◆Error Codes Wiki

HTTP 103 Early Hints — Preloading and Link Header Configuration Issues

Informational1xx informational

Overview

Troubleshoot HTTP 103 Early Hints implementation errors including browser compatibility issues, incorrect Link headers, and preload failures.

Key Details

  • HTTP 103 Early Hints allows servers to send preload hints before the final response is ready
  • The server sends 103 with Link headers pointing to critical resources (CSS, JS, fonts)
  • Browsers can start fetching these resources while waiting for the full response
  • Not all browsers support 103 Early Hints — check compatibility before relying on it
  • Early Hints are sent as an informational response before the final 200 response

Common Causes

  • Browser not supporting HTTP 103 Early Hints (Safari, older browsers)
  • Link headers in 103 response not matching the resources actually used in the final response
  • Proxy or CDN stripping the 103 intermediate response
  • Preloaded resources served with mismatched CORS or content-type causing browser rejection

Steps

  1. 1Verify browser support: Chrome 103+ and Firefox 102+ support Early Hints, Safari does not yet
  2. 2Send 103 with Link headers for critical render-blocking resources: Link: </style.css>; rel=preload; as=style
  3. 3Ensure preloaded resources match the actual resources in the HTML — mismatches cause double downloads
  4. 4Configure your CDN to forward 103 responses: Cloudflare supports Early Hints natively
  5. 5Use the Performance API to measure if Early Hints improve your Largest Contentful Paint (LCP)

Tags

early-hints103preloadperformancelink-header

More in 1xx Informational

http-100-continueHTTP 100 Continue — What It Means & How to Fix It
Informational
http-101-switching-protocolsHTTP 101 Switching Protocols — What It Means & How to Fix It
Informational
http-102-processingHTTP 102 Processing — What It Means & How to Fix It
Informational
http-103-early-hintsHTTP 103 Early Hints — What It Means & How to Fix It
Informational
http-100HTTP 100 Continue — What It Means
Informational
http-101HTTP 101 Switching Protocols Explained
Informational

Frequently Asked Questions

Early Hints allow browsers to start loading CSS, fonts, and scripts before the HTML response arrives. This can improve LCP by 100-500ms, especially for pages with slow server-side rendering.