fix: don't override user-agent in fetch if specified

packages/next/server/web/sandbox/context.ts

@@ -154,7 +154,10 @@ async function createModuleContext(options: {
     const prevs = init.headers.get(`x-middleware-subrequest`)?.split(':') || []
     const value = prevs.concat(options.module).join(':')
     init.headers.set('x-middleware-subrequest', value)
-    init.headers.set(`user-agent`, `Next.js Middleware`)
+
+    if (!init.headers.has('user-agent')) {
+      init.headers.set(`user-agent`, `Next.js Middleware`)
+    }
 
     if (typeof input === 'object' && 'url' in input) {
       return fetch(input.url, {

test/integration/middleware/core/pages/api/headers.js

@@ -0,0 +1,3 @@
+export default function handler(req, res) {
+  res.json({ url: req.url, headers: req.headers })
+}

test/integration/middleware/core/pages/interface/_middleware.js

@@ -31,6 +31,52 @@ export async function middleware(request) {
     }
   }
 
+  if (url.pathname.includes('/fetchUserAgentDefault')) {
+    try {
+      const apiRoute = new URL(url)
+      apiRoute.pathname = '/api/headers'
+      const res = await fetch(apiRoute)
+      return new Response(await res.text(), {
+        status: 200,
+        headers: {
+          'content-type': 'application/json',
+        },
+      })
+    } catch (err) {
+      return new Response(JSON.stringify({ error: err.message }), {
+        status: 500,
+        headers: {
+          'content-type': 'application/json',
+        },
+      })
+    }
+  }
+
+  if (url.pathname.includes('/fetchUserAgentCustom')) {
+    try {
+      const apiRoute = new URL(url)
+      apiRoute.pathname = '/api/headers'
+      const res = await fetch(apiRoute, {
+        headers: {
+          'user-agent': 'custom-agent',
+        },
+      })
+      return new Response(await res.text(), {
+        status: 200,
+        headers: {
+          'content-type': 'application/json',
+        },
+      })
+    } catch (err) {
+      return new Response(JSON.stringify({ error: err.message }), {
+        status: 500,
+        headers: {
+          'content-type': 'application/json',
+        },
+      })
+    }
+  }
+
   if (url.pathname.endsWith('/webcrypto')) {
     const response = {}
     try {

test/integration/middleware/core/test/index.test.js

@@ -139,6 +139,20 @@ describe('Middleware base tests', () => {
 })
 
 function urlTests(_log, locale = '') {
+  it('should set fetch user agent correctly', async () => {
+    const res = await fetchViaHTTP(
+      context.appPort,
+      `${locale}/interface/fetchUserAgentDefault`
+    )
+    expect((await res.json()).headers['user-agent']).toBe('Next.js Middleware')
+
+    const res2 = await fetchViaHTTP(
+      context.appPort,
+      `${locale}/interface/fetchUserAgentCustom`
+    )
+    expect((await res2.json()).headers['user-agent']).toBe('custom-agent')
+  })
+
   it('rewrites by default to a target location', async () => {
     const res = await fetchViaHTTP(context.appPort, `${locale}/urls`)
     const html = await res.text()