fix(nuxt): disable x-nuxt-no-ssr header by default (#20024)

packages/nuxt/src/core/nitro.ts

@@ -189,6 +189,15 @@ export async function initNitro (nuxt: Nuxt & { _nitro?: Nitro }) {
     }
   }
 
+  // Add backward-compatible middleware to respect `x-nuxt-no-ssr` header
+  if (nuxt.options.experimental.respectNoSSRHeader) {
+    nitroConfig.handlers = nitroConfig.handlers || []
+    nitroConfig.handlers.push({
+      handler: resolve(distDir, 'core/runtime/nitro/no-ssr'),
+      middleware: true
+    })
+  }
+
   // Register nuxt protection patterns
   nitroConfig.rollupConfig!.plugins = await nitroConfig.rollupConfig!.plugins || []
   nitroConfig.rollupConfig!.plugins = Array.isArray(nitroConfig.rollupConfig!.plugins) ? nitroConfig.rollupConfig!.plugins : [nitroConfig.rollupConfig!.plugins]

packages/nuxt/src/core/runtime/nitro/no-ssr.ts

@@ -0,0 +1,8 @@
+import { defineEventHandler, getRequestHeader } from 'h3'
+
+export default defineEventHandler((event) => {
+  if (getRequestHeader(event, 'x-nuxt-no-ssr')) {
+    event.context.nuxt = event.context.nuxt || {}
+    event.context.nuxt.noSSR = true
+  }
+})

packages/nuxt/src/core/runtime/nitro/renderer.ts

@@ -213,7 +213,7 @@ export default defineRenderHandler(async (event) => {
     runtimeConfig: useRuntimeConfig() as NuxtSSRContext['runtimeConfig'],
     noSSR:
       !!(process.env.NUXT_NO_SSR) ||
-      !!(event.node.req.headers['x-nuxt-no-ssr']) ||
+      event.context.nuxt?.noSSR ||
       routeOptions.ssr === false ||
       (process.env.prerender ? PRERENDER_NO_SSR_ROUTES.has(url) : false),
     error: !!ssrError,

packages/schema/src/config/experimental.ts

@@ -158,6 +158,9 @@ export default defineUntypedSchema({
      *
      * This can be disabled for most Nuxt sites to reduce the client-side bundle by ~0.5kb.
      */
-    polyfillVueUseHead: true
+    polyfillVueUseHead: true,
+
+    /** Allow disabling Nuxt SSR responses by setting the `x-nuxt-no-ssr` header. */
+    respectNoSSRHeader: false,
   }
 })

test/fixtures/basic/nuxt.config.ts

@@ -184,6 +184,7 @@ export default defineNuxtConfig({
     }
   },
   experimental: {
+    respectNoSSRHeader: true,
     clientFallback: true,
     restoreState: true,
     inlineSSRStyles: id => !!id && !id.includes('assets.vue'),