diff --git a/index.d.ts b/index.d.ts
index bb1396c..6b4bcf9 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -245,19 +245,19 @@ declare namespace OAuth2Server {
          * Invoked to generate a new access token.
          *
          */
-        generateAccessToken?(client: Client, user: User, scope: string[]): Promise<string>;
+        generateAccessToken?(client: Client, user: User, scope: string[]): string | Promise<string>;
 
         /**
          * Invoked to retrieve a client using a client id or a client id/client secret combination, depending on the grant type.
          *
          */
-        getClient(clientId: string, clientSecret: string): Promise<Client | Falsey>;
+        getClient(clientId: string, clientSecret: string): Client | Falsey | Promise<Client | Falsey>;
 
         /**
          * Invoked to save an access token and optionally a refresh token, depending on the grant type.
          *
          */
-        saveToken(token: Token, client: Client, user: User): Promise<Token | Falsey>;
+        saveToken(token: Token, client: Client, user: User): Token | Falsey | Promise<Token | Falsey>;
     }
 
     interface RequestAuthenticationModel {
@@ -265,14 +265,14 @@ declare namespace OAuth2Server {
          * Invoked to retrieve an existing access token previously saved through Model#saveToken().
          *
          */
-        getAccessToken(accessToken: string): Promise<Token | Falsey>;
+        getAccessToken(accessToken: string): Token | Falsey | Promise<Token | Falsey>;
 
         /**
          * Invoked during request authentication to check if the provided access token was authorized the requested scopes.
          * Optional, if a custom authenticateHandler is used or if there is no scope part of the request.
          *
          */
-        verifyScope?(token: Token, scope: string[]): Promise<boolean>;
+        verifyScope?(token: Token, scope: string[]): boolean | Promise<boolean>;
     }
 
     interface AuthorizationCodeModel extends BaseModel, RequestAuthenticationModel {
@@ -280,19 +280,19 @@ declare namespace OAuth2Server {
          * Invoked to generate a new refresh token.
          *
          */
-        generateRefreshToken?(client: Client, user: User, scope: string[]): Promise<string>;
+        generateRefreshToken?(client: Client, user: User, scope: string[]): string | Promise<string>;
 
         /**
          * Invoked to generate a new authorization code.
          *
          */
-        generateAuthorizationCode?(client: Client, user: User, scope: string[]): Promise<string>;
+        generateAuthorizationCode?(client: Client, user: User, scope: string[]): string | Promise<string>;
 
         /**
          * Invoked to retrieve an existing authorization code previously saved through Model#saveAuthorizationCode().
          *
          */
-        getAuthorizationCode(authorizationCode: string): Promise<AuthorizationCode | Falsey>;
+        getAuthorizationCode(authorizationCode: string): AuthorizationCode | Falsey | Promise<AuthorizationCode | Falsey>;
 
         /**
          * Invoked to save an authorization code.
@@ -302,25 +302,25 @@ declare namespace OAuth2Server {
           code: Pick<AuthorizationCode, 'authorizationCode' | 'expiresAt' | 'redirectUri' | 'scope' | 'codeChallenge' | 'codeChallengeMethod'>,
           client: Client,
           user: User
-        ): Promise<AuthorizationCode | Falsey>;
+        ): AuthorizationCode | Falsey | Promise<AuthorizationCode | Falsey>;
 
         /**
          * Invoked to revoke an authorization code.
          *
          */
-        revokeAuthorizationCode(code: AuthorizationCode): Promise<boolean>;
+        revokeAuthorizationCode(code: AuthorizationCode): boolean | Promise<boolean>;
 
         /**
          * Invoked to check if the requested scope is valid for a particular client/user combination.
          *
          */
-        validateScope?(user: User, client: Client, scope?: string[]): Promise<string[] | Falsey>;
+        validateScope?(user: User, client: Client, scope?: string[]): string[] | Falsey | Promise<string[] | Falsey>;
 
         /**
          * Invoked to check if the provided `redirectUri` is valid for a particular `client`.
          *
          */
-        validateRedirectUri?(redirect_uri: string, client: Client): Promise<boolean>;
+        validateRedirectUri?(redirect_uri: string, client: Client): boolean | Promise<boolean>;
     }
 
     interface PasswordModel extends BaseModel, RequestAuthenticationModel {
@@ -328,19 +328,19 @@ declare namespace OAuth2Server {
          * Invoked to generate a new refresh token.
          *
          */
-        generateRefreshToken?(client: Client, user: User, scope: string[]): Promise<string>;
+        generateRefreshToken?(client: Client, user: User, scope: string[]): string | Promise<string>;
 
         /**
          * Invoked to retrieve a user using a username/password combination.
          *
          */
-        getUser(username: string, password: string, client: Client): Promise<User | Falsey>;
+        getUser(username: string, password: string, client: Client): User | Falsey | Promise<User | Falsey>;
 
         /**
          * Invoked to check if the requested scope is valid for a particular client/user combination.
          *
          */
-        validateScope?(user: User, client: Client, scope?: string[]): Promise<string[] | Falsey>;
+        validateScope?(user: User, client: Client, scope?: string[]): string[] | Falsey | Promise<string[] | Falsey>;
     }
 
     interface RefreshTokenModel extends BaseModel, RequestAuthenticationModel {
@@ -348,19 +348,19 @@ declare namespace OAuth2Server {
          * Invoked to generate a new refresh token.
          *
          */
-        generateRefreshToken?(client: Client, user: User, scope: string[]): Promise<string>;
+        generateRefreshToken?(client: Client, user: User, scope: string[]): string | Promise<string>;
 
         /**
          * Invoked to retrieve an existing refresh token previously saved through Model#saveToken().
          *
          */
-        getRefreshToken(refreshToken: string): Promise<RefreshToken | Falsey>;
+        getRefreshToken(refreshToken: string): RefreshToken | Falsey | Promise<RefreshToken | Falsey>;
 
         /**
          * Invoked to revoke a refresh token.
          *
          */
-        revokeToken(token: RefreshToken): Promise<boolean>;
+        revokeToken(token: RefreshToken): boolean | Promise<boolean>;
     }
 
     interface ClientCredentialsModel extends BaseModel, RequestAuthenticationModel {
@@ -368,13 +368,13 @@ declare namespace OAuth2Server {
          * Invoked to retrieve the user associated with the specified client.
          *
          */
-        getUserFromClient(client: Client): Promise<User | Falsey>;
+        getUserFromClient(client: Client): User | Falsey | Promise<User | Falsey>;
 
         /**
          * Invoked to check if the requested scope is valid for a particular client/user combination.
          *
          */
-        validateScope?(user: User, client: Client, scope?: string[]): Promise<string[] | Falsey>;
+        validateScope?(user: User, client: Client, scope?: string[]): string[] | Falsey | Promise<string[] | Falsey>;
     }
 
     interface ExtensionModel extends BaseModel, RequestAuthenticationModel {}