Astro on Vercel: Cannot use server-rendered pages without an adapter

The error

The build stops with NoAdapterInstalled: Cannot use server-rendered pages without an adapter. Please install and configure the appropriate server adapter for your final deployment. Astro's docs put the rule plainly: to render any page on demand, you need to add an adapter.

When an Astro site renders on demand

By default, your entire Astro site is prerendered at build time, and a static Astro site deploys to Vercel with zero configuration. Two settings change that. output: server in astro.config renders every page on demand by default. export const prerender = false in a page or an endpoint renders that one route on demand. Either one needs an adapter.

// astro.config.mjs
import { defineConfig } from "astro/config";

export default defineConfig({
  output: "server",
  // no adapter: astro build stops with NoAdapterInstalled
});

The fix: add Astro's Vercel adapter

Run npx astro add vercel. It installs @astrojs/vercel and adds the adapter to astro.config. To do it by hand, install @astrojs/vercel and set adapter: vercel() yourself.

// astro.config.mjs
import { defineConfig } from "astro/config";
import vercel from "@astrojs/vercel";

export default defineConfig({
  output: "server",
  adapter: vercel(),
});

Another platform's adapter on Vercel

A project that started on another host can still have @astrojs/node, @astrojs/netlify or @astrojs/cloudflare in astro.config. The build then passes, but the output is shaped for that platform. Vercel's docs for Astro say that to deploy a server-rendered Astro app, you must add Astro's Vercel adapter. Replace the old adapter with @astrojs/vercel and remove it from package.json.

A static site needs no adapter

If nothing renders on demand, you do not need an adapter to deploy to Vercel. Astro's Vercel adapter docs say you only need it for a static site if you use additional Vercel services, such as Web Analytics or Image Optimization through the adapter. Removing output: server and every prerender = false is also a valid fix.

How DeployDoctor catches it

DeployDoctor reads astro.config and the JavaScript and TypeScript files under src/pages in the Build configuration check. When output is server, or hybrid on Astro 4, or a route sets prerender to false, and astro.config has no adapter, it reports a failing check that points at the line. When the adapter is @astrojs/node, @astrojs/netlify or @astrojs/cloudflare, it reports a warning with the switch to @astrojs/vercel. The same check covers SvelteKit adapters built for another platform. Scan your repository for free on the home page, or run it on every pull request with the GitHub Action.