fix(nextjs): Update proxy template wrapping (#18086)

This fixes an issue when we are wrapping Next.js 16+ proxy.ts files, the
webpack loader was exporting both middleware and proxy named exports,
causing Next.js to potentially treat proxy files incorrectly as
middleware files.

closes #18001

Author: chargome

packages/nextjs/src/config/templates/middlewareWrapperTemplate.ts

@@ -26,13 +26,17 @@ const userApiModule = origModule as NextApiModule;
 // the case Next.js wil crash during runtime but the Sentry SDK should definitely not crash so we need tohandle it.
 let userProvidedNamedHandler: EdgeRouteHandler | undefined = undefined;
 let userProvidedDefaultHandler: EdgeRouteHandler | undefined = undefined;
+let userProvidedMiddleware = false;
+let userProvidedProxy = false;
 
 if ('middleware' in userApiModule && typeof userApiModule.middleware === 'function') {
   // Handle when user defines via named ESM export: `export { middleware };`
   userProvidedNamedHandler = userApiModule.middleware;
+  userProvidedMiddleware = true;
 } else if ('proxy' in userApiModule && typeof userApiModule.proxy === 'function') {
   // Handle when user defines via named ESM export (Next.js 16): `export { proxy };`
   userProvidedNamedHandler = userApiModule.proxy;
+  userProvidedProxy = true;
 } else if ('default' in userApiModule && typeof userApiModule.default === 'function') {
   // Handle when user defines via ESM export: `export default myFunction;`
   userProvidedDefaultHandler = userApiModule.default;
@@ -41,10 +45,14 @@ if ('middleware' in userApiModule && typeof userApiModule.middleware === 'functi
   userProvidedDefaultHandler = userApiModule;
 }
 
-export const middleware = userProvidedNamedHandler
-  ? Sentry.wrapMiddlewareWithSentry(userProvidedNamedHandler)
-  : undefined;
-export const proxy = userProvidedNamedHandler ? Sentry.wrapMiddlewareWithSentry(userProvidedNamedHandler) : undefined;
+// Wrap the handler that the user provided (middleware, proxy, or default)
+// We preserve the original export names so Next.js can handle its internal renaming logic
+const wrappedHandler = userProvidedNamedHandler ? Sentry.wrapMiddlewareWithSentry(userProvidedNamedHandler) : undefined;
+
+// Only export the named export that the user actually provided
+// This ensures Next.js sees the same export structure and can apply its renaming logic
+export const middleware = userProvidedMiddleware ? wrappedHandler : undefined;
+export const proxy = userProvidedProxy ? wrappedHandler : undefined;
 export default userProvidedDefaultHandler ? Sentry.wrapMiddlewareWithSentry(userProvidedDefaultHandler) : undefined;
 
 // Re-export anything exported by the page module we're wrapping. When processing this code, Rollup is smart enough to

packages/nextjs/test/config/wrappingLoader.test.ts

@@ -102,4 +102,151 @@ describe('wrappingLoader', () => {
 
     expect(callback).toHaveBeenCalledWith(null, expect.stringContaining("'/my/route'"), expect.anything());
   });
+
+  describe('middleware wrapping', () => {
+    it('should export proxy when user exports named "proxy" export', async () => {
+      const callback = vi.fn();
+
+      const userCode = `
+        export function proxy(request) {
+          return new Response('ok');
+        }
+      `;
+      const userCodeSourceMap = undefined;
+
+      const loaderPromise = new Promise<void>(resolve => {
+        const loaderThis = {
+          ...defaultLoaderThis,
+          resourcePath: '/my/src/proxy.ts',
+          callback: callback.mockImplementation(() => {
+            resolve();
+          }),
+          getOptions() {
+            return {
+              pagesDir: '/my/pages',
+              appDir: '/my/app',
+              pageExtensionRegex: DEFAULT_PAGE_EXTENSION_REGEX,
+              excludeServerRoutes: [],
+              wrappingTargetKind: 'middleware',
+              vercelCronsConfig: undefined,
+              nextjsRequestAsyncStorageModulePath: '/my/request-async-storage.js',
+            };
+          },
+        } satisfies LoaderThis<WrappingLoaderOptions>;
+
+        wrappingLoader.call(loaderThis, userCode, userCodeSourceMap);
+      });
+
+      await loaderPromise;
+
+      const wrappedCode = callback.mock.calls[0][1];
+
+      // Verify both exports are present in export statement (Rollup bundles this way)
+      expect(wrappedCode).toMatch(/export \{[^}]*\bmiddleware\b[^}]*\bproxy\b[^}]*\}/);
+
+      // Should detect proxy export
+      expect(wrappedCode).toContain('userProvidedProxy = true');
+
+      // Proxy should be wrapped, middleware should be undefined
+      expect(wrappedCode).toMatch(/const proxy = userProvidedProxy \? wrappedHandler : undefined/);
+      expect(wrappedCode).toMatch(/const middleware = userProvidedMiddleware \? wrappedHandler : undefined/);
+    });
+
+    it('should export middleware when user exports named "middleware" export', async () => {
+      const callback = vi.fn();
+
+      const userCode = `
+        export function middleware(request) {
+          return new Response('ok');
+        }
+      `;
+      const userCodeSourceMap = undefined;
+
+      const loaderPromise = new Promise<void>(resolve => {
+        const loaderThis = {
+          ...defaultLoaderThis,
+          resourcePath: '/my/src/middleware.ts',
+          callback: callback.mockImplementation(() => {
+            resolve();
+          }),
+          getOptions() {
+            return {
+              pagesDir: '/my/pages',
+              appDir: '/my/app',
+              pageExtensionRegex: DEFAULT_PAGE_EXTENSION_REGEX,
+              excludeServerRoutes: [],
+              wrappingTargetKind: 'middleware',
+              vercelCronsConfig: undefined,
+              nextjsRequestAsyncStorageModulePath: '/my/request-async-storage.js',
+            };
+          },
+        } satisfies LoaderThis<WrappingLoaderOptions>;
+
+        wrappingLoader.call(loaderThis, userCode, userCodeSourceMap);
+      });
+
+      await loaderPromise;
+
+      const wrappedCode = callback.mock.calls[0][1];
+
+      // Should detect middleware export
+      expect(wrappedCode).toContain('userProvidedMiddleware = true');
+
+      // Should NOT detect proxy export
+      expect(wrappedCode).toContain('userProvidedProxy = false');
+
+      // Middleware should be wrapped, proxy should be undefined
+      expect(wrappedCode).toMatch(/const middleware = userProvidedMiddleware \? wrappedHandler : undefined/);
+      expect(wrappedCode).toMatch(/const proxy = userProvidedProxy \? wrappedHandler : undefined/);
+    });
+
+    it('should export undefined middleware/proxy when user only exports default', async () => {
+      const callback = vi.fn();
+
+      const userCode = `
+        export default function(request) {
+          return new Response('ok');
+        }
+      `;
+      const userCodeSourceMap = undefined;
+
+      const loaderPromise = new Promise<void>(resolve => {
+        const loaderThis = {
+          ...defaultLoaderThis,
+          resourcePath: '/my/src/middleware.ts',
+          callback: callback.mockImplementation(() => {
+            resolve();
+          }),
+          getOptions() {
+            return {
+              pagesDir: '/my/pages',
+              appDir: '/my/app',
+              pageExtensionRegex: DEFAULT_PAGE_EXTENSION_REGEX,
+              excludeServerRoutes: [],
+              wrappingTargetKind: 'middleware',
+              vercelCronsConfig: undefined,
+              nextjsRequestAsyncStorageModulePath: '/my/request-async-storage.js',
+            };
+          },
+        } satisfies LoaderThis<WrappingLoaderOptions>;
+
+        wrappingLoader.call(loaderThis, userCode, userCodeSourceMap);
+      });
+
+      await loaderPromise;
+
+      const wrappedCode = callback.mock.calls[0][1];
+
+      // Should export default
+      expect(wrappedCode).toMatch(/export \{[^}]* as default[^}]*\}/);
+
+      // Both flags should be false (no named exports provided by user)
+      expect(wrappedCode).toContain('userProvidedMiddleware = false');
+      expect(wrappedCode).toContain('userProvidedProxy = false');
+
+      // Both middleware and proxy should be undefined (conditionals evaluate to false)
+      expect(wrappedCode).toMatch(/const middleware = userProvidedMiddleware \? wrappedHandler : undefined/);
+      expect(wrappedCode).toMatch(/const proxy = userProvidedProxy \? wrappedHandler : undefined/);
+    });
+  });
 });