Accept-Encoding header missing when using rewrites() in next.config.js

[allysheng]

Summary

When using rewrites() in next.config.js to proxy API requests to an internal backend server, the Accept-Encoding header is lost before it reaches the destination. This prevents our backend from returning compressed responses (e.g. gzip or br).

I confirmed the request passes through Next.js middleware with Accept-Encoding still present, but by the time it hits the internal server, it's gone.

Is this expected?
What’s the correct way to preserve the original Accept-Encoding?

Additional information

// next.config.js

module.exports = {
  async rewrites() {
    return [
      {
        source: '/api/internal/:path*',
        destination: 'http://internal-api.mycompany.local/:path*',
      },
    ];
  },
};

// Client request:

fetch('/api/internal/some-endpoint');

Request is confirmed to go through Next.js middleware, and Accept-Encoding is visible at that point.
However, when it reaches http://internal-api.mycompany.local, Accept-Encoding is missing.

We rely on this header to return compressed responses, so we'd like to know how to preserve it correctly when using rewrites.

Next.js 15 with App Router and Middleware enabled.

Example

No response