diff --git a/scripts/windows-concurrency-tests.mjs b/scripts/windows-concurrency-tests.mjs
index 7185a1028e..0c39d21277 100644
--- a/scripts/windows-concurrency-tests.mjs
+++ b/scripts/windows-concurrency-tests.mjs
@@ -18,7 +18,7 @@ export function readInventory(directory) {
       assert(Array.isArray(group.tests) && group.tests.length > 0, `${file}: empty tests`);
       const tests = required.get(group.package) ?? new Set();
       for (const test of group.tests) {
-        assert(typeof test === 'string' && /^Test[A-Za-z0-9_]+(?:\/[^\s]+)*$/.test(test), `${file}: invalid test`);
+        assert(typeof test === 'string' && /^Test[A-Za-z0-9_]+(?:\/[^\s]+)?$/.test(test), `${file}: invalid test`);
         tests.add(test);
       }
       required.set(group.package, tests);
diff --git a/scripts/windows-concurrency-tests.test.mjs b/scripts/windows-concurrency-tests.test.mjs
index e774a2d389..fab3f327db 100644
--- a/scripts/windows-concurrency-tests.test.mjs
+++ b/scripts/windows-concurrency-tests.test.mjs
@@ -2,9 +2,30 @@ import assert from 'node:assert/strict';
 import { mkdtempSync, rmSync, writeFileSync } from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
+import { spawnSync } from 'node:child_process';
 import test from 'node:test';
 import { readInventory, verifyInventory } from './windows-concurrency-tests.mjs';
 
+test('long invalid subtest suffix is rejected without repeated slash partitioning', () => {
+  const dir = mkdtempSync(join(tmpdir(), 'concurrency-inventory-'));
+  try {
+    const inventory = tests => JSON.stringify([{ package: 'reasonix/internal/example', tests }]);
+    const file = join(dir, 'names.json');
+    const valid = ['Test0', 'Test0/a/b', 'Test0/!/', 'Test0//'];
+    writeFileSync(file, inventory(valid));
+    assert.deepEqual([...readInventory(dir).values()][0], new Set(valid));
+    writeFileSync(file, inventory(['Test0', `Test0/${'!/'.repeat(10000)} `]));
+    const script = `import assert from 'node:assert/strict';
+      import { readInventory } from ${JSON.stringify(new URL('./windows-concurrency-tests.mjs', import.meta.url).href)};
+      assert.throws(() => readInventory(process.argv[1]), /invalid test/);`;
+    const result = spawnSync(process.execPath, ['--input-type=module', '-e', script, dir], { timeout: 5000, encoding: 'utf8' });
+    assert.ifError(result.error);
+    assert.equal(result.status, 0, result.stderr);
+  } finally {
+    rmSync(dir, { recursive: true, force: true });
+  }
+});
+
 test('additive manifests keep package-qualified requirements', () => {
   const dir = mkdtempSync(join(tmpdir(), 'concurrency-inventory-'));
   try {