diff --git a/src/vs/workbench/api/common/extHostNotebook.ts b/src/vs/workbench/api/common/extHostNotebook.ts
index 2e76b5fe7c9..347418f4927 100644
--- a/src/vs/workbench/api/common/extHostNotebook.ts
+++ b/src/vs/workbench/api/common/extHostNotebook.ts
@@ -729,7 +729,7 @@ export class ExtHostNotebookController implements ExtHostNotebookShape, ExtHostN
 					emitDocumentMetadataChange(event: vscode.NotebookDocumentMetadataChangeEvent): void {
 						that._onDidChangeNotebookDocumentMetadata.fire(event);
 					}
-				}, viewType, modelData.contentOptions, { ...notebookDocumentMetadataDefaults, ...modelData.metadata }, uri, storageRoot);
+				}, viewType, modelData.contentOptions, new extHostTypes.NotebookDocumentMetadata().with(modelData.metadata ?? {}), uri, storageRoot);
 
 				document.acceptModelChanged({
 					versionId: modelData.versionId,
diff --git a/src/vs/workbench/api/common/extHostNotebookDocument.ts b/src/vs/workbench/api/common/extHostNotebookDocument.ts
index 0fe42bd9de4..b8a9bc12073 100644
--- a/src/vs/workbench/api/common/extHostNotebookDocument.ts
+++ b/src/vs/workbench/api/common/extHostNotebookDocument.ts
@@ -12,7 +12,8 @@ import { URI } from 'vs/base/common/uri';
 import { CellKind, INotebookDocumentPropertiesChangeData } from 'vs/workbench/api/common/extHost.protocol';
 import { ExtHostDocumentsAndEditors, IExtHostModelAddedData } from 'vs/workbench/api/common/extHostDocumentsAndEditors';
 import * as extHostTypeConverters from 'vs/workbench/api/common/extHostTypeConverters';
-import { IMainCellDto, IOutputDto, NotebookCellMetadata, NotebookCellsChangedEventDto, NotebookCellsChangeType, NotebookCellsSplice2, notebookDocumentMetadataDefaults } from 'vs/workbench/contrib/notebook/common/notebookCommon';
+import * as extHostTypes from 'vs/workbench/api/common/extHostTypes';
+import { IMainCellDto, IOutputDto, NotebookCellMetadata, NotebookCellsChangedEventDto, NotebookCellsChangeType, NotebookCellsSplice2 } from 'vs/workbench/contrib/notebook/common/notebookCommon';
 import * as vscode from 'vscode';
 
 class RawContentChangeEvent {
@@ -47,7 +48,7 @@ export class ExtHostCell {
 	readonly onDidDispose: Event<void> = this._onDidDispose.event;
 
 	private _outputs: IOutputDto[];
-	private _metadata: vscode.NotebookCellMetadata;
+	private _metadata: extHostTypes.NotebookCellMetadata;
 
 	readonly handle: number;
 	readonly uri: URI;
@@ -64,7 +65,7 @@ export class ExtHostCell {
 		this.uri = URI.revive(_cellData.uri);
 		this.cellKind = _cellData.cellKind;
 		this._outputs = _cellData.outputs;
-		this._metadata = _cellData.metadata ?? {};
+		this._metadata = new extHostTypes.NotebookCellMetadata().with(_cellData.metadata ?? {});
 	}
 
 	dispose() {
@@ -99,8 +100,8 @@ export class ExtHostCell {
 		this._outputs = newOutputs;
 	}
 
-	setMetadata(newMetadata: vscode.NotebookCellMetadata): void {
-		this._metadata = newMetadata;
+	setMetadata(newMetadata: NotebookCellMetadata): void {
+		this._metadata = this._metadata.with(newMetadata);
 	}
 }
 
@@ -138,7 +139,7 @@ export class ExtHostNotebookDocument extends Disposable {
 		private readonly _emitter: INotebookEventEmitter,
 		private readonly _viewType: string,
 		private readonly _contentOptions: vscode.NotebookDocumentContentOptions,
-		private _metadata: Required<vscode.NotebookDocumentMetadata>,
+		private _metadata: extHostTypes.NotebookDocumentMetadata,
 		public readonly uri: URI,
 		private readonly _storagePath: URI | undefined
 	) {
@@ -190,11 +191,9 @@ export class ExtHostNotebookDocument extends Disposable {
 	}
 
 	acceptDocumentPropertiesChanged(data: INotebookDocumentPropertiesChangeData) {
-		const newMetadata = {
-			...notebookDocumentMetadataDefaults,
-			...data.metadata
-		};
-		this._metadata = newMetadata;
+		if (data.metadata) {
+			this._metadata = this._metadata.with(data.metadata);
+		}
 		this._emitter.emitDocumentMetadataChange({ document: this.notebookDocument });
 	}