Nuxt ServerError: An invalid response was received from the upstream server

This error typically indicates that the Sanity API is returning an invalid or unexpected response during your Nuxt build or runtime, causing Nitro (Nuxt's server engine) to fail. Since nothing changed in your repo but deployments suddenly stopped working both locally and on Netlify, this strongly points to an upstream issue rather than a code problem.

Check Sanity's Service Status

First, check if there's an ongoing incident affecting Sanity's services. Look for Sanity's official status page or check their social media channels and community forums for any service disruption announcements. If the API or CDN is experiencing issues, you'll likely see reports of 502 errors or degraded service from other users.

Enable Debug Logging

To get more details about what's failing, enable debug logging in your Nuxt app:

# Locally
NITRO_DEBUG=1 DEBUG=nitro:* npm run dev

# Or for more detailed fetch logging
DEBUG=ofetch:* npm run dev

This will show you exactly which Sanity API requests are failing and what responses you're receiving.

Check Your Sanity Client Configuration

Even though nothing changed in your repo, verify your Sanity client setup. If you're using the @nuxtjs/sanity module, ensure your configuration is correct:

// nuxt.config.ts
export default defineNuxtConfig({
  modules: ['@nuxtjs/sanity'],
  sanity: {
    projectId: 'your-project-id',
    dataset: 'production',
    apiVersion: '2024-01-01', // Use a specific date
    useCdn: true
  }
})

Common Causes When Code Hasn't Changed

1. API Version Issues: According to the API versioning documentation, Sanity uses date-based API versions. If you're not explicitly specifying an apiVersion, you might be affected by changes in default behavior. Always specify a version to avoid unexpected changes.

2. CDN/Network Issues: Sanity uses a CDN, and 502 errors often originate from CDN-level problems during incidents or network disruptions.

3. Query Timeout: If you have complex GROQ queries, they might be timing out intermittently due to upstream load or temporary performance issues.

Temporary Workarounds

While waiting for upstream issues to resolve, try these approaches:

Add Timeout Configuration

// If using the Sanity client directly
import { createClient } from '@sanity/client'

const client = createClient({
  projectId: 'your-project-id',
  dataset: 'production',
  apiVersion: '2024-01-01',
  useCdn: true,
  timeout: 30000, // 30 seconds
})

Bypass the CDN Temporarily

sanity: {
  projectId: 'your-project-id',
  dataset: 'production',
  apiVersion: '2024-01-01',
  useCdn: false // Temporarily disable CDN
}

This bypasses the CDN and hits the API directly, which can help isolate CDN-specific issues.

If It's Not a Sanity Outage

If there are no reported service issues, try:

1. Clear Netlify build cache: Deploy with "Clear cache and retry deploy"
2. Check environment variables: Ensure your SANITY_PROJECT_ID, SANITY_DATASET, and any authentication tokens are set correctly in Netlify
3. Verify API version: Make sure you're specifying an explicit apiVersion in your configuration rather than relying on defaults
4. Inspect specific GROQ queries: If you have particularly complex queries, try simplifying them temporarily to see if one specific query is causing timeouts
5. Check network connectivity: Test if you can reach Sanity's API directly using curl or a browser to rule out local network issues

Since you're experiencing this both locally and on Netlify with no code changes, and the last deployment from Sanity Studio worked fine, this strongly suggests a temporary API or network connectivity issue. The fact that it's happening in multiple environments simultaneously is a strong indicator of an upstream problem rather than a configuration issue in your code.

Monitor for any service status updates and try redeploying once any incidents are resolved. In the meantime, the debug logging should help you identify exactly which requests are failing and provide more context for troubleshooting or reporting the issue to Sanity support.