diff --git a/src/vs/vscode.proposed.d.ts b/src/vs/vscode.proposed.d.ts
index 4245c92aeb0..d9ea32966c3 100644
--- a/src/vs/vscode.proposed.d.ts
+++ b/src/vs/vscode.proposed.d.ts
@@ -1062,12 +1062,19 @@ declare module 'vscode' {
 		readonly uri: Uri;
 		readonly version: number;
 
+		/** @deprecated Use `uri` instead */
 		// todo@API don't have this...
 		readonly fileName: string;
 
 		readonly isDirty: boolean;
 		readonly isUntitled: boolean;
 
+		/**
+		 * `true` if the notebook has been closed. A closed notebook isn't synchronized anymore
+		 * and won't be re-used when the same resource is opened again.
+		 */
+		readonly isClosed: boolean;
+
 		readonly metadata: NotebookDocumentMetadata;
 
 		// todo@API should we really expose this?
diff --git a/src/vs/workbench/api/common/extHostNotebookDocument.ts b/src/vs/workbench/api/common/extHostNotebookDocument.ts
index aaf7203dfd2..4db521e3880 100644
--- a/src/vs/workbench/api/common/extHostNotebookDocument.ts
+++ b/src/vs/workbench/api/common/extHostNotebookDocument.ts
@@ -177,6 +177,7 @@ export class ExtHostNotebookDocument extends Disposable {
 				get viewType() { return that._viewType; },
 				get isDirty() { return that._isDirty; },
 				get isUntitled() { return that.uri.scheme === Schemas.untitled; },
+				get isClosed() { return that._disposed; },
 				get metadata() { return that._metadata; },
 				set metadata(_value: Required<vscode.NotebookDocumentMetadata>) { throw new Error('Use WorkspaceEdit to update metadata.'); },
 				get cells(): ReadonlyArray<vscode.NotebookCell> { return that._cells.map(cell => cell.cell); },