diff --git a/extensions/vscode-notebook-tests/package.json b/extensions/vscode-notebook-tests/package.json
index 0a6b5ee625de6e132502481b78d964951bf8ecc7..83f73669e0a977c844ba2bbaf98b6343218b5a5f 100644
--- a/extensions/vscode-notebook-tests/package.json
+++ b/extensions/vscode-notebook-tests/package.json
@@ -54,6 +54,15 @@
           }
         ]
       }
+    ],
+    "notebookOutputRenderer": [
+      {
+        "viewType": "notebookCoreTestRenderer",
+        "displayName": "Notebook Core Test Renderer",
+        "mimeTypes": [
+          "text/custom"
+        ]
+      }
     ]
   }
 }
diff --git a/extensions/vscode-notebook-tests/src/customRenderer.js b/extensions/vscode-notebook-tests/src/customRenderer.js
new file mode 100644
index 0000000000000000000000000000000000000000..75e2ec1eb7ad4c42523e90991089f340133af36d
--- /dev/null
+++ b/extensions/vscode-notebook-tests/src/customRenderer.js
@@ -0,0 +1,13 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+const vscode = acquireVsCodeApi();
+
+vscode.postMessage({
+	type: 'custom_renderer_initialize',
+	payload: {
+		firstMessage: true
+	}
+});
diff --git a/extensions/vscode-notebook-tests/src/notebook.test.ts b/extensions/vscode-notebook-tests/src/notebook.test.ts
index a54d9121703a464fed2f3f7a5492b0ab4d7aa032..1c296a79e6370151628ccd2faca48fa339f97a00 100644
--- a/extensions/vscode-notebook-tests/src/notebook.test.ts
+++ b/extensions/vscode-notebook-tests/src/notebook.test.ts
@@ -689,7 +689,7 @@ suite('regression', () => {
 
 });
 
-suite('webview resource uri', () => {
+suite('webview', () => {
 	test('asWebviewUri', async function () {
 		const resource = vscode.Uri.parse(join(vscode.workspace.rootPath || '', './first.vsctestnb'));
 		await vscode.commands.executeCommand('vscode.openWith', resource, 'notebookCoreTest');
@@ -698,4 +698,23 @@ suite('webview resource uri', () => {
 		assert.equal(uri.scheme, 'vscode-webview-resource');
 		await vscode.commands.executeCommand('workbench.action.closeAllEditors');
 	});
+
+	test('custom renderer message', async function () {
+		const resource = vscode.Uri.parse(join(vscode.workspace.rootPath || '', './customRenderer.vsctestnb'));
+		await vscode.commands.executeCommand('vscode.openWith', resource, 'notebookCoreTest');
+
+		const editor = vscode.notebook.activeNotebookEditor;
+		const promise = new Promise(resolve => {
+			const messageEmitter = editor?.onDidReceiveMessage(e => {
+				if (e.type === 'custom_renderer_initialize') {
+					resolve();
+					messageEmitter?.dispose();
+				}
+			});
+		});
+
+		await vscode.commands.executeCommand('notebook.cell.execute');
+		await promise;
+		await vscode.commands.executeCommand('workbench.action.closeAllEditors');
+	});
 });
diff --git a/extensions/vscode-notebook-tests/src/notebookTestMain.ts b/extensions/vscode-notebook-tests/src/notebookTestMain.ts
index 155a8e2c847669b6713831da5dba80e8ba022d11..059f998acae6910fceda65e614945094212d8ee1 100644
--- a/extensions/vscode-notebook-tests/src/notebookTestMain.ts
+++ b/extensions/vscode-notebook-tests/src/notebookTestMain.ts
@@ -4,6 +4,7 @@
  *--------------------------------------------------------------------------------------------*/
 
 import * as vscode from 'vscode';
+import * as path from 'path';
 import { smokeTestActivate } from './notebookSmokeTestMain';
 
 export function activate(context: vscode.ExtensionContext): any {
@@ -66,13 +67,38 @@ export function activate(context: vscode.ExtensionContext): any {
 				_cell = _document.cells[0];
 			}
 
+			if (_document.uri.path.endsWith('customRenderer.vsctestnb')) {
+				_cell.outputs = [{
+					outputKind: vscode.CellOutputKind.Rich,
+					data: {
+						'text/custom': 'test'
+					}
+				}];
+
+				return;
+			}
+
 			_cell.outputs = [{
 				outputKind: vscode.CellOutputKind.Rich,
 				data: {
 					'text/plain': ['my output']
 				}
 			}];
+
 			return;
 		}
 	}));
+
+	const preloadUri = vscode.Uri.file(path.resolve(__dirname, '../src/customRenderer.js'));
+	context.subscriptions.push(vscode.notebook.registerNotebookOutputRenderer('notebookCoreTestRenderer', {
+		type: 'display_data',
+		subTypes: [
+			'text/custom'
+		]
+	}, {
+		preloads: [preloadUri],
+		render(_document: vscode.NotebookDocument, _output: vscode.CellDisplayOutput, _mimeType: string): string {
+			return '<div>test</div>';
+		}
+	}));
 }
diff --git a/extensions/vscode-notebook-tests/test/customRenderer.vsctestnb b/extensions/vscode-notebook-tests/test/customRenderer.vsctestnb
new file mode 100644
index 0000000000000000000000000000000000000000..a4a092d83492f73541c48127da38563b8fce621d
--- /dev/null
+++ b/extensions/vscode-notebook-tests/test/customRenderer.vsctestnb
@@ -0,0 +1,4 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/