microsoft
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
@@ -7403,7 +7403,7 @@ export interface CommandLineOptionBase {
     isFilePath?: boolean;                                   // True if option value is a path or fileName
     shortName?: string;                                     // A short mnemonic for convenience - for instance, 'h' can be used in place of 'help'
     description?: DiagnosticMessage;                        // The message describing what the command line switch does.
-    defaultValueDescription?: string | number | boolean | DiagnosticMessage;   // The message describing what the dafault value is. string type is prepared for fixed chosen like "false" which do not need I18n.
+    defaultValueDescription?: string | number | boolean | DiagnosticMessage | undefined;   // The message describing what the dafault value is. string type is prepared for fixed chosen like "false" which do not need I18n.
     paramType?: DiagnosticMessage;                          // The name to be used for a non-boolean option's parameter
     isTSConfigOnly?: boolean;                               // True if option can only be specified via tsconfig.json file
     isCommandLineOnly?: boolean;

diff --git a/src/server/project.ts b/src/server/project.ts
@@ -347,7 +347,7 @@ export abstract class Project implements LanguageServiceHost, ModuleResolutionHo
     readonly realpath?: (path: string) => string;
 
     /** @internal */
-    hasInvalidatedResolutions: HasInvalidatedResolutions | undefined;
+    hasInvalidatedResolutions?: HasInvalidatedResolutions | undefined;
 
     /** @internal */
     hasInvalidatedLibResolutions: HasInvalidatedLibResolutions | undefined;

diff --git a/src/services/types.ts b/src/services/types.ts
@@ -340,7 +340,7 @@ export interface LanguageServiceHost extends GetEffectiveTypeRootsHost, MinimalR
      */
     readDirectory?(path: string, extensions?: readonly string[], exclude?: readonly string[], include?: readonly string[], depth?: number): string[];
     realpath?(path: string): string;
-    /** @internal */ createHash?(data: string): string;
+    /** @internal */ createHash?: ((data: string) => string) | undefined;
 
     /*
      * Unlike `realpath and `readDirectory`, `readFile` and `fileExists` are now _required_
@@ -393,9 +393,9 @@ export interface LanguageServiceHost extends GetEffectiveTypeRootsHost, MinimalR
      * If provided along with custom resolveLibrary, used to determine if we should redo library resolutions
      * @internal
      */
-    hasInvalidatedLibResolutions?(libFileName: string): boolean;
+    hasInvalidatedLibResolutions?: ((libFileName: string) => boolean) | undefined;
 
-    /** @internal */ hasInvalidatedResolutions?: HasInvalidatedResolutions;
+    /** @internal */ hasInvalidatedResolutions?: HasInvalidatedResolutions | undefined;
     /** @internal */ hasChangedAutomaticTypeDirectiveNames?: HasChangedAutomaticTypeDirectiveNames;
     /** @internal */ getGlobalTypingsCacheLocation?(): string | undefined;
     /** @internal */ getSymlinkCache?(files?: readonly SourceFile[]): SymlinkCache;
@@ -432,7 +432,7 @@ export interface LanguageServiceHost extends GetEffectiveTypeRootsHost, MinimalR
     /** @internal */ onReleaseParsedCommandLine?(configFileName: string, oldResolvedRef: ResolvedProjectReference | undefined, optionOptions: CompilerOptions): void;
     /** @internal */ getIncompleteCompletionsCache?(): IncompleteCompletionsCache;
 
-    jsDocParsingMode?: JSDocParsingMode;
+    jsDocParsingMode?: JSDocParsingMode | undefined;
 }
 
 /** @internal */

diff --git a/tests/baselines/reference/api/typescript.d.ts b/tests/baselines/reference/api/typescript.d.ts
@@ -10420,7 +10420,7 @@ declare namespace ts {
         installPackage?(options: InstallPackageOptions): Promise<ApplyCodeActionCommandResult>;
         writeFile?(fileName: string, content: string): void;
         getParsedCommandLine?(fileName: string): ParsedCommandLine | undefined;
-        jsDocParsingMode?: JSDocParsingMode;
+        jsDocParsingMode?: JSDocParsingMode | undefined;
     }
     type WithMetadata<T> = T & {
         metadata?: unknown;

diff --git a/tests/cases/compiler/APILibCheck.ts b/tests/cases/compiler/APILibCheck.ts
@@ -2,6 +2,7 @@
 // @noImplicitAny: true
 // @strictNullChecks: true
 // @lib: es2018
+// @exactOptionalPropertyTypes: true
 
 // @filename: node_modules/typescript/package.json
 {