diff --git a/extensions/typescript-language-features/src/features/bufferSyncSupport.ts b/extensions/typescript-language-features/src/features/bufferSyncSupport.ts
index 5d874198b15..7cd17426fc8 100644
--- a/extensions/typescript-language-features/src/features/bufferSyncSupport.ts
+++ b/extensions/typescript-language-features/src/features/bufferSyncSupport.ts
@@ -329,7 +329,7 @@ export default class BufferSyncSupport extends Disposable {
 
 	constructor(
 		client: ITypeScriptServiceClient,
-		modeIds: string[]
+		modeIds: readonly string[]
 	) {
 		super();
 		this.client = client;
diff --git a/extensions/typescript-language-features/src/typescriptServiceClient.ts b/extensions/typescript-language-features/src/typescriptServiceClient.ts
index 9484a80ac53..e1817afefb6 100644
--- a/extensions/typescript-language-features/src/typescriptServiceClient.ts
+++ b/extensions/typescript-language-features/src/typescriptServiceClient.ts
@@ -121,7 +121,7 @@ export default class TypeScriptServiceClient extends Disposable implements IType
 		private readonly onDidChangeTypeScriptVersion: (version: TypeScriptVersion) => void,
 		public readonly pluginManager: PluginManager,
 		private readonly logDirectoryProvider: LogDirectoryProvider,
-		allModeIds: string[]
+		allModeIds: readonly string[]
 	) {
 		super();
 		this.pathSeparator = path.sep;