diff --git a/testing/web-platform/meta/MANIFEST.json b/testing/web-platform/meta/MANIFEST.json
index 002d478e7a8c..2ebef2ea9000 100644
--- a/testing/web-platform/meta/MANIFEST.json
+++ b/testing/web-platform/meta/MANIFEST.json
@@ -369396,6 +369396,12 @@
      {}
     ]
    ],
+   "xhr/open-parameters-toString.htm": [
+    [
+     "/xhr/open-parameters-toString.htm",
+     {}
+    ]
+   ],
    "xhr/open-referer.htm": [
     [
      "/xhr/open-referer.htm",
@@ -606541,6 +606547,10 @@
    "8b1721532c80a0732462bc090db236256bf1bf38",
    "testharness"
   ],
+  "xhr/open-parameters-toString.htm": [
+   "bd1a660def50557f2a7ee896aa29583504be9ae3",
+   "testharness"
+  ],
   "xhr/open-referer.htm": [
    "7343a7af06cda485e60af3bb5d86e1376a89fd68",
    "testharness"
diff --git a/testing/web-platform/tests/xhr/open-parameters-toString.htm b/testing/web-platform/tests/xhr/open-parameters-toString.htm
new file mode 100644
index 000000000000..c059482cbf66
--- /dev/null
+++ b/testing/web-platform/tests/xhr/open-parameters-toString.htm
@@ -0,0 +1,54 @@
+<!doctype html>
+<title>XMLHttpRequest: open() attempts to toString its string parameters</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id="log"></div>
+<script>
+test(() => {
+  let log = [];
+  let expected = [
+    'method',
+    'url',
+    // NOTE: 'async' intentionally missing
+    'username',
+    'password',
+  ];
+
+  let xhr = new XMLHttpRequest;
+  xhr.open(
+    {
+      toString() {
+        log.push('method');
+        return 'get';
+      },
+    },
+    {
+      toString() {
+        log.push('url');
+        return location.href;
+      },
+    },
+    // NOTE: ToBoolean should not invoke valueOf
+    {
+      valueOf() {
+        log.push('async');
+        return true;
+      },
+    },
+    {
+      toString() {
+        log.push('username');
+        return 'username';
+      },
+    },
+    {
+      toString() {
+        log.push('password');
+        return 'password';
+      },
+    }
+  );
+
+  assert_array_equals(log, expected);
+});
+</script>
