From 9957205ab4800478ee2ea8443830271d51b6b9e7 Mon Sep 17 00:00:00 2001
From: M-YasirGhaffar <yasirghaffar.nust@gmail.com>
Date: Fri, 6 Feb 2026 01:27:35 +0500
Subject: [PATCH] websocket: provide descriptive error on abnormal closure
 (fixes #4625)

Signed-off-by: M-YasirGhaffar <yasirghaffar.nust@gmail.com>
---
 lib/web/websocket/websocket.js |  2 +-
 test/websocket/issue-4625.js   | 46 ++++++++++++++++++++++++++++++++++
 2 files changed, 47 insertions(+), 1 deletion(-)
 create mode 100644 test/websocket/issue-4625.js

diff --git a/lib/web/websocket/websocket.js b/lib/web/websocket/websocket.js
index 331497677a3..3138a030f3d 100644
--- a/lib/web/websocket/websocket.js
+++ b/lib/web/websocket/websocket.js
@@ -587,7 +587,7 @@ class WebSocket extends EventTarget {
       code = 1006
 
       fireEvent('error', this, (type, init) => new ErrorEvent(type, init), {
-        error: new TypeError(reason)
+        error: new TypeError(reason || 'WebSocket connection closed abnormally')
       })
     }
 
diff --git a/test/websocket/issue-4625.js b/test/websocket/issue-4625.js
new file mode 100644
index 00000000000..5c6a5288fde
--- /dev/null
+++ b/test/websocket/issue-4625.js
@@ -0,0 +1,46 @@
+'use strict'
+
+const { test } = require('node:test')
+const net = require('node:net')
+const { WebSocket } = require('../..')
+
+// Test for https://github.com/nodejs/undici/issues/4625
+// When WebSocket closes abnormally (code 1006), error message should be descriptive
+test('abnormal closure should have descriptive error message', async (t) => {
+  // Create a raw TCP server that accepts the connection but then abruptly closes
+  const server = net.createServer((socket) => {
+    // Send a valid WebSocket handshake response
+    socket.write(
+      'HTTP/1.1 101 Switching Protocols\r\n' +
+            'Upgrade: websocket\r\n' +
+            'Connection: Upgrade\r\n' +
+            'Sec-WebSocket-Accept: invalid\r\n' +
+            '\r\n'
+    )
+    // Immediately destroy the socket to simulate abnormal closure
+    socket.destroy()
+  })
+
+  await new Promise((resolve) => server.listen(0, resolve))
+  const port = server.address().port
+
+  const ws = new WebSocket(`ws://localhost:${port}`)
+
+  await new Promise((resolve) => {
+    ws.addEventListener('error', (event) => {
+      t.assert.ok(event.error instanceof TypeError)
+      t.assert.ok(
+        event.error.message.length > 0,
+        'error message should not be empty'
+      )
+      t.assert.ok(
+        event.error.message.includes('abnormally') ||
+                event.error.message.includes('closed'),
+        'error message should describe the closure'
+      )
+      resolve()
+    })
+  })
+
+  server.close()
+})