diff --git a/src/base.ts b/src/base.ts
index b14fbdc..0149d24 100644
--- a/src/base.ts
+++ b/src/base.ts
@@ -1,3 +1,5 @@
+'use strict';
+
 import https, { RequestOptions } from 'https';
 import { URL } from 'url';
 import {
diff --git a/src/client/auth_client.ts b/src/client/auth_client.ts
index d61095a..a38ea6a 100644
--- a/src/client/auth_client.ts
+++ b/src/client/auth_client.ts
@@ -1,3 +1,5 @@
+'use strict';
+
 /**
  * Defines the main interface for all clients that generate credentials.
  */
diff --git a/src/utils.ts b/src/utils.ts
index 94ded1c..05c44fc 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -1,3 +1,5 @@
+'use strict';
+
 import { promises as fs } from 'fs';
 import crypto from 'crypto';
 import path from 'path';
diff --git a/tests/client/credentials_json_client.test.ts b/tests/client/credentials_json_client.test.ts
index 78f71f4..63f8587 100644
--- a/tests/client/credentials_json_client.test.ts
+++ b/tests/client/credentials_json_client.test.ts
@@ -1,3 +1,5 @@
+'use strict';
+
 import 'mocha';
 import { expect } from 'chai';
 
diff --git a/tests/client/workload_identity_client.test.ts b/tests/client/workload_identity_client.test.ts
index 8c13910..e4a7dc9 100644
--- a/tests/client/workload_identity_client.test.ts
+++ b/tests/client/workload_identity_client.test.ts
@@ -1,3 +1,5 @@
+'use strict';
+
 import 'mocha';
 import { expect } from 'chai';