diff --git a/src/vs/vscode.proposed.d.ts b/src/vs/vscode.proposed.d.ts
index 59df165ca0224bcde80aa1e353c2f6bf302ce106..95f1ede0e40eea8247c5b45a10654c9a338e6fb8 100644
--- a/src/vs/vscode.proposed.d.ts
+++ b/src/vs/vscode.proposed.d.ts
@@ -133,6 +133,13 @@ declare module 'vscode' {
 
 	//#region @alexdima - resolvers
 
+	export interface MessageOptions {
+		/**
+		 * Do not render a native message box.
+		 */
+		useCustom?: boolean;
+	}
+
 	export interface RemoteAuthorityResolverContext {
 		resolveAttempt: number;
 	}
diff --git a/src/vs/workbench/api/browser/mainThreadMessageService.ts b/src/vs/workbench/api/browser/mainThreadMessageService.ts
index c956da47b954913aa56683a81b7c9df40b78b737..134d8488dfd347883063f5e53ae9012086c6ae41 100644
--- a/src/vs/workbench/api/browser/mainThreadMessageService.ts
+++ b/src/vs/workbench/api/browser/mainThreadMessageService.ts
@@ -33,6 +33,7 @@ export class MainThreadMessageService implements MainThreadMessageServiceShape {
 
 	$showMessage(severity: Severity, message: string, options: MainThreadMessageOptions, commands: { title: string; isCloseAffordance: boolean; handle: number; }[]): Promise<number | undefined> {
 		if (options.modal) {
+			// TODO: Handle options.useCustom
 			return this._showModalMessage(severity, message, commands);
 		} else {
 			return this._showMessage(severity, message, commands, options.extension);
diff --git a/src/vs/workbench/api/common/extHost.protocol.ts b/src/vs/workbench/api/common/extHost.protocol.ts
index 9ed524ac3b52caf65dea0686c510d569d38eae82..b2a4e235b0019d8d638540042adc7dfcfe1b2274 100644
--- a/src/vs/workbench/api/common/extHost.protocol.ts
+++ b/src/vs/workbench/api/common/extHost.protocol.ts
@@ -420,6 +420,7 @@ export interface MainThreadLanguagesShape extends IDisposable {
 export interface MainThreadMessageOptions {
 	extension?: IExtensionDescription;
 	modal?: boolean;
+	useCustom?: boolean;
 }
 
 export interface MainThreadMessageServiceShape extends IDisposable {
diff --git a/src/vs/workbench/api/common/extHostMessageService.ts b/src/vs/workbench/api/common/extHostMessageService.ts
index ff75e2f3393ed33986a478919425cb7af0242b38..847a27a2b5ef3fac53322fb3ffa4dd9d2c1fb8a5 100644
--- a/src/vs/workbench/api/common/extHostMessageService.ts
+++ b/src/vs/workbench/api/common/extHostMessageService.ts
@@ -8,6 +8,7 @@ import type * as vscode from 'vscode';
 import { MainContext, MainThreadMessageServiceShape, MainThreadMessageOptions, IMainContext } from './extHost.protocol';
 import { IExtensionDescription } from 'vs/platform/extensions/common/extensions';
 import { ILogService } from 'vs/platform/log/common/log';
+import { checkProposedApiEnabled } from 'vs/workbench/services/extensions/common/extensions';
 
 function isMessageItem(item: any): item is vscode.MessageItem {
 	return item && item.title;
@@ -37,9 +38,14 @@ export class ExtHostMessageService {
 			items = [optionsOrFirstItem, ...rest];
 		} else {
 			options.modal = optionsOrFirstItem && optionsOrFirstItem.modal;
+			options.useCustom = optionsOrFirstItem && optionsOrFirstItem.useCustom;
 			items = rest;
 		}
 
+		if (options.useCustom) {
+			checkProposedApiEnabled(extension);
+		}
+
 		const commands: { title: string; isCloseAffordance: boolean; handle: number; }[] = [];
 
 		for (let handle = 0; handle < items.length; handle++) {