? HTTP 426 – Upgrade Required

? Quick Overview

The 426 Upgrade Required status code indicates that the server refuses to process the request using the current protocol. The client must upgrade to a different protocol (for example, switching from HTTP/1.1 to a newer version) before retrying the request.

? Key Concepts

Part of the 4xx Client Error HTTP status code family
Introduced with protocol negotiation mechanisms
Commonly associated with WebSocket or protocol upgrades
Server responds with an Upgrade header

? Syntax / Theory

When a client sends a request using an unsupported or outdated protocol, the server may respond with status code 426 and include headers that specify which protocol(s) are acceptable.

? View Code Example

// HTTP response indicating protocol upgrade is required
HTTP/1.1 426 Upgrade Required
Upgrade: HTTP/2.0
Connection: Upgrade

? Code Example (Server Side)

? View Code Example

// Node.js example sending a 426 response
res.status(426);
res.set("Upgrade", "HTTP/2.0");
res.send("Please upgrade your protocol");

? Live Output / Explanation

The client receives a 426 response and understands that the request will only succeed after switching to the protocol specified in the Upgrade header.

? Interactive Example

Imagine a browser trying to establish a connection over an outdated protocol. Click below to simulate a protocol handshake.

Ready to Request

> System idle...

? View Logic Example

// Client-side fetch showing a failed request due to protocol mismatch fetch("https://example.com/socket") .then(res => { if (res.status === 426) { console.log("Upgrade required to: " + res.headers.get("Upgrade")); } });

? Use Cases

Forcing clients to upgrade to HTTP/2 or newer
WebSocket handshake enforcement
Legacy protocol deprecation
Security-driven protocol upgrades

✅ Tips & Best Practices

Always include a clear Upgrade header
Provide human-readable guidance in the response body
Use 426 sparingly and only when protocol change is mandatory

? Try It Yourself

Create a small server that rejects HTTP/1.1 requests
Return status code 426 with an Upgrade header
Test the behavior using browser dev tools or curl