How to Fix: `fetch` having a different behavior on Vercel with input request

Why does fetch(request) work locally but behave differently on Vercel? In this bug, the problem is not the HTTP method itself. The real issue is how a Request object is forwarded inside a serverless runtime. On local development, the runtime can appear more forgiving, but on Vercel the request body stream, method semantics, headers, and runtime-specific fetch handling can expose incorrect assumptions immediately.

Understanding the Root Cause

This issue typically happens when code forwards an incoming Web Request directly into fetch(), such as fetch(url, request) or fetch(new Request(...)), expecting identical behavior across local dev and Vercel deployment.

That assumption breaks for a few important reasons:

• Request bodies are streams. A request body can usually be consumed only once. If the body has already been read, logged, cloned incorrectly, or transformed by middleware, the forwarded request may fail or behave inconsistently.
• GET and HEAD requests should not carry a body. Some local environments tolerate this. Production runtimes and platform proxies often do not.
• Headers from the original request may be invalid to forward directly. Headers like host, content-length, or certain platform-managed values can cause mismatches when replayed upstream.
• Node.js dev runtime and Vercel runtime are not identical. Even when both support the Fetch API, implementation details differ, especially around streaming, cloning, compression, and body parsing.
• Framework request wrappers are not always safe fetch init objects. In frameworks like Next.js, the inbound request object is not meant to be passed blindly as the second argument to fetch(). You should construct a clean fetch options object instead.

In short, the root cause is usually reusing the inbound request object as outbound fetch configuration. That can work accidentally in development and then fail on Vercel, where the runtime is stricter.

Step-by-Step Solution

The reliable fix is to normalize the outgoing request instead of forwarding the original input request object directly.

1. Extract only the parts you actually need

Read the method, selected headers, query params, and body explicitly. Do not pass the incoming request object as-is.

export async function GET(req) {
  const url = new URL(req.url);
  const method = url.searchParams.get('method') || 'GET';

  const response = await fetch('https://example.com/api', {
    method,
    headers: {
      'accept': 'application/json'
    }
  });

  const data = await response.text();
  return new Response(data, { status: response.status });
}

2. Only attach a body for methods that support it

If you proxy POST, PUT, PATCH, or DELETE, read the body once and forward that value. For GET and HEAD, never send a body.

export async function POST(req) {
  const bodyText = await req.text();

  const response = await fetch('https://example.com/api', {
    method: 'POST',
    headers: {
      'content-type': req.headers.get('content-type') || 'application/json',
      'accept': 'application/json'
    },
    body: bodyText
  });

  return new Response(await response.text(), {
    status: response.status,
    headers: {
      'content-type': response.headers.get('content-type') || 'text/plain'
    }
  });
}

3. If one route handles multiple methods, branch explicitly

This is often the safest pattern for reproductions like /api?method=get or /api?method=post.

export async function GET(req) {
  const url = new URL(req.url);
  const simulatedMethod = (url.searchParams.get('method') || 'GET').toUpperCase();

  const init = {
    method: simulatedMethod,
    headers: {
      'accept': 'application/json'
    }
  };

  if (simulatedMethod !== 'GET' && simulatedMethod !== 'HEAD') {
    init.headers['content-type'] = 'application/json';
    init.body = JSON.stringify({ hello: 'world' });
  }

  const upstream = await fetch('https://httpbin.org/anything', init);
  const text = await upstream.text();

  return new Response(text, {
    status: upstream.status,
    headers: {
      'content-type': upstream.headers.get('content-type') || 'application/json'
    }
  });
}

4. Strip problematic headers when proxying

If your endpoint behaves like a proxy, forward only safe headers. Do not blindly pass every inbound header upstream.

function buildForwardHeaders(req) {
  const headers = new Headers();
  const contentType = req.headers.get('content-type');
  const authorization = req.headers.get('authorization');
  const accept = req.headers.get('accept');

  if (contentType) headers.set('content-type', contentType);
  if (authorization) headers.set('authorization', authorization);
  if (accept) headers.set('accept', accept);

  return headers;
}

export async function POST(req) {
  const body = await req.text();

  const upstream = await fetch('https://example.com/api', {
    method: 'POST',
    headers: buildForwardHeaders(req),
    body
  });

  return new Response(await upstream.text(), { status: upstream.status });
}

5. Avoid reading the request body more than once

This is one of the most common hidden causes. The following pattern is fragile:

const body = await req.text();
console.log(body);

await fetch('https://example.com/api', {
  method: req.method,
  body: req.body
});

After await req.text(), the original stream is already consumed. Forward the saved text instead:

const body = await req.text();
console.log(body);

await fetch('https://example.com/api', {
  method: req.method,
  headers: {
    'content-type': req.headers.get('content-type') || 'text/plain'
  },
  body
});

6. Prefer explicit route handlers over request simulation via query strings

If you are using a single endpoint to simulate different HTTP methods with ?method=get, remember that the actual inbound request may still be a GET. That can create confusing body and cache behavior. A clearer design is to test real HTTP methods directly.

export async function GET() {
  return Response.json({ ok: true, method: 'GET' });
}

export async function POST(req) {
  const json = await req.json();
  return Response.json({ ok: true, method: 'POST', json });
}

7. If needed, force consistent runtime behavior

If your framework supports multiple runtimes, verify whether the route is running in Node.js or the Edge runtime. Streaming and request handling can differ.

export const runtime = 'nodejs';

Use that only when it matches your deployment needs. The main fix is still proper request reconstruction.

Common Edge Cases

• Forwarding req.body after calling req.json() or req.text(): the stream has already been consumed.
• Sending a body with GET or HEAD: some environments ignore it, others reject it.
• Incorrect content-length: manually forwarding this header can break requests when the body changes.
• Compressed or transformed payloads: middleware, proxies, or platform layers may alter transfer behavior.
• Using framework-specific request objects as fetch init: always create a plain object for fetch().
• Accidentally cached fetch responses: depending on framework defaults, a request may appear inconsistent because caching is involved rather than method handling.
• Differences between local dev server and deployed serverless execution: local tooling can mask invalid request forwarding patterns.

If the endpoint is meant to act like a proxy, the safest mental model is: read inbound request, validate it, rebuild outbound request, return upstream response.

FAQ

Why does it work locally but fail only on Vercel?

Local development often runs through a different server implementation with looser behavior. Vercel executes in a production runtime where stream consumption, header validation, and HTTP semantics are enforced more strictly.

Can I pass the original request object directly to fetch()?

You should avoid that for proxy-style handlers. While parts of the Fetch API allow constructing a new request from an old one, framework request wrappers and already-consumed streams make this unreliable. Build a new fetch init object explicitly.

What is the safest way to proxy a request body?

Read the body once using await req.text(), await req.json(), or await req.arrayBuffer() depending on payload type, then send that exact serialized value in the outbound fetch() call with a matching content-type.

Bottom line: the fix for this GitHub issue is to stop forwarding the incoming request object blindly. Reconstruct the outgoing request with an explicit method, curated headers, and a body only when the method supports one. That makes behavior consistent across local development and Vercel deployment.