diff --git a/tests/helpers-jsx-legacy-register.test.mjs b/tests/helpers-jsx-legacy-register.test.mjs
new file mode 100644
index 00000000..18d32dca
--- /dev/null
+++ b/tests/helpers-jsx-legacy-register.test.mjs
@@ -0,0 +1,90 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import { registerHooks } from 'node:module';
+
+// `tests/helpers-jsx.mjs` picks one of two module-customization APIs:
+//
+//   registerHooks(await import(HOOKS_URL.href))   // Node >= 22.15
+//   register(HOOKS_URL)                           // everything older
+//
+// package.json pins `engines.node` to `22.x`, and `registerHooks` only landed
+// in 22.15, so the second arm is live code for a supported interpreter rather
+// than dead compatibility ballast. It is also unreachable from inside a test
+// process, which always runs a Node where `registerHooks` exists — so the arm
+// that the oldest supported Node would take was the one nothing checked. A
+// regression there (a typo in the `register` call, the wrong argument shape)
+// would be invisible in CI and would break `npm run test:unit` outright on
+// 22.0-22.14, because every test that imports a `src/**` React file depends on
+// those hooks being installed.
+//
+// The branch is driven by redirecting the helper's own `node:module` import at
+// a stub, using a `registerHooks` resolve hook — public API, no flags. The
+// redirect is keyed on the importing module, so only the cache-busted copy of
+// the helper loaded below sees the stub; `node:module` resolves normally
+// everywhere else in this process, including for the hook machinery itself.
+const STUB_URL = new URL('./tools/legacy-module-stub.mjs', import.meta.url)
+  .href;
+
+// A fresh query string defeats the module cache, which matters twice over: the
+// helper memoizes its registration in a module-scoped `registration`, and the
+// instance under test has to be the one resolved through the redirect.
+const HELPER_URL = new URL(
+  './helpers-jsx.mjs?legacy-register-fallback',
+  import.meta.url,
+).href;
+
+// `run` is awaited before the hook is removed. Module resolution inside a
+// dynamic `import()` happens in a later microtask, so deregistering on the
+// synchronous return of `run()` would tear the hook down before the helper's
+// `node:module` specifier is ever resolved -- the redirect would silently not
+// apply and the real builtin (with a working `registerHooks`) would load.
+async function withModuleStubbedFor(importerUrl, run) {
+  const hooks = registerHooks({
+    resolve(specifier, context, nextResolve) {
+      if (specifier === 'node:module' && context.parentURL === importerUrl) {
+        return { url: STUB_URL, shortCircuit: true };
+      }
+      return nextResolve(specifier, context);
+    },
+  });
+  try {
+    return await run();
+  } finally {
+    hooks.deregister();
+  }
+}
+
+test('importSource falls back to register() when registerHooks is absent', async () => {
+  const { calls, importSource } = await withModuleStubbedFor(
+    HELPER_URL,
+    async () => {
+      const helper = await import(HELPER_URL);
+      const stub = await import(STUB_URL);
+      return { calls: stub.registerCalls, importSource: helper.importSource };
+    },
+  );
+
+  assert.deepEqual(calls, [], 'the helper must not register at import time');
+
+  // `src/lib/profile-links.mjs` is plain ESM, so it imports successfully even
+  // though the stubbed `register` installed nothing. That keeps the assertion
+  // on the helper's own contract -- the fallback still resolves the requested
+  // path against the repository root and returns its namespace -- rather than
+  // on what the hooks would have done.
+  const mod = await importSource('src/lib/profile-links.mjs');
+  assert.equal(typeof mod.websiteUrl, 'function');
+
+  assert.equal(calls.length, 1, 'the legacy arm registers exactly once');
+  // Asserted as the exact `file:` URL, not a suffix: `register()` resolves a
+  // bare string against the caller, so handing it a plain filesystem path
+  // instead of a URL would not register anything.
+  assert.equal(
+    calls[0],
+    new URL('./tools/jsx-hooks.mjs', import.meta.url).href,
+  );
+
+  // The memoized `registration` is shared by the fallback arm too: a second
+  // call must reuse it rather than register the hooks again.
+  await importSource('src/lib/profile-links.mjs');
+  assert.equal(calls.length, 1, 'a second import reuses the registration');
+});
diff --git a/tests/tools/legacy-module-stub.mjs b/tests/tools/legacy-module-stub.mjs
new file mode 100644
index 00000000..dd828422
--- /dev/null
+++ b/tests/tools/legacy-module-stub.mjs
@@ -0,0 +1,16 @@
+// A stand-in for `node:module` as it looked before Node 22.15, used by
+// tests/helpers-jsx-legacy-register.test.mjs to reach the `register()` arm of
+// tests/helpers-jsx.mjs on an interpreter where `registerHooks` exists.
+//
+// `registerHooks` is exported as `undefined` rather than omitted so the import
+// in helpers-jsx.mjs still binds: a missing named export is a link-time error,
+// which would fail the import instead of exercising the fallback.
+
+export const registerHooks = undefined;
+
+/** URLs passed to `register`, in call order. */
+export const registerCalls = [];
+
+export function register(url) {
+  registerCalls.push(String(url));
+}