fix: don't double-prefix base when checking for prerendered paths in server fetch

.changeset/fetch-prerendered-base.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: correctly detect prerendered paths in server `fetch` when `paths.base` is set

packages/kit/src/runtime/server/fetch.js

@@ -117,7 +117,7 @@ export function create_fetch({ event, options, manifest, state, get_cookie_heade
 					return await fetch(request);
 				}
 
-				if (has_prerendered_path(manifest, paths.base + decoded)) {
+				if (has_prerendered_path(manifest, decoded)) {
 					// The path of something prerendered could match a different route
 					// that is still in the manifest, leading to the wrong route being loaded.
 					// We therefore bail early here. The prerendered logic is different for

packages/kit/test/apps/options-2/src/routes/fetch-prerendered/+page.server.js

@@ -0,0 +1,10 @@
+/** @type {import('./$types').PageServerLoad} */
+export async function load({ fetch }) {
+	const response = await fetch('/basepath/fetch-prerendered/shadowed/static');
+	const text = await response.text();
+
+	return {
+		status: response.status,
+		found: text.includes('this page was prerendered')
+	};
+}

packages/kit/test/apps/options-2/src/routes/fetch-prerendered/+page.svelte

@@ -0,0 +1,6 @@
+<script>
+	/** @type {import('./$types').PageProps} */
+	let { data } = $props();
+</script>
+
+<h1>{data.status} {data.found}</h1>

packages/kit/test/apps/options-2/src/routes/fetch-prerendered/shadowed/[...rest]/+page.svelte

@@ -0,0 +1 @@
+<h1>catch-all</h1>

packages/kit/test/apps/options-2/src/routes/fetch-prerendered/shadowed/static/+page.js

@@ -0,0 +1 @@
+export const prerender = true;

packages/kit/test/apps/options-2/src/routes/fetch-prerendered/shadowed/static/+page.svelte

@@ -0,0 +1 @@
+<h1>this page was prerendered</h1>

packages/kit/test/apps/options-2/test/test.js

@@ -61,6 +61,11 @@ test.describe('paths', () => {
 		expect(new URL(page.url()).pathname).toBe('/basepath/hello');
 	});
 
+	test('serves a prerendered page for a server-side fetch of its path', async ({ page }) => {
+		await page.goto('/basepath/fetch-prerendered');
+		expect(await page.textContent('h1')).toBe('200 true');
+	});
+
 	test('query remote function from client accounts for base path', async ({
 		page,
 		javaScriptEnabled