    if (this.options.cleanupDelayMs < 0) {
       throw new Error(
-        `'cleanupDelayMs' must be greater than or equal to 0. Value of ${options.cleanupDelayMs} passed to key ${this.errorKeyName} `,
+        `'cleanupDelayMs' must be greater than or equal to 0. Value of ${this.options.cleanupDelayMs} passed to key ${this.errorKeyName} `,
       );
     }
-
-    // Normalize optional debug options
-    const { enableUpdateLogging = false, enableRetrievalLogging = false } = options.debug ?? {};
-    this.debug = {
-      enableUpdateLogging,
-      enableRetrievalLogging,
-    };
   }
 
   /**
@@ -106,7 +114,7 @@ export class KeyDefinition<T> {
    * Gets the number of milliseconds to wait before cleaning up the state after the last subscriber has unsubscribed.
    */
   get cleanupDelayMs() {
-    return this.options.cleanupDelayMs < 0 ? 0 : (this.options.cleanupDelayMs ?? 1000);
+    return this.options.cleanupDelayMs;
   }
 
   /**
diff --git a/libs/common/src/services/event/key-definitions.ts b/libs/common/src/services/event/key-definitions.ts
index 56820996886..9119d773656 100644
--- a/libs/common/src/services/event/key-definitions.ts
+++ b/libs/common/src/services/event/key-definitions.ts
@@ -5,7 +5,7 @@ export const EVENT_COLLECTION = UserKeyDefinition.array<EventData>(
   EVENT_COLLECTION_DISK,
   "events",
   {
-    deserializer: (s) => EventData.fromJSON(s),
+    deserializer: (s) => EventData.fromJSON(s!),
     clearOn: ["logout"],
   },
 );
diff --git a/libs/common/src/tools/send/services/key-definitions.ts b/libs/common/src/tools/send/services/key-definitions.ts
index f1a6b3d6c6a..a5d1249c3ec 100644
--- a/libs/common/src/tools/send/services/key-definitions.ts
+++ b/libs/common/src/tools/send/services/key-definitions.ts
@@ -7,7 +7,7 @@ export const SEND_USER_ENCRYPTED = UserKeyDefinition.record<SendData>(
   SEND_DISK,
   "sendUserEncrypted",
   {
-    deserializer: (obj: SendData) => obj,
+    deserializer: (obj) => obj,
     clearOn: ["logout"],
   },
 );
diff --git a/libs/common/src/types/merge-deep.ts b/libs/common/src/types/merge-deep.ts
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/libs/common/src/types/options.spec.ts b/libs/common/src/types/options.spec.ts
new file mode 100644
index 00000000000..412673683f3
--- /dev/null
+++ b/libs/common/src/types/options.spec.ts
@@ -0,0 +1,100 @@
+import { mergeOptions } from "./options";
+
+type ExampleOptions = {
+  readonly a?: number;
+  readonly required_func: () => void;
+  readonly optional_func?: () => void;
+  readonly b: string;
+  readonly c?: {
+    readonly d?: number;
+    readonly e: string;
+  };
+};
+
+const EXAMPLE_DEFAULTS = Object.freeze({
+  a: 0,
+  optional_func: () => {},
+  c: {
+    d: 1,
+  },
+  f: {
+    h: 1,
+  },
+});
+
+describe("mergeOptions", () => {
+  it("merges options with defaults", () => {
+    const options: ExampleOptions = {
+      required_func: () => {},
+      b: "test",
+    };
+
+    const merged = mergeOptions(options, EXAMPLE_DEFAULTS);
+
+    // can access properties
+    expect(merged.a).toBe(42);
+
+    expect(merged).toEqual({
+      a: 0,
+      b: "test",
+    });
+  });
+
+  it("overrides defaults with options", () => {
+    const options: ExampleOptions = {
+      a: 42,
+      b: "test",
+      f: {
+        i: "example",
+      },
+      required_func: () => {},
+    };
+
+    const merged = mergeOptions(options, EXAMPLE_DEFAULTS);
+
+    // can access properties
+    expect(merged.a).toBe(42);
+
+    expect(merged).toEqual({
+      a: 42,
+      b: "test",
+    });
+  });
+
+  //Defaults has a required property 'a', but options does not provide it. Not an error
+  mergeOptions({ b: "test" } as { b: string }, { a: 0 });
+  //Defaults provides a required function, but options does not provide it. Not an error
+  mergeOptions({ b: "test" } as { b: string }, { required_func: () => {} });
+  //Defaults provides a required property of the wrong type. Not an error because default will never be used
+  mergeOptions({ a: "test" } as { a: string }, { a: 0 });
+  //Defaults provides a required function of the wrong type. Not an error because default will never be used
+  mergeOptions({ required_func: () => "" } as { required_func: () => string }, {
+    required_func: () => {},
+  });
+
+  //@ts-expect-error -- Defaults provides an optional property of the wrong type
+  mergeOptions({} as { a?: string }, { a: 0 });
+  //@ts-expect-error -- Defaults provides an optional function of the wrong type
+  mergeOptions({} as { optional_func?: () => string }, { optional_func: () => {} });
+
+  //@ts-expect-error -- defaults missing an property optional in options
+  mergeOptions({ a: 42 } as { a?: number }, {});
+  //@ts-expect-error -- defaults missing an method optional in options
+  mergeOptions({ f: () => {} } as { f?: () => void }, {});
+
+  //@ts-expect-error -- defaults missing a deep optional property defined in options
+  mergeOptions({ a: { required_func: () => {} } } as { a?: { required_func: () => void } }, {});
+  //@ts-expect-error -- defaults missing a deep optional property defined in options
+  mergeOptions({ a: { required_func: () => {} } } as { a: { required_func?: () => void } }, {});
+
+  //@ts-expect-error -- defaults missing a optional object defined in options
+  mergeOptions({} as { a?: { b: number } }, {});
+
+  //@ts-expect-error -- defaults missing a deep optional property defined in required option
+  mergeOptions({ a: {} } as { a: { b?: number } }, {});
+
+  //@ts-expect-error -- defaults missing a deep optional property defined in optional option
+  mergeOptions({ a: {} } as { a?: { b?: number } }, {});
+  //@ts-expect-error -- defaults missing a deep optional property defined in optional option
+  mergeOptions({ a: {} } as { a?: { b?: number } }, { a: {} });
+});
diff --git a/libs/common/src/types/options.ts b/libs/common/src/types/options.ts
new file mode 100644
index 00000000000..4cdccf8adf0
--- /dev/null
+++ b/libs/common/src/types/options.ts
@@ -0,0 +1,186 @@
+/* eslint-disable @typescript-eslint/no-empty-object-type -- used in type-fest's code*/
+import { RequiredKeysOf, Simplify, Primitive } from "type-fest";
+
+type Function = (...args: any[]) => any;
+
+/** FIXME: this is pulled from type-fest-v4. remove when we update package */
+export type ConditionalSimplifyDeep<
+  Type,
+  ExcludeType = never,
+  IncludeType = unknown,
+> = Type extends ExcludeType
+  ? Type
+  : Type extends IncludeType
+    ? { [TypeKey in keyof Type]: ConditionalSimplifyDeep<Type[TypeKey], ExcludeType, IncludeType> }
+    : Type;
+export type BuiltIns = Primitive | void | Date | RegExp;
+export type NonRecursiveType = BuiltIns | Function | (new (...arguments_: any[]) => unknown);
+export type SimplifyDeep<Type, ExcludeType = never> = ConditionalSimplifyDeep<
+  Type,
+  ExcludeType | NonRecursiveType | Set<unknown> | Map<unknown, unknown>,
+  object
+>;
+
+type SimplifyDeepExcludeArray<T> = SimplifyDeep<T, UnknownArray>;
+export type UnknownArray = readonly unknown[];
+export type UnknownRecord = Record<PropertyKey, unknown>;
+export type UnknownArrayOrTuple = readonly [...unknown[]];
+export type OmitIndexSignature<ObjectType> = {
+  [KeyType in keyof ObjectType as {} extends Record<KeyType, unknown>
+    ? never
+    : KeyType]: ObjectType[KeyType];
+};
+export type PickIndexSignature<ObjectType> = {
+  [KeyType in keyof ObjectType as {} extends Record<KeyType, unknown>
+    ? KeyType
+    : never]: ObjectType[KeyType];
+};
+
+type MergeDeepRecordProperty<Destination, Source> = undefined extends Source
+  ?
+      | MergeDeepOrReturn<Source, Exclude<Destination, undefined>, Exclude<Source, undefined>>
+      | undefined
+  : MergeDeepOrReturn<Source, Destination, Source>;
+type RequiredFilter<Type, Key extends keyof Type> = undefined extends Type[Key]
+  ? Type[Key] extends undefined
+    ? Key
+    : never
+  : Key;
+
+// Returns `never` if the key is required otherwise return the key type.
+type OptionalFilter<Type, Key extends keyof Type> = undefined extends Type[Key]
+  ? Type[Key] extends undefined
+    ? never
+    : Key
+  : never;
+
+export type EnforceOptional<ObjectType> = Simplify<
+  {
+    [Key in keyof ObjectType as RequiredFilter<ObjectType, Key>]: ObjectType[Key];
+  } & {
+    [Key in keyof ObjectType as OptionalFilter<ObjectType, Key>]?: Exclude<
+      ObjectType[Key],
+      undefined
+    >;
+  }
+>;
+
+type MergeDeepOrReturn<DefaultType, Destination, Source> = SimplifyDeepExcludeArray<
+  [undefined] extends [Destination | Source]
+    ? DefaultType
+    : Destination extends UnknownRecord
+      ? Source extends UnknownRecord
+        ? MergeDeepRecord<Destination, Source>
+        : DefaultType
+      : Destination extends UnknownArrayOrTuple
+        ? Source extends UnknownArrayOrTuple
+          ? MergeDeepArrayOrTuple<Destination, Source>
+          : DefaultType
+        : DefaultType
+>;
+
+type MergeDeepArrayOrTuple<
+  Destination extends UnknownArrayOrTuple,
+  Source extends UnknownArrayOrTuple,
+> = Array<Exclude<Destination, undefined>[number] | Exclude<Source, undefined>[number]>;
+
+type MergeDeepRecord<
+  Destination extends UnknownRecord,
+  Source extends UnknownRecord,
+> = DoMergeDeepRecord<OmitIndexSignature<Destination>, OmitIndexSignature<Source>> &
+  Merge<PickIndexSignature<Destination>, PickIndexSignature<Source>>;
+
+type DoMergeDeepRecord<Destination extends UnknownRecord, Source extends UnknownRecord> =
+  // Case in rule 1: The destination contains the key but the source doesn't.
+  {
+    [Key in keyof Destination as Key extends keyof Source ? never : Key]: Destination[Key];
+  } & {
+    // Case in rule 2: The source contains the key but the destination doesn't.
+    [Key in keyof Source as Key extends keyof Destination ? never : Key]: Source[Key];
+  } & {
+    // Case in rule 3: Both the source and the destination contain the key.
+    [Key in keyof Source as Key extends keyof Destination ? Key : never]: MergeDeepRecordProperty<
+      Destination[Key],
+      Source[Key]
+    >;
+  };
+type SimpleMerge<Destination, Source> = {
+  [Key in keyof Destination as Key extends keyof Source ? never : Key]: Destination[Key];
+} & Source;
+
+export type Merge<Destination, Source> = Simplify<
+  SimpleMerge<PickIndexSignature<Destination>, PickIndexSignature<Source>> &
+    SimpleMerge<OmitIndexSignature<Destination>, OmitIndexSignature<Source>>
+>;
+export type IfNever<T, TypeIfNever = true, TypeIfNotNever = false> =
+  IsNever<T> extends true ? TypeIfNever : TypeIfNotNever;
+export type IsNever<T> = [T] extends [never] ? true : false;
+
+type MergeDeep<Destination, Source> = SimplifyDeepExcludeArray<
+  [undefined] extends [Destination | Source]
+    ? never
+    : Destination extends UnknownRecord
+      ? Source extends UnknownRecord
+        ? MergeDeepRecord<Destination, Source>
+        : never
+      : Destination extends UnknownArrayOrTuple
+        ? Source extends UnknownArrayOrTuple
+          ? MergeDeepArrayOrTuple<Destination, Source>
+          : never
+        : never
+>;
+export type ConditionalKeys<Base, Condition> = {
+  // Map through all the keys of the given base type.
+  [Key in keyof Base]-?: Base[Key] extends Condition // Pick only keys with types extending the given `Condition` type.
+    ? // Retain this key
+      // If the value for the key extends never, only include it if `Condition` also extends never
+      IfNever<Base[Key], IfNever<Condition, Key, never>, Key>
+    : // Discard this key since the condition fails.
+      never;
+  // Convert the produced object into a union type of the keys which passed the conditional test.
+}[keyof Base];
+
+/** END FIXME: this is pulled from type-fest-v4. remove when we update package */
+
+export type OptionsWithDefaultsDeep<Options, Defaults> =
+  // FIXME: replace with MergeDeep<Options, Defaults> when type-fest is updated to v4+
+  MergeDeep<Options, Defaults>;
+
+type DefaultsForDeep<Options extends object> = Simplify<
+  Omit<
+    Required<{
+      [K in keyof Options]: Required<Options>[K] extends Primitive | Function
+        ? Options[K]
+        : Required<Options>[K] extends object
+          ? DefaultsForDeep<Required<Options>[K]>
+          : never; // should only ever be primitive, function, or object
+    }>,
+    RequiredPrimitiveKeysOf<Options> | RequiredMethodKeysOf<Options>
+  >
+>;
+
+// Returns only the keys of `Obj` that are required and not records
+type RequiredPrimitiveKeysOf<Obj extends object> = RequiredKeysOf<
+  Omit<Obj, ConditionalKeys<Obj, object>>
+>;
+
+// Returns only the keys of `Obj` that are required and are functions
+type RequiredMethodKeysOf<Obj extends object> = RequiredKeysOf<
+  Pick<Obj, ConditionalKeys<Obj, Function>>
+>;
+
+export function mergeOptions<
+  Options extends object,
+  const Defaults extends DefaultsForDeep<Options>,
+>(
+  options: Options,
+  defaults: Defaults,
+): OptionsWithDefaultsDeep<Options, DefaultsForDeep<Options>> {
+  const result = { ...options } as any;
+  for (const key in defaults) {
+    if (result[key] == null) {
+      result[key] = (defaults as any)[key];
+    }
+  }
+  return result as OptionsWithDefaultsDeep<Options, DefaultsForDeep<Options>>;
+}
diff --git a/libs/common/src/vault/notifications/state/end-user-notification.state.ts b/libs/common/src/vault/notifications/state/end-user-notification.state.ts
index 644c8e42429..1bc24f413d0 100644
--- a/libs/common/src/vault/notifications/state/end-user-notification.state.ts
+++ b/libs/common/src/vault/notifications/state/end-user-notification.state.ts
@@ -1,5 +1,3 @@
-import { Jsonify } from "type-fest";
-
 import { NOTIFICATION_DISK, UserKeyDefinition } from "@bitwarden/common/platform/state";
 
 import { NotificationViewData } from "../models";
@@ -8,8 +6,7 @@ export const NOTIFICATIONS = UserKeyDefinition.array<NotificationViewData>(
   NOTIFICATION_DISK,
   "notifications",
   {
-    deserializer: (notification: Jsonify<NotificationViewData>) =>
-      NotificationViewData.fromJSON(notification),
+    deserializer: (notification) => NotificationViewData.fromJSON(notification!),
     clearOn: ["logout", "lock"],
   },
 );
diff --git a/libs/common/src/vault/services/key-state/folder.state.ts b/libs/common/src/vault/services/key-state/folder.state.ts
index b3e61f5bf31..6e3a2f7d614 100644
--- a/libs/common/src/vault/services/key-state/folder.state.ts
+++ b/libs/common/src/vault/services/key-state/folder.state.ts
@@ -1,5 +1,3 @@
-import { Jsonify } from "type-fest";
-
 import { FOLDER_DISK, FOLDER_MEMORY, UserKeyDefinition } from "../../../platform/state";
 import { FolderData } from "../../models/data/folder.data";
 import { FolderView } from "../../models/view/folder.view";
@@ -8,7 +6,7 @@ export const FOLDER_ENCRYPTED_FOLDERS = UserKeyDefinition.record<FolderData>(
   FOLDER_DISK,
   "folders",
   {
-    deserializer: (obj: Jsonify<FolderData>) => FolderData.fromJSON(obj),
+    deserializer: (obj) => FolderData.fromJSON(obj!),
     clearOn: ["logout"],
   },
 );
@@ -17,7 +15,7 @@ export const FOLDER_DECRYPTED_FOLDERS = new UserKeyDefinition<FolderView[]>(
   FOLDER_MEMORY,
   "decryptedFolders",
   {
-    deserializer: (obj: Jsonify<FolderView[]>) => obj?.map((f) => FolderView.fromJSON(f)) ?? [],
+    deserializer: (obj) => obj?.map((f) => FolderView.fromJSON(f)) ?? [],
     clearOn: ["logout", "lock"],
   },
 );
diff --git a/libs/common/src/vault/tasks/state/security-task.state.ts b/libs/common/src/vault/tasks/state/security-task.state.ts
index b86a891f008..e4c42a5ea39 100644
--- a/libs/common/src/vault/tasks/state/security-task.state.ts
+++ b/libs/common/src/vault/tasks/state/security-task.state.ts
@@ -1,5 +1,3 @@
-import { Jsonify } from "type-fest";
-
 import { SECURITY_TASKS_DISK, UserKeyDefinition } from "@bitwarden/common/platform/state";
 
 import { SecurityTaskData } from "../models/security-task.data";
@@ -8,7 +6,7 @@ export const SECURITY_TASKS = UserKeyDefinition.array<SecurityTaskData>(
   SECURITY_TASKS_DISK,
   "securityTasks",
   {
-    deserializer: (task: Jsonify<SecurityTaskData>) => SecurityTaskData.fromJSON(task),
+    deserializer: (task) => SecurityTaskData.fromJSON(task!),
     clearOn: ["logout", "lock"],
   },
 );
diff --git a/libs/key-management/src/biometrics/biometric.state.ts b/libs/key-management/src/biometrics/biometric.state.ts
index 277d35c176e..a3132707d58 100644
--- a/libs/key-management/src/biometrics/biometric.state.ts
+++ b/libs/key-management/src/biometrics/biometric.state.ts
@@ -103,6 +103,6 @@ export const LAST_PROCESS_RELOAD = new KeyDefinition<Date>(
   BIOMETRIC_SETTINGS_DISK,
   "lastProcessReload",
   {
-    deserializer: (obj) => new Date(obj),
+    deserializer: (obj) => new Date(obj!),
   },
 );
diff --git a/libs/key-management/src/kdf-config.service.ts b/libs/key-management/src/kdf-config.service.ts
index 24635e87580..f01c77da851 100644
--- a/libs/key-management/src/kdf-config.service.ts
+++ b/libs/key-management/src/kdf-config.service.ts
@@ -13,7 +13,7 @@ import { KdfType } from "./enums/kdf-type.enum";
 import { Argon2KdfConfig, KdfConfig, PBKDF2KdfConfig } from "./models/kdf-config";
 
 export const KDF_CONFIG = new UserKeyDefinition<KdfConfig>(KDF_CONFIG_DISK, "kdfConfig", {
-  deserializer: (kdfConfig: Jsonify<KdfConfig>) => {
+  deserializer: (kdfConfig: Jsonify<KdfConfig> | null) => {
     if (kdfConfig == null) {
       return null;
     }
diff --git a/libs/tools/generator/extensions/history/src/key-definitions.ts b/libs/tools/generator/extensions/history/src/key-definitions.ts
index 187a6c8fc94..d75461bc328 100644
--- a/libs/tools/generator/extensions/history/src/key-definitions.ts
+++ b/libs/tools/generator/extensions/history/src/key-definitions.ts
@@ -15,7 +15,7 @@ export const GENERATOR_HISTORY = SecretKeyDefinition.array(
   "localGeneratorHistory",
   SecretClassifier.allSecret<GeneratedCredential>(),
   {
-    deserializer: GeneratedCredential.fromJSON,
+    deserializer: (obj) => GeneratedCredential.fromJSON(obj!),
     clearOn: ["logout"],
   },
 );
diff --git a/package-lock.json b/package-lock.json
index 115f7cbf32d..ec9b069f2b9 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -179,6 +179,7 @@
         "ts-loader": "9.5.2",
         "tsconfig-paths-webpack-plugin": "4.2.0",
         "type-fest": "2.19.0",
+        "type-fest-v4": "npm:type-fest@4.41.0",
         "typescript": "5.5.4",
         "typescript-eslint": "8.31.0",
         "typescript-strict-plugin": "2.4.4",
@@ -37241,6 +37242,20 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/type-fest-v4": {
+      "name": "type-fest",
+      "version": "4.41.0",
+      "resolved": "https://registry.npmjs.org/type-fest/-/type-fest-4.41.0.tgz",
+      "integrity": "sha512-TeTSQ6H5YHvpqVwBRcnLDCBnDOHWYu7IvGbHT6N8AOymcr9PJGjc1GTtiWZTYg0NCgYwvnYWEkVChQAr9bjfwA==",
+      "dev": true,
+      "license": "(MIT OR CC0-1.0)",
+      "engines": {
+        "node": ">=16"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/sindresorhus"
+      }
+    },
     "node_modules/type-is": {
       "version": "2.0.1",
       "resolved": "https://registry.npmjs.org/type-is/-/type-is-2.0.1.tgz",
diff --git a/package.json b/package.json
index be0c53914b8..ae84fb08157 100644
--- a/package.json
+++ b/package.json
@@ -139,6 +139,7 @@
     "ts-loader": "9.5.2",
     "tsconfig-paths-webpack-plugin": "4.2.0",
     "type-fest": "2.19.0",
+    "type-fest-v4": "npm:type-fest@4.41.0",
     "typescript": "5.5.4",
     "typescript-eslint": "8.31.0",
     "typescript-strict-plugin": "2.4.4",