Fix: The Middleware Returned Something That Is Not a Response in Astro

What Causes This Error?

This error occurs when your middleware returns a value that is not a valid Response object. Middleware must return either a Response object or the result of calling next().

The Problem

// src/middleware.ts
import { defineMiddleware } from 'astro:middleware';

export const onRequest = defineMiddleware((context, next) => {
  // ❌ Returning an object instead of Response
  return { data: 'hello' };
});

export const onRequest = defineMiddleware((context, next) => {
  // ❌ Returning a string
  return 'Hello World';
});

The Fix

Return a Response Object

import { defineMiddleware } from 'astro:middleware';

export const onRequest = defineMiddleware((context, next) => {
  // ✅ Proper Response
  return new Response(JSON.stringify({ data: 'hello' }), {
    headers: { 'Content-Type': 'application/json' },
  });
});

Or Return next()

import { defineMiddleware } from 'astro:middleware';

export const onRequest = defineMiddleware((context, next) => {
  // ✅ Continue to route handler
  return next();
});

Common Scenarios

JSON Response

import { defineMiddleware } from 'astro:middleware';

export const onRequest = defineMiddleware((context, next) => {
  if (context.url.pathname === '/api/health') {
    // ❌ Wrong - plain object
    // return { status: 'ok' };

    // ✅ Proper JSON Response
    return Response.json({ status: 'ok' });
  }
  return next();
});

Redirect

import { defineMiddleware } from 'astro:middleware';

export const onRequest = defineMiddleware((context, next) => {
  // ❌ Wrong - returning URL string
  // return '/login';

  // ✅ Use context.redirect
  if (needsAuth(context)) {
    return context.redirect('/login');
  }

  return next();
});

Error Response

import { defineMiddleware } from 'astro:middleware';

export const onRequest = defineMiddleware((context, next) => {
  // ❌ Wrong - throwing error object
  // return { error: 'Not found' };

  // ✅ Proper error Response
  return new Response('Not Found', {
    status: 404,
    statusText: 'Not Found',
  });
});

Async Middleware

import { defineMiddleware } from 'astro:middleware';

export const onRequest = defineMiddleware(async (context, next) => {
  const data = await fetchData();

  // ❌ Returning the data directly
  // return data;

  // ✅ Return Response or next()
  context.locals.data = data;
  return next();
});

Modifying and Returning Response

import { defineMiddleware } from 'astro:middleware';

export const onRequest = defineMiddleware(async (context, next) => {
  const response = await next();

  // ❌ Returning modified headers object
  // return response.headers;

  // ✅ Return the Response object
  response.headers.set('X-Custom', 'value');
  return response;
});

Forgot to Await next()

import { defineMiddleware } from 'astro:middleware';

export const onRequest = defineMiddleware(async (context, next) => {
  // ❌ Not awaiting - returns Promise
  // return next();

  // ✅ Await the response
  const response = await next();
  return response;

  // Or simpler:
  return await next();

  // Or just:
  return next(); // Also works as it returns a Promise<Response>
});

Creating Responses

// Various ways to create valid Response objects

// Text response
new Response('Hello World');

// JSON response
Response.json({ data: 'value' });

// With status
new Response('Created', { status: 201 });

// Redirect
Response.redirect('https://example.com', 302);

// From context
context.redirect('/path');

// Empty response
new Response(null, { status: 204 });

Type Safety

import { defineMiddleware } from 'astro:middleware';
import type { MiddlewareHandler } from 'astro';

// TypeScript will catch incorrect return types
export const onRequest: MiddlewareHandler = async (context, next) => {
  // TypeScript error if returning wrong type
  return next();
};

Quick Checklist

• Return Response object or next() result
• Use Response.json() for JSON responses
• Use context.redirect() for redirects
• Use new Response() for custom responses
• Don’t return plain objects, strings, or numbers