fix(app-router): router.prefetch throws on invalid URL

packages/vinext/src/shims/navigation.ts

@@ -1511,6 +1511,17 @@ const _appRouter = {
   prefetch(href: string, options?: PrefetchOptions): void {
     assertSafeNavigationUrl(href);
     if (isServer) return;
+    // Validate the URL is parseable. Mirrors Next.js's createPrefetchURL:
+    // `packages/next/src/client/components/app-router-utils.ts` — when the URL
+    // cannot be converted, Next.js throws so the call site (and its surrounding
+    // error boundary, in the App Router) surfaces the failure. Without this
+    // guard, vinext silently swallows unparseable hrefs and the test app's
+    // error boundary never renders.
+    try {
+      new URL(href, window.location.href);
+    } catch {
+      throw new Error(`Cannot prefetch '${href}' because it cannot be converted to a URL.`);
+    }
     void (async () => {
       // Normalize same-origin absolute URLs to local paths; no-op for external
       // origins so we don't pollute the prefetch cache with a same-path .rsc on

tests/router-prefetch-invalid-url.test.ts

@@ -0,0 +1,83 @@
+/**
+ * App Router router.prefetch invalid-URL parity.
+ *
+ * Ported from Next.js: test/e2e/app-dir/app-prefetch/prefetching.test.ts
+ * https://github.com/vercel/next.js/blob/canary/test/e2e/app-dir/app-prefetch/prefetching.test.ts
+ *
+ * Next.js validates the href passed to `router.prefetch` via
+ * `createPrefetchURL` and throws an Error when the value cannot be parsed
+ * into a URL relative to `window.location.href`. The thrown error is
+ * surfaced by the surrounding error boundary in the test app, and the
+ * upstream test asserts the rendered `<h1>` reads "A prefetch threw an
+ * error". The canonical message thrown by Next.js is:
+ *
+ *   Cannot prefetch '<href>' because it cannot be converted to a URL.
+ *
+ * See: packages/next/src/client/components/app-router-utils.ts:27-29.
+ *
+ * Vinext previously swallowed unparseable hrefs (caught inside the async
+ * prefetch IIFE and reported as `console.error("[vinext] RSC prefetch setup
+ * error:", error)`), so the upstream test failed. This file pins the parity
+ * fix: an invalid URL throws synchronously from `router.prefetch`, while
+ * normal hrefs continue to work.
+ */
+import { describe, it, expect, beforeEach, afterEach, vi } from "vite-plus/test";
+
+type Navigation = typeof import("../packages/vinext/src/shims/navigation.js");
+let appRouterInstance: Navigation["appRouterInstance"];
+
+beforeEach(async () => {
+  // Set window BEFORE importing so isServer evaluates to false in the shim.
+  // INVALID_URL = "///" needs `new URL("///", window.location.href)` to throw,
+  // which it does for both http:// and https:// base URLs in modern runtimes.
+  (globalThis as any).window = {
+    __VINEXT_RSC_PREFETCH_CACHE__: new Map(),
+    __VINEXT_RSC_PREFETCHED_URLS__: new Set(),
+    location: {
+      origin: "http://localhost",
+      pathname: "/",
+      search: "",
+      hash: "",
+      href: "http://localhost/",
+    },
+    addEventListener: () => {},
+    history: { pushState: () => {}, replaceState: () => {}, state: null },
+    dispatchEvent: () => {},
+  };
+  vi.resetModules();
+  const nav = await import("../packages/vinext/src/shims/navigation.js");
+  appRouterInstance = nav.appRouterInstance;
+});
+
+afterEach(() => {
+  vi.restoreAllMocks();
+  delete (globalThis as any).window;
+  delete (globalThis as any).fetch;
+});
+
+describe("App Router router.prefetch with an invalid URL", () => {
+  // Mirrors Next.js's INVALID_URL fixture:
+  // .nextjs-ref/test/e2e/app-dir/app-prefetch/app/invalid-url/invalid-url.js
+  const INVALID_URL = "///";
+
+  it("throws the canonical error message", () => {
+    expect(() => appRouterInstance.prefetch(INVALID_URL)).toThrowError(
+      `Cannot prefetch '${INVALID_URL}' because it cannot be converted to a URL.`,
+    );
+  });
+
+  it("does not throw on a normal pathname", () => {
+    // Stub fetch so the async prefetch IIFE doesn't try to hit the network.
+    (globalThis as any).fetch = vi
+      .fn()
+      .mockResolvedValue(new Response(new ArrayBuffer(0), { status: 200 }));
+    expect(() => appRouterInstance.prefetch("/safe")).not.toThrow();
+  });
+
+  it("does not throw on an absolute http URL", () => {
+    (globalThis as any).fetch = vi
+      .fn()
+      .mockResolvedValue(new Response(new ArrayBuffer(0), { status: 200 }));
+    expect(() => appRouterInstance.prefetch("http://localhost/safe")).not.toThrow();
+  });
+});