diff --git a/src/vs/editor/common/model/mirrorTextModel.ts b/src/vs/editor/common/model/mirrorTextModel.ts
index 26a31412fa732..4fefa551a1aca 100644
--- a/src/vs/editor/common/model/mirrorTextModel.ts
+++ b/src/vs/editor/common/model/mirrorTextModel.ts
@@ -23,6 +23,14 @@ export interface IModelChangedEvent {
 	 * The new version id the model has transitioned to.
 	 */
 	readonly versionId: number;
+	/**
+	 * Flag that indicates that this event was generated while undoing.
+	 */
+	readonly isUndoing: boolean;
+	/**
+	 * Flag that indicates that this event was generated while redoing.
+	 */
+	readonly isRedoing: boolean;
 }
 
 export interface IMirrorTextModel {
diff --git a/src/vs/vscode.d.ts b/src/vs/vscode.d.ts
index f5cdab665a5ad..f6022c9f52f79 100644
--- a/src/vs/vscode.d.ts
+++ b/src/vs/vscode.d.ts
@@ -10130,6 +10130,14 @@ declare module 'vscode' {
 		readonly text: string;
 	}
 
+	export enum TextDocumentChangeReason {
+		/** The text change is caused by an undo operation. */
+		Undo = 1,
+
+		/** The text change is caused by an redo operation. */
+		Redo = 2,
+	}
+
 	/**
 	 * An event describing a transactional {@link TextDocument document} change.
 	 */
@@ -10144,6 +10152,12 @@ declare module 'vscode' {
 		 * An array of content changes.
 		 */
 		readonly contentChanges: readonly TextDocumentContentChangeEvent[];
+
+		/**
+		 * The reason why the document was changed.
+		 * Is undefined if the reason is not known.
+		*/
+		readonly reason?: TextDocumentChangeReason;
 	}
 
 	/**
diff --git a/src/vs/workbench/api/common/extHost.api.impl.ts b/src/vs/workbench/api/common/extHost.api.impl.ts
index 97823cf6c3f40..22080ab47fc62 100644
--- a/src/vs/workbench/api/common/extHost.api.impl.ts
+++ b/src/vs/workbench/api/common/extHost.api.impl.ts
@@ -1239,6 +1239,7 @@ export function createApiFactoryAndRegisterActors(accessor: ServicesAccessor): I
 			TextEditorLineNumbersStyle: extHostTypes.TextEditorLineNumbersStyle,
 			TextEditorRevealType: extHostTypes.TextEditorRevealType,
 			TextEditorSelectionChangeKind: extHostTypes.TextEditorSelectionChangeKind,
+			TextDocumentChangeReason: extHostTypes.TextDocumentChangeReason,
 			ThemeColor: extHostTypes.ThemeColor,
 			ThemeIcon: extHostTypes.ThemeIcon,
 			TreeItem: extHostTypes.TreeItem,
diff --git a/src/vs/workbench/api/common/extHostDocuments.ts b/src/vs/workbench/api/common/extHostDocuments.ts
index dcf170acd9253..4f13102856602 100644
--- a/src/vs/workbench/api/common/extHostDocuments.ts
+++ b/src/vs/workbench/api/common/extHostDocuments.ts
@@ -14,6 +14,7 @@ import * as TypeConverters from 'vs/workbench/api/common/extHostTypeConverters';
 import type * as vscode from 'vscode';
 import { assertIsDefined } from 'vs/base/common/types';
 import { deepFreeze } from 'vs/base/common/objects';
+import { TextDocumentChangeReason } from 'vs/workbench/api/common/extHostTypes';
 
 export class ExtHostDocuments implements ExtHostDocumentsShape {
 
@@ -134,7 +135,8 @@ export class ExtHostDocuments implements ExtHostDocumentsShape {
 		data._acceptIsDirty(isDirty);
 		this._onDidChangeDocument.fire({
 			document: data.document,
-			contentChanges: []
+			contentChanges: [],
+			reason: undefined
 		});
 	}
 
@@ -146,6 +148,14 @@ export class ExtHostDocuments implements ExtHostDocumentsShape {
 		}
 		data._acceptIsDirty(isDirty);
 		data.onEvents(events);
+
+		let reason: vscode.TextDocumentChangeReason | undefined = undefined;
+		if (events.isUndoing) {
+			reason = TextDocumentChangeReason.Undo;
+		} else if (events.isRedoing) {
+			reason = TextDocumentChangeReason.Redo;
+		}
+
 		this._onDidChangeDocument.fire(deepFreeze({
 			document: data.document,
 			contentChanges: events.changes.map((change) => {
@@ -155,7 +165,8 @@ export class ExtHostDocuments implements ExtHostDocumentsShape {
 					rangeLength: change.rangeLength,
 					text: change.text
 				};
-			})
+			}),
+			reason
 		}));
 	}
 
diff --git a/src/vs/workbench/api/common/extHostTypes.ts b/src/vs/workbench/api/common/extHostTypes.ts
index 704417acf9c22..2cc35df7c9882 100644
--- a/src/vs/workbench/api/common/extHostTypes.ts
+++ b/src/vs/workbench/api/common/extHostTypes.ts
@@ -1590,6 +1590,11 @@ export enum TextEditorSelectionChangeKind {
 	Command = 3
 }
 
+export enum TextDocumentChangeReason {
+	Undo = 1,
+	Redo = 2,
+}
+
 /**
  * These values match very carefully the values of `TrackedRangeStickiness`
  */
diff --git a/src/vs/workbench/test/browser/api/extHostDocumentData.test.ts b/src/vs/workbench/test/browser/api/extHostDocumentData.test.ts
index c1a5af90538a4..228f929ca15fb 100644
--- a/src/vs/workbench/test/browser/api/extHostDocumentData.test.ts
+++ b/src/vs/workbench/test/browser/api/extHostDocumentData.test.ts
@@ -104,6 +104,8 @@ suite('ExtHostDocumentData', () => {
 			}],
 			eol: undefined!,
 			versionId: undefined!,
+			isRedoing: false,
+			isUndoing: false,
 		});
 
 		// line didn't change
@@ -161,6 +163,8 @@ suite('ExtHostDocumentData', () => {
 			}],
 			eol: undefined!,
 			versionId: undefined!,
+			isRedoing: false,
+			isUndoing: false,
 		});
 
 		assertOffsetAt(0, 1, 1);
@@ -179,6 +183,8 @@ suite('ExtHostDocumentData', () => {
 			}],
 			eol: undefined!,
 			versionId: undefined!,
+			isRedoing: false,
+			isUndoing: false,
 		});
 
 		assertOffsetAt(0, 1, 1);
@@ -197,6 +203,8 @@ suite('ExtHostDocumentData', () => {
 			}],
 			eol: undefined!,
 			versionId: undefined!,
+			isRedoing: false,
+			isUndoing: false,
 		});
 
 		assertOffsetAt(0, 1, 1);
@@ -218,6 +226,8 @@ suite('ExtHostDocumentData', () => {
 			}],
 			eol: undefined!,
 			versionId: undefined!,
+			isRedoing: false,
+			isUndoing: false,
 		});
 
 		assertOffsetAt(0, 1, 1);
@@ -398,6 +408,8 @@ suite('ExtHostDocumentData updates line mapping', () => {
 			}],
 			eol: eol!,
 			versionId: undefined!,
+			isRedoing: false,
+			isUndoing: false,
 		};
 	}
 
@@ -423,7 +435,7 @@ suite('ExtHostDocumentData updates line mapping', () => {
 			'and this is line number two',
 			'it is followed by #3',
 			'and finished with the fourth.',
-		], { changes: [], eol: undefined!, versionId: 7 });
+		], { changes: [], eol: undefined!, versionId: 7, isRedoing: false, isUndoing: false });
 	});
 
 	test('after remove', () => {
diff --git a/src/vs/workbench/test/browser/api/extHostDocumentSaveParticipant.test.ts b/src/vs/workbench/test/browser/api/extHostDocumentSaveParticipant.test.ts
index b41dc067fb32b..c8973417b2337 100644
--- a/src/vs/workbench/test/browser/api/extHostDocumentSaveParticipant.test.ts
+++ b/src/vs/workbench/test/browser/api/extHostDocumentSaveParticipant.test.ts
@@ -306,7 +306,9 @@ suite('ExtHostDocumentSaveParticipant', () => {
 					text: 'bar'
 				}],
 				eol: undefined!,
-				versionId: 2
+				versionId: 2,
+				isRedoing: false,
+				isUndoing: false,
 			}, true);
 
 			e.waitUntil(Promise.resolve([TextEdit.insert(new Position(0, 0), 'bar')]));
@@ -338,7 +340,9 @@ suite('ExtHostDocumentSaveParticipant', () => {
 							rangeLength: undefined!,
 						}],
 						eol: undefined!,
-						versionId: documents.getDocumentData(uri)!.version + 1
+						versionId: documents.getDocumentData(uri)!.version + 1,
+						isRedoing: false,
+						isUndoing: false,
 					}, true);
 					// }
 				}
diff --git a/src/vs/workbench/test/browser/api/extHostNotebookConcatDocument.test.ts b/src/vs/workbench/test/browser/api/extHostNotebookConcatDocument.test.ts
index b9c704ae5bfa1..6ff7b0f14af5c 100644
--- a/src/vs/workbench/test/browser/api/extHostNotebookConcatDocument.test.ts
+++ b/src/vs/workbench/test/browser/api/extHostNotebookConcatDocument.test.ts
@@ -334,7 +334,9 @@ suite('NotebookConcatDocument', function () {
 				rangeLength: 6,
 				rangeOffset: 12,
 				text: 'Hi'
-			}]
+			}],
+			isRedoing: false,
+			isUndoing: false,
 		}, false);
 		assertLines(doc, 'Hello', 'World', 'Hi World!', 'Hallo', 'Welt', 'Hallo Welt!');
 		assertLocation(doc, new Position(2, 12), new Location(notebook.apiNotebook.cellAt(0).document.uri, new Position(2, 9)), false);