@@ -1,7 +1,22 @@
 'use strict';
 
+const assert = require('assert');
+const { basename } = require('path');
 const { WPTRunner } = require('../common/wpt');
 
 const runner = new WPTRunner('html/webappapis/timers', { concurrency: 1 });
 
+runner.setScriptModifier((script) => {
+  if (!['type-long-settimeout.any.js', 'type-long-setinterval.any.js']
+    .includes(basename(script.filename))) return;
+
+  // Cancel the failure timer when the test completes so it cannot throw
+  // while the runner is still processing the completion message.
+  const failureTimer = 'setTimeout(assert_unreached, 100);';
+  assert(script.code.includes(failureTimer), `Unexpected contents of ${script.filename}`);
+  script.code = script.code.replace(failureTimer,
+                                    'const failureTimer = setTimeout(assert_unreached, 100);\n' +
+                                    'add_completion_callback(() => clearTimeout(failureTimer));');
+});
+
 runner.runJsTests();