diff --git a/docs/examples/proxy/index.js b/docs/examples/proxy/index.js
index eeb8c330b38..355b17de3cc 100644
--- a/docs/examples/proxy/index.js
+++ b/docs/examples/proxy/index.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const { Pool, Client } = require('../../../')
 const http = require('node:http')
 const proxy = require('./proxy')
diff --git a/docs/examples/proxy/websocket.js b/docs/examples/proxy/websocket.js
index e71d8f0768c..8cb9ef86d87 100644
--- a/docs/examples/proxy/websocket.js
+++ b/docs/examples/proxy/websocket.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const { Pool, Client } = require('../../../')
 const http = require('node:http')
 const proxy = require('./proxy')
diff --git a/lib/api/abort-signal.js b/lib/api/abort-signal.js
index 2afe747d3a1..608170b4316 100644
--- a/lib/api/abort-signal.js
+++ b/lib/api/abort-signal.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const { addAbortListener } = require('../core/util')
 const { RequestAbortedError } = require('../core/errors')
 
diff --git a/lib/api/util.js b/lib/api/util.js
index 2f983bf424c..5512636f339 100644
--- a/lib/api/util.js
+++ b/lib/api/util.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const assert = require('node:assert')
 const {
   ResponseStatusCodeError
diff --git a/lib/core/symbols.js b/lib/core/symbols.js
index 1c3732620b1..f3b563a5419 100644
--- a/lib/core/symbols.js
+++ b/lib/core/symbols.js
@@ -1,3 +1,5 @@
+'use strict'
+
 module.exports = {
   kClose: Symbol('close'),
   kDestroy: Symbol('destroy'),
diff --git a/lib/dispatcher/pool-stats.js b/lib/dispatcher/pool-stats.js
index 8c7e8c9572e..c739211f098 100644
--- a/lib/dispatcher/pool-stats.js
+++ b/lib/dispatcher/pool-stats.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const { kFree, kConnected, kPending, kQueued, kRunning, kSize } = require('../core/symbols')
 const kPool = Symbol('pool')
 
diff --git a/scripts/generate-pem.js b/scripts/generate-pem.js
index 0d7e628e209..88ac5c8392f 100644
--- a/scripts/generate-pem.js
+++ b/scripts/generate-pem.js
@@ -1,3 +1,4 @@
+'use strict'
 /* istanbul ignore file */
 
 require('https-pem/install')
diff --git a/scripts/generate-undici-types-package-json.js b/scripts/generate-undici-types-package-json.js
index 78095ae6d5e..ea2dd07b5f9 100644
--- a/scripts/generate-undici-types-package-json.js
+++ b/scripts/generate-undici-types-package-json.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const fs = require('node:fs')
 const path = require('node:path')
 
diff --git a/scripts/verifyVersion.js b/scripts/verifyVersion.js
index 8ad2d192a66..dbaafa245d5 100644
--- a/scripts/verifyVersion.js
+++ b/scripts/verifyVersion.js
@@ -1,3 +1,5 @@
+'use strict'
+
 /* istanbul ignore file */
 
 const [major, minor, patch] = process.versions.node.split('.').map(v => Number(v))
diff --git a/test/fixtures/fetch.js b/test/fixtures/fetch.js
index e6792ed1dbd..36e572cd3d1 100644
--- a/test/fixtures/fetch.js
+++ b/test/fixtures/fetch.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const { createServer } = require('node:http')
 const { fetch } = require('../..')
 
diff --git a/test/fixtures/undici.js b/test/fixtures/undici.js
index 6ce69aa6785..643142501ff 100644
--- a/test/fixtures/undici.js
+++ b/test/fixtures/undici.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const { createServer } = require('node:http')
 const { request } = require('../..')
 
diff --git a/test/fixtures/websocket.js b/test/fixtures/websocket.js
index 67d283aa96e..708808bd59f 100644
--- a/test/fixtures/websocket.js
+++ b/test/fixtures/websocket.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const { WebSocketServer } = require('ws')
 const { WebSocket } = require('../..')
 
diff --git a/test/jest/mock-scope.test.js b/test/jest/mock-scope.test.js
index e841e48c473..dca8fca343e 100644
--- a/test/jest/mock-scope.test.js
+++ b/test/jest/mock-scope.test.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const { MockAgent, setGlobalDispatcher, request } = require('../../index')
 
 /* global afterAll, expect, it, AbortController */
diff --git a/test/node-fetch/utils/server.js b/test/node-fetch/utils/server.js
index 548f351a88a..233c30da106 100644
--- a/test/node-fetch/utils/server.js
+++ b/test/node-fetch/utils/server.js
@@ -1,3 +1,5 @@
+'use strict'
+
 const http = require('node:http')
 const zlib = require('node:zlib')
 const { once } = require('node:events')