diff --git a/src/vs/vscode.proposed.d.ts b/src/vs/vscode.proposed.d.ts
index 108a6515e425f47dba88203ebd90fc1f4280cf6d..893c80ca37fd56df37d33deb8d0204720492bf08 100644
--- a/src/vs/vscode.proposed.d.ts
+++ b/src/vs/vscode.proposed.d.ts
@@ -2207,4 +2207,26 @@ declare module 'vscode' {
 		}): Disposable;
 	}
 	//#endregion
+
+	//#region
+
+	export interface FileSystem {
+		/**
+		 * Check if a given file system supports writing files.
+		 *
+		 * Keep in mind that just because a file system supports writing, that does
+		 * not mean that writes will always succeed. There may be permissions issues
+		 * or other errors that prevent writing a file.
+		 *
+		 * @param scheme The scheme of the filesystem, for example `file` or `git`.
+		 *
+		 * @return `true` if the file system supports writing, `false` if it does not
+		 * support writing (i.e. it is readonly), and `undefined` if VS Code does not
+		 * know about the filesystem.
+		 */
+		isWritableFileSystem(scheme: string): boolean | undefined;
+	}
+
+
+	//#endregion
 }
diff --git a/src/vs/workbench/api/common/extHost.api.impl.ts b/src/vs/workbench/api/common/extHost.api.impl.ts
index 46a0b734f5be1810fa70723d7cf5dff3c5ba93f0..f4d07676ed2ee505c4f5f9a29d93dd0904830913 100644
--- a/src/vs/workbench/api/common/extHost.api.impl.ts
+++ b/src/vs/workbench/api/common/extHost.api.impl.ts
@@ -135,7 +135,7 @@ export function createApiFactoryAndRegisterActors(accessor: ServicesAccessor): I
 	const extHostEditorInsets = rpcProtocol.set(ExtHostContext.ExtHostEditorInsets, new ExtHostEditorInsets(rpcProtocol.getProxy(MainContext.MainThreadEditorInsets), extHostEditors, initData.environment));
 	const extHostDiagnostics = rpcProtocol.set(ExtHostContext.ExtHostDiagnostics, new ExtHostDiagnostics(rpcProtocol, extHostLogService));
 	const extHostLanguageFeatures = rpcProtocol.set(ExtHostContext.ExtHostLanguageFeatures, new ExtHostLanguageFeatures(rpcProtocol, uriTransformer, extHostDocuments, extHostCommands, extHostDiagnostics, extHostLogService, extHostApiDeprecation));
-	const extHostFileSystem = rpcProtocol.set(ExtHostContext.ExtHostFileSystem, new ExtHostFileSystem(rpcProtocol, extHostLanguageFeatures));
+	const extHostFileSystem = rpcProtocol.set(ExtHostContext.ExtHostFileSystem, new ExtHostFileSystem(rpcProtocol, extHostLanguageFeatures, extHostConsumerFileSystem));
 	const extHostFileSystemEvent = rpcProtocol.set(ExtHostContext.ExtHostFileSystemEventService, new ExtHostFileSystemEventService(rpcProtocol, extHostLogService, extHostDocumentsAndEditors));
 	const extHostQuickOpen = rpcProtocol.set(ExtHostContext.ExtHostQuickOpen, new ExtHostQuickOpen(rpcProtocol, extHostWorkspace, extHostCommands));
 	const extHostSCM = rpcProtocol.set(ExtHostContext.ExtHostSCM, new ExtHostSCM(rpcProtocol, extHostCommands, extHostLogService));
diff --git a/src/vs/workbench/api/common/extHostFileSystem.ts b/src/vs/workbench/api/common/extHostFileSystem.ts
index 34d7d3862edb54a1dbc2d600432457002872ab5c..c6330dd6e8b2cb4d5fe675b469086ca6d272ce08 100644
--- a/src/vs/workbench/api/common/extHostFileSystem.ts
+++ b/src/vs/workbench/api/common/extHostFileSystem.ts
@@ -16,6 +16,7 @@ import { State, StateMachine, LinkComputer, Edge } from 'vs/editor/common/modes/
 import { commonPrefixLength } from 'vs/base/common/strings';
 import { CharCode } from 'vs/base/common/charCode';
 import { VSBuffer } from 'vs/base/common/buffer';
+import { IExtHostConsumerFileSystem } from 'vs/workbench/api/common/extHostFileSystemConsumer';
 
 class FsLinkProvider {
 
@@ -119,7 +120,7 @@ export class ExtHostFileSystem implements ExtHostFileSystemShape {
 	private _linkProviderRegistration?: IDisposable;
 	private _handlePool: number = 0;
 
-	constructor(mainContext: IMainContext, private _extHostLanguageFeatures: ExtHostLanguageFeatures) {
+	constructor(mainContext: IMainContext, private _extHostLanguageFeatures: ExtHostLanguageFeatures, private readonly _fileSystemConsumer: IExtHostConsumerFileSystem) {
 		this._proxy = mainContext.getProxy(MainContext.MainThreadFileSystem);
 
 		// register used schemes
@@ -142,6 +143,8 @@ export class ExtHostFileSystem implements ExtHostFileSystemShape {
 			throw new Error(`a provider for the scheme '${scheme}' is already registered`);
 		}
 
+		const schemeRegistration = this._fileSystemConsumer._registerScheme(scheme, options);
+
 		//
 		this._registerLinkProviderIfNotYetRegistered();
 
@@ -197,6 +200,7 @@ export class ExtHostFileSystem implements ExtHostFileSystemShape {
 
 		return toDisposable(() => {
 			subscription.dispose();
+			schemeRegistration.dispose();
 			this._linkProvider.delete(scheme);
 			this._usedSchemes.delete(scheme);
 			this._fsProvider.delete(handle);
diff --git a/src/vs/workbench/api/common/extHostFileSystemConsumer.ts b/src/vs/workbench/api/common/extHostFileSystemConsumer.ts
index 67c8bb77d5e40fc66b885a93b41cb6deafbe065e..cf3d309dbfbcec8ad11ef010ca1782cb45938086 100644
--- a/src/vs/workbench/api/common/extHostFileSystemConsumer.ts
+++ b/src/vs/workbench/api/common/extHostFileSystemConsumer.ts
@@ -10,6 +10,7 @@ import { FileSystemError } from 'vs/workbench/api/common/extHostTypes';
 import { VSBuffer } from 'vs/base/common/buffer';
 import { createDecorator } from 'vs/platform/instantiation/common/instantiation';
 import { IExtHostRpcService } from 'vs/workbench/api/common/extHostRpcService';
+import { IDisposable, toDisposable } from 'vs/base/common/lifecycle';
 
 export class ExtHostConsumerFileSystem implements vscode.FileSystem {
 
@@ -17,6 +18,8 @@ export class ExtHostConsumerFileSystem implements vscode.FileSystem {
 
 	private readonly _proxy: MainThreadFileSystemShape;
 
+	private readonly _schemes = new Map<string, { readonly isReadonly?: boolean }>();
+
 	constructor(@IExtHostRpcService extHostRpc: IExtHostRpcService) {
 		this._proxy = extHostRpc.getProxy(MainContext.MainThreadFileSystem);
 	}
@@ -45,6 +48,14 @@ export class ExtHostConsumerFileSystem implements vscode.FileSystem {
 	copy(source: vscode.Uri, destination: vscode.Uri, options?: { overwrite?: boolean; }): Promise<void> {
 		return this._proxy.$copy(source, destination, { ...{ overwrite: false }, ...options }).catch(ExtHostConsumerFileSystem._handleError);
 	}
+	isWritableFileSystem(scheme: string): boolean | undefined {
+		const entry = this._schemes.get(scheme);
+		if (entry) {
+			return !entry.isReadonly;
+		}
+		return undefined;
+	}
+
 	private static _handleError(err: any): never {
 		// generic error
 		if (!(err instanceof Error)) {
@@ -68,6 +79,14 @@ export class ExtHostConsumerFileSystem implements vscode.FileSystem {
 			default: throw new FileSystemError(err.message, err.name as files.FileSystemProviderErrorCode);
 		}
 	}
+
+	/* internal */ _registerScheme(scheme: string, options: { readonly isReadonly?: boolean }): IDisposable {
+		this._schemes.set(scheme, options);
+
+		return toDisposable(() => {
+			return this._schemes.delete(scheme);
+		});
+	}
 }
 
 export interface IExtHostConsumerFileSystem extends ExtHostConsumerFileSystem { }