fix: resolve fetch issue with same host URLs differing from paths.base

Author: goulinkh

.changeset/honest-cobras-jog.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': major
+---
+
+fix: `fetch` not working when URL is same host but different than `paths.base`

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

@@ -285,10 +285,9 @@ export async function internal_respond(request, options, manifest, state) {
 	let route = null;
 
 	if (base && !state.prerendering?.fallback) {
-		if (!resolved_path.startsWith(base)) {
-			return text('Not found', { status: 404 });
+		if (resolved_path.startsWith(base)) {
+			resolved_path = resolved_path.slice(base.length) || '/';
 		}
-		resolved_path = resolved_path.slice(base.length) || '/';
 	}
 
 	if (is_route_resolution_request) {

packages/kit/test/apps/options/source/pages/routing/link-outside-base/+page.server.js

@@ -0,0 +1,6 @@
+export async function load({ fetch }) {
+	const response = await fetch('/not-base-path/');
+	return {
+		responseContentType: response.headers.get('content-type')
+	};
+}

packages/kit/test/apps/options/source/pages/routing/link-outside-base/+page.svelte

@@ -0,0 +1,6 @@
+<script>
+	/** @type {{ data: { responseContentType: string } }} */
+	const { data } = $props();
+</script>
+
+<p>{data.responseContentType}</p>

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

@@ -236,6 +236,12 @@ test.describe('Routing', () => {
 		await page.click('[href="/path-base/routing/link-outside-app-target/target/"]');
 		await expect(page.locator('h2')).toHaveText('target: 0');
 	});
+
+	test('fetch outside base path succeeds', async ({ request }) => {
+		const response = await request.get('/path-base/routing/link-outside-base/');
+		expect(response.status()).toBe(200);
+		expect(await response.text()).toContain('text/plain');
+	});
 });
 
 test.describe('Async', () => {