withastro · matthewp · Aug 14, 2026 · Aug 13, 2026 · Aug 13, 2026
diff --git a/.changeset/curly-donkeys-say.md b/.changeset/curly-donkeys-say.md
@@ -0,0 +1,5 @@
+---
+'@astrojs/vercel': patch
+---
+
+Fixes server islands returning 404 responses in Vercel deployments using `output: "static"`
diff --git a/packages/integrations/vercel/src/index.ts b/packages/integrations/vercel/src/index.ts
@@ -127,6 +127,7 @@ function getAdapter({
 		adapterFeatures: {
 			buildOutput,
 			middlewareMode,
+			preserveBuildServerDir: true,
 			staticHeaders,
 		},
 		supportedAstroFeatures: {
@@ -266,6 +267,7 @@ export default function vercelAdapter({
 	let _buildTempFolder: URL;
 	let _serverEntry: string;
 	let _middlewareEntryPoint: URL | undefined;
+	let _hasServerBuild = false;
 	let _routeToHeaders: RouteToHeaders | undefined = undefined;
 	// Extra files to be merged with `includeFiles` during build
 	const extraFilesToInclude: URL[] = [];
@@ -295,6 +297,9 @@ export default function vercelAdapter({
 					build: {
 						format: 'directory',
 						redirects: false,
+						...(config.output === 'static'
+							? { server: new URL('./.vercel/output/server/', config.root) }
+							: {}),
 					},
 					integrations: [
 						{
@@ -393,10 +398,12 @@ export default function vercelAdapter({
 				_serverEntry = config.build.serverEntry;
 			},
 			'astro:build:start': async () => {
+				_hasServerBuild = false;
 				// Ensure to have `.vercel/output` empty.
 				await emptyDir(new URL('./.vercel/output/', _config.root));
 			},
 			'astro:build:ssr': async ({ middlewareEntryPoint }) => {
+				_hasServerBuild = true;
 				_middlewareEntryPoint = middlewareEntryPoint;
 			},
 
@@ -405,6 +412,7 @@ export default function vercelAdapter({
 			},
 			'astro:build:done': async ({ logger }: HookParameters<'astro:build:done'>) => {
 				const outDir = new URL('./.vercel/output/', _config.root);
+
 				if (staticDir) {
 					if (existsSync(staticDir)) {
 						await emptyDir(staticDir);
@@ -428,9 +436,9 @@ export default function vercelAdapter({
 					middlewarePath?: string;
 				}> = [];
 
-				if (_buildOutput === 'server') {
+				if (_hasServerBuild) {
 					// Merge any includes from `vite.assetsInclude
-					if (_config.vite.assetsInclude) {
+					if (_buildOutput === 'server' && _config.vite.assetsInclude) {
 						const mergeGlobbedIncludes = (globPattern: unknown) => {
 							if (typeof globPattern === 'string') {
 								const entries = globSync(globPattern).map((p) => pathToFileURL(p));
@@ -460,7 +468,7 @@ export default function vercelAdapter({
 					);
 
 					const entryFile = new URL(_serverEntry, _buildTempFolder);
-					if (isr) {
+					if (_buildOutput === 'server' && isr) {
 						const isrConfig = typeof isr === 'object' ? isr : {};
 						await builder.buildServerlessFolder(entryFile, NODE_PATH, _config.root);
 						if (isrConfig.exclude?.length) {
@@ -538,7 +546,7 @@ export default function vercelAdapter({
 						continue: true,
 					},
 				];
-				if (_buildOutput === 'server') {
+				if (_hasServerBuild) {
 					finalRoutes.push(...routeDefinitions);
 				}
 
@@ -638,7 +646,7 @@ export default function vercelAdapter({
 				});
 
 				// Remove temporary folder
-				if (_buildOutput === 'server') {
+				if (_hasServerBuild) {
 					await removeDir(_buildTempFolder);
 				}
 			},

diff --git a/packages/integrations/vercel/test/fixtures/server-islands-static/astro.config.mjs b/packages/integrations/vercel/test/fixtures/server-islands-static/astro.config.mjs
@@ -0,0 +1,7 @@
+import vercel from '@astrojs/vercel';
+import { defineConfig } from 'astro/config';
+
+export default defineConfig({
+	output: 'static',
+	adapter: vercel(),
+});
diff --git a/packages/integrations/vercel/test/fixtures/server-islands-static/package.json b/packages/integrations/vercel/test/fixtures/server-islands-static/package.json
@@ -0,0 +1,9 @@
+{
+	"name": "@test/vercel-server-islands-static",
+	"version": "0.0.0",
+	"private": true,
+	"dependencies": {
+		"@astrojs/vercel": "workspace:*",
+		"astro": "workspace:*"
+	}
+}
diff --git a/packages/integrations/vercel/test/fixtures/server-islands-static/src/components/Island.astro b/packages/integrations/vercel/test/fixtures/server-islands-static/src/components/Island.astro
@@ -0,0 +1 @@
+<h1>I'm an island</h1>
diff --git a/packages/integrations/vercel/test/fixtures/server-islands-static/src/pages/index.astro b/packages/integrations/vercel/test/fixtures/server-islands-static/src/pages/index.astro
@@ -0,0 +1,12 @@
+---
+import Island from '../components/Island.astro';
+---
+<html>
+	<head>
+		<title>One</title>
+	</head>
+	<body>
+		<h1>One</h1>
+		<Island server:defer />
+	</body>
+</html>
diff --git a/packages/integrations/vercel/test/server-islands.test.ts b/packages/integrations/vercel/test/server-islands.test.ts
@@ -1,4 +1,5 @@
 import assert from 'node:assert/strict';
+import { existsSync } from 'node:fs';
 import { before, describe, it } from 'node:test';
 import { type Fixture, loadFixture, getVercelConfig } from './test-utils.ts';
 
@@ -24,3 +25,65 @@ describe('Server Islands', () => {
 		assert.notEqual(found, null, 'Default server islands route included');
 	});
 });
+
+describe('Server Islands (static output)', () => {
+	let fixture: Fixture;
+	let renderFunction: { default: { fetch(request: Request): Promise<Response> } };
+
+	before(async () => {
+		fixture = await loadFixture({
+			root: './fixtures/server-islands-static/',
+		});
+		await fixture.build({});
+
+		const functionConfig = JSON.parse(
+			await fixture.readFile('../.vercel/output/functions/_render.func/.vc-config.json'),
+		);
+		const functionEntry = new URL(
+			`../.vercel/output/functions/_render.func/${functionConfig.handler}`,
+			fixture.config.outDir,
+		);
+		renderFunction = await import(functionEntry.href);
+	});
+
+	it('creates _render.func for server islands in static builds', { timeout: 30000 }, async () => {
+		const renderFuncDir = new URL('.vercel/output/functions/_render.func/', fixture.config.root);
+		assert.ok(existsSync(renderFuncDir), '_render.func directory should exist');
+		assert.ok(
+			existsSync(new URL('.vc-config.json', renderFuncDir)),
+			'.vc-config.json should exist in _render.func',
+		);
+	});
+
+	it('renders the server island', { timeout: 30000 }, async () => {
+		const html = await fixture.readFile('../.vercel/output/static/index.html');
+		const islandUrl = /fetch\((["'])(\/_server-islands\/[^"']+)\1/.exec(html)?.[2];
+		assert.ok(islandUrl, 'prerendered HTML should include the server island URL');
+
+		const response = await renderFunction.default.fetch(
+			new Request(new URL(islandUrl, 'https://example.com')),
+		);
+		assert.equal(response.status, 200);
+		assert.match(await response.text(), /I'm an island/);
+	});
+
+	it('does not publish the server entry as a static file', () => {
+		assert.equal(
+			existsSync(new URL('.vercel/output/static/entry.mjs', fixture.config.root)),
+			false,
+		);
+	});
+
+	it('includes server islands route in config', { timeout: 30000 }, async () => {
+		const config = await getVercelConfig(fixture);
+		let found = null;
+		for (const route of config.routes) {
+			if (route.src?.includes('_server-islands')) {
+				found = route;
+				break;
+			}
+		}
+		assert.ok(found, 'Server islands route should be in config');
+		assert.equal(found.dest, '_render', 'Server islands route should point to _render');
+	});
+});
diff --git a/packages/integrations/vercel/test/static.test.ts b/packages/integrations/vercel/test/static.test.ts
@@ -1,4 +1,5 @@
 import assert from 'node:assert/strict';
+import { existsSync } from 'node:fs';
 import { before, describe, it } from 'node:test';
 import { type Fixture, loadFixture } from './test-utils.ts';
 
@@ -21,4 +22,11 @@ describe('static routing', () => {
 			status: 404,
 		});
 	});
+
+	it('does not create a serverless function', () => {
+		assert.equal(
+			existsSync(new URL('.vercel/output/functions/_render.func/', fixture.config.root)),
+			false,
+		);
+	});
 });
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml