perf(runtime): deduplicate concurrent client manifest loads

packages/void/src/runtime/routing-assets.ts

@@ -54,6 +54,7 @@ export type ManifestLoader = (
 ) => Promise<ClientManifest | null>;
 export function createManifestLoader(): ManifestLoader {
   let cached: ClientManifest | null = null;
+  let pending: Promise<ClientManifest | null> | null = null;
   return async (request, env) => {
     if (cached) {
       return cached;
@@ -68,14 +69,21 @@ export function createManifestLoader(): ManifestLoader {
     if (!assets || typeof assets.fetch !== 'function') {
       return null;
     }
-    const response = await assets.fetch(
-      new Request(new URL('/.vite/manifest.json', request.url).toString()),
-    );
-    if (!response.ok) {
-      return null;
-    }
-    cached = (await response.json()) as ClientManifest;
-    return cached;
+    // Concurrent cold requests share one fetch and parse. A missing or failed
+    // manifest is not cached, so a later request retries.
+    pending ??= (async () => {
+      const response = await assets.fetch(
+        new Request(new URL('/.vite/manifest.json', request.url).toString()),
+      );
+      if (!response.ok) {
+        return null;
+      }
+      cached = (await response.json()) as ClientManifest;
+      return cached;
+    })().finally(() => {
+      pending = null;
+    });
+    return pending;
   };
 }
 export type ClientAssetOptions = {

packages/void/test/unit/routing-runtime.test.ts

@@ -638,6 +638,42 @@ describe('typed client asset tags', () => {
     expect(islandTags.preloads).not.toContain('/md.js');
     expect(binding).toHaveBeenCalledTimes(1);
   });
+
+  it('shares one manifest fetch between concurrent cold loads', async () => {
+    const binding = vi.fn(async () => Response.json({ page: { file: 'page.js' } }));
+    const loader = createManifestLoader();
+    const env = { ASSETS: { fetch: binding } };
+    const request = new Request('https://app.test/page');
+    const manifests = await Promise.all(Array.from({ length: 8 }, () => loader(request, env)));
+    expect(binding).toHaveBeenCalledTimes(1);
+    expect(new Set(manifests).size).toBe(1);
+    expect(manifests[0]).toEqual({ page: { file: 'page.js' } });
+    expect(await loader(request, env)).toBe(manifests[0]);
+    expect(binding).toHaveBeenCalledTimes(1);
+  });
+
+  it('retries the manifest after a missing response or a rejected fetch', async () => {
+    const manifest = { page: { file: 'page.js' } };
+    const binding = vi
+      .fn<() => Promise<Response>>()
+      .mockResolvedValueOnce(new Response('missing', { status: 404 }))
+      .mockRejectedValueOnce(new Error('assets unavailable'))
+      .mockImplementation(async () => Response.json(manifest));
+    const loader = createManifestLoader();
+    const env = { ASSETS: { fetch: binding } };
+    const request = new Request('https://app.test/page');
+    expect(await loader(request, env)).toBeNull();
+    expect(binding).toHaveBeenCalledTimes(1);
+    const rejected = [loader(request, env), loader(request, env)];
+    await expect(rejected[0]).rejects.toThrow('assets unavailable');
+    await expect(rejected[1]).rejects.toThrow('assets unavailable');
+    expect(binding).toHaveBeenCalledTimes(2);
+    expect(await loader(request, env)).toEqual(manifest);
+    expect(binding).toHaveBeenCalledTimes(3);
+    await loader(request, env);
+    expect(binding).toHaveBeenCalledTimes(3);
+  });
+
   it('loads development CSS with active roots and omits client tags when production has no manifest', async () => {
     const devCss = vi.fn(
       async (