diff --git a/src/vs/platform/extensions/common/extensions.ts b/src/vs/platform/extensions/common/extensions.ts
index a1eae1bb07d..0bfebc689dd 100644
--- a/src/vs/platform/extensions/common/extensions.ts
+++ b/src/vs/platform/extensions/common/extensions.ts
@@ -257,31 +257,33 @@ export const EXTENSION_CATEGORIES = [
 	'Other',
 ];
 
-export interface IExtensionManifest {
-	readonly name: string;
-	readonly displayName?: string;
-	readonly publisher: string;
-	readonly version: string;
-	readonly engines: { readonly vscode: string };
-	readonly description?: string;
-	readonly main?: string;
-	readonly browser?: string;
-	readonly icon?: string;
-	readonly categories?: string[];
-	readonly keywords?: string[];
-	readonly activationEvents?: string[];
-	readonly extensionDependencies?: string[];
-	readonly extensionPack?: string[];
-	readonly extensionKind?: ExtensionKind | ExtensionKind[];
-	readonly contributes?: IExtensionContributions;
-	readonly repository?: { url: string };
-	readonly bugs?: { url: string };
-	readonly enabledApiProposals?: readonly string[];
-	readonly api?: string;
-	readonly scripts?: { [key: string]: string };
-	readonly capabilities?: IExtensionCapabilities;
+export interface IRelaxedExtensionManifest {
+	name: string;
+	displayName?: string;
+	publisher: string;
+	version: string;
+	engines: { readonly vscode: string };
+	description?: string;
+	main?: string;
+	browser?: string;
+	icon?: string;
+	categories?: string[];
+	keywords?: string[];
+	activationEvents?: string[];
+	extensionDependencies?: string[];
+	extensionPack?: string[];
+	extensionKind?: ExtensionKind | ExtensionKind[];
+	contributes?: IExtensionContributions;
+	repository?: { url: string };
+	bugs?: { url: string };
+	enabledApiProposals?: readonly string[];
+	api?: string;
+	scripts?: { [key: string]: string };
+	capabilities?: IExtensionCapabilities;
 }
 
+export type IExtensionManifest = Readonly<IRelaxedExtensionManifest>;
+
 export const enum ExtensionType {
 	System,
 	User
@@ -375,16 +377,19 @@ export class ExtensionIdentifier {
 	}
 }
 
-export interface IExtensionDescription extends IExtensionManifest {
-	readonly identifier: ExtensionIdentifier;
-	readonly uuid?: string;
-	readonly targetPlatform: TargetPlatform;
-	readonly isBuiltin: boolean;
-	readonly isUserBuiltin: boolean;
-	readonly isUnderDevelopment: boolean;
-	readonly extensionLocation: URI;
+export interface IRelaxedExtensionDescription extends IRelaxedExtensionManifest {
+	id?: string;
+	identifier: ExtensionIdentifier;
+	uuid?: string;
+	targetPlatform: TargetPlatform;
+	isBuiltin: boolean;
+	isUserBuiltin: boolean;
+	isUnderDevelopment: boolean;
+	extensionLocation: URI;
 }
 
+export type IExtensionDescription = Readonly<IRelaxedExtensionDescription>;
+
 export function isLanguagePackExtension(manifest: IExtensionManifest): boolean {
 	return manifest.contributes && manifest.contributes.localizations ? manifest.contributes.localizations.length > 0 : false;
 }
diff --git a/src/vs/workbench/services/extensions/common/extensionPoints.ts b/src/vs/workbench/services/extensions/common/extensionPoints.ts
index eaca238a299..ca143db3b94 100644
--- a/src/vs/workbench/services/extensions/common/extensionPoints.ts
+++ b/src/vs/workbench/services/extensions/common/extensionPoints.ts
@@ -14,7 +14,7 @@ import * as types from 'vs/base/common/types';
 import { URI } from 'vs/base/common/uri';
 import { getGalleryExtensionId, groupByExtension, getExtensionId, ExtensionKey } from 'vs/platform/extensionManagement/common/extensionManagementUtil';
 import { isValidExtensionVersion } from 'vs/platform/extensions/common/extensionValidator';
-import { ExtensionIdentifier, IExtensionDescription, TargetPlatform, UNDEFINED_PUBLISHER } from 'vs/platform/extensions/common/extensions';
+import { ExtensionIdentifier, IExtensionDescription, IRelaxedExtensionDescription, TargetPlatform, UNDEFINED_PUBLISHER } from 'vs/platform/extensions/common/extensions';
 
 const MANIFEST_FILE = 'package.json';
 
@@ -115,9 +115,15 @@ abstract class ExtensionManifestHandler {
 	}
 }
 
+interface ILocalExtensionMetadata {
+	id?: string;
+	targetPlatform?: TargetPlatform;
+	isBuiltin?: boolean;
+}
+
 class ExtensionManifestParser extends ExtensionManifestHandler {
 
-	private static _fastParseJSON(text: string, errors: json.ParseError[]): any {
+	private static _fastParseJSON<T>(text: string, errors: json.ParseError[]): T {
 		try {
 			return JSON.parse(text);
 		} catch (err) {
@@ -126,10 +132,10 @@ class ExtensionManifestParser extends ExtensionManifestHandler {
 		}
 	}
 
-	public parse(): Promise<IExtensionDescription> {
+	public parse(): Promise<IExtensionDescription | null> {
 		return this._host.readFile(this._absoluteManifestPath).then((manifestContents) => {
 			const errors: json.ParseError[] = [];
-			const manifest = ExtensionManifestParser._fastParseJSON(manifestContents, errors);
+			const manifest = ExtensionManifestParser._fastParseJSON<IRelaxedExtensionDescription & { __metadata?: ILocalExtensionMetadata }>(manifestContents, errors);
 			if (json.getNodeType(manifest) !== 'object') {
 				this._error(this._absoluteFolderPath, nls.localize('jsonParseInvalidType', "Invalid manifest file {0}: Not an JSON object.", this._absoluteManifestPath));
 			} else if (errors.length === 0) {
@@ -360,25 +366,6 @@ class ExtensionManifestNLSReplacer extends ExtensionManifestHandler {
 	}
 }
 
-// Relax the readonly properties here, it is the one place where we check and normalize values
-export interface IRelaxedExtensionDescription {
-	id: string;
-	uuid?: string;
-	targetPlatform: TargetPlatform;
-	identifier: ExtensionIdentifier;
-	name: string;
-	version: string;
-	publisher: string;
-	isBuiltin: boolean;
-	isUserBuiltin: boolean;
-	isUnderDevelopment: boolean;
-	extensionLocation: URI;
-	engines: {
-		vscode: string;
-	};
-	main?: string;
-}
-
 class ExtensionManifestValidator extends ExtensionManifestHandler {
 	validate(_extensionDescription: IExtensionDescription): IExtensionDescription | null {
 		let extensionDescription = <IRelaxedExtensionDescription>_extensionDescription;
diff --git a/src/vs/workbench/services/extensions/electron-sandbox/cachedExtensionScanner.ts b/src/vs/workbench/services/extensions/electron-sandbox/cachedExtensionScanner.ts
index 18c5b791334..6d55c713a8b 100644
--- a/src/vs/workbench/services/extensions/electron-sandbox/cachedExtensionScanner.ts
+++ b/src/vs/workbench/services/extensions/electron-sandbox/cachedExtensionScanner.ts
@@ -12,11 +12,11 @@ import * as platform from 'vs/base/common/platform';
 import { joinPath, originalFSPath } from 'vs/base/common/resources';
 import { URI } from 'vs/base/common/uri';
 import { INativeWorkbenchEnvironmentService } from 'vs/workbench/services/environment/electron-sandbox/environmentService';
-import { BUILTIN_MANIFEST_CACHE_FILE, MANIFEST_CACHE_FOLDER, USER_MANIFEST_CACHE_FILE, IExtensionDescription } from 'vs/platform/extensions/common/extensions';
+import { BUILTIN_MANIFEST_CACHE_FILE, MANIFEST_CACHE_FOLDER, USER_MANIFEST_CACHE_FILE, IExtensionDescription, IRelaxedExtensionDescription } from 'vs/platform/extensions/common/extensions';
 import { IProductService } from 'vs/platform/product/common/productService';
 import { INotificationService, Severity } from 'vs/platform/notification/common/notification';
 import { IHostService } from 'vs/workbench/services/host/browser/host';
-import { Translations, ILog, ExtensionScanner, ExtensionScannerInput, IExtensionReference, IExtensionResolver, IExtensionScannerHost, IRelaxedExtensionDescription } from 'vs/workbench/services/extensions/common/extensionPoints';
+import { Translations, ILog, ExtensionScanner, ExtensionScannerInput, IExtensionReference, IExtensionResolver, IExtensionScannerHost } from 'vs/workbench/services/extensions/common/extensionPoints';
 import { dedupExtensions } from 'vs/workbench/services/extensions/common/extensionsUtil';
 import { FileOperationResult, IFileService, toFileOperationResult } from 'vs/platform/files/common/files';
 import { VSBuffer } from 'vs/base/common/buffer';