HTTP/2 + HTTP/3 checker. Protocol version + Alt-Svc.
Paste raw response headers. We identify the HTTP protocol version (1.1 / 2 / 3), validate the Alt-Svc header for HTTP/3 advertising, and recommend an upgrade path if you're still on HTTP/1.1.
Paste HTTP response headers (the first line is the status line: HTTP/2 200 etc.). The checker reads the protocol version, parses the Alt-Svc header for HTTP/3 advertising, and recommends an upgrade path. Browser-only.
How to capture the protocol version
- curl:
curl -I --http2 https://example.comshows HTTP/2;curl -I --http3 https://example.comtries HTTP/3 (newer curl required) - Browser DevTools: F12 → Network → click document request → Headers → look at General → the response line (HTTP/2 200, HTTP/3 200, etc.). Or right-click a column header in the Network table → enable Protocol column for at-a-glance view
Sources used
- RFC 9113 — HTTP/2 specification
- RFC 9114 — HTTP/3 specification
- RFC 9000 — QUIC transport (HTTP/3 underlying transport)
- RFC 7838 — Alt-Svc header (alternative services)
- MDN Alt-Svc header reference
- web.dev HTTP/2 performance guide
No data is sent to any external service. Detection happens entirely in your browser by parsing the status line and Alt-Svc header from your pasted text.
Privacy: parsing happens in your browser. Nothing is sent or logged.
HTTP/2 is table stakes in 2026.
HTTP/2 (RFC 9113) ships multiplexing + HPACK header compression. HTTP/3 (RFC 9114) runs over QUIC and eliminates transport-layer head-of-line blocking. Sites still on HTTP/1.1 leave 15-30% of page-load performance on the table for free. The checker above identifies your protocol from the status line, validates Alt-Svc for HTTP/3 advertising, and recommends an upgrade path. Modern web servers (nginx, Apache, IIS, Caddy) and all major CDNs (Cloudflare, Fastly, Vercel, Netlify, Cloudfront) support HTTP/2 + HTTP/3 — usually a single config flag.
Three operating points. One, HTTP/2 requires TLS in browsers (HTTP/2 cleartext is not implemented in any browser; HTTPS required). Two, HTTP/3 also requires TLS 1.3 + UDP — some corporate firewalls block UDP, in which case browsers fall back to HTTP/2 over TCP. Three, Alt-Svc caching means the first visit to an HTTP/3-capable origin uses HTTP/2; subsequent visits upgrade. Don't worry if a single page-load shows HTTP/2 in DevTools — that's expected on the very first connection.
Tools in the same cluster: HTTP Headers Checker for the broader headers grade. CDN Detector for the edge tier (CDNs auto-handle protocol upgrades). Compression Detector for the Brotli/gzip view.
Five answers.
Why does HTTP/2 matter for performance?
HTTP/2 (RFC 9113) ships three performance wins over HTTP/1.1: multiplexing (multiple requests over one connection, eliminating head-of-line blocking), header compression via HPACK (typical 30-60% header size reduction), and server push (largely deprecated but originally allowed origins to push critical resources before requested). For typical websites with 50-100 sub-resources, HTTP/2 cuts page load time 15-30% over HTTP/1.1 with no application changes.
What does HTTP/3 add over HTTP/2?
HTTP/3 (RFC 9114) runs over QUIC (RFC 9000) instead of TCP. The big win is transport-layer head-of-line blocking elimination — TCP's reliable-ordered delivery means one lost packet stalls all streams; QUIC's per-stream reliability means one stream's packet loss only stalls that stream. The benefit is most visible on lossy mobile networks (cellular, congested WiFi) where HTTP/3 cuts page load 10-20% over HTTP/2. Sites advertise HTTP/3 via the Alt-Svc response header.
How do I tell if I'm on HTTP/2 or HTTP/3?
The first line of the HTTP response (the status line) declares the protocol version. HTTP/1.1 200 OK means HTTP/1.1. HTTP/2 200 means HTTP/2. HTTP/3 200 means HTTP/3 (newer curl + browsers needed to negotiate). Note that browsers always show HTTP/2 in DevTools when negotiated even if Alt-Svc advertises HTTP/3 — the first connection uses HTTP/2 and only subsequent connections may upgrade. The checker above also flags Alt-Svc headers that indicate HTTP/3 availability even when the current connection is HTTP/2.
What is the Alt-Svc header?
Alt-Svc (Alternative Services, RFC 7838) tells clients that the same origin is also reachable over a different protocol. The most common use today is advertising HTTP/3 availability. A typical header: alt-svc: h3=":443"; ma=86400 means HTTP/3 is available on the same port for 86400 seconds (24 hours). Modern browsers cache this and use HTTP/3 for subsequent connections to the origin. Without Alt-Svc, even an HTTP/3-capable origin server is reached via HTTP/2 first.
Does this tool save my data?
No. Header parsing happens in your browser. Nothing is sent to any server. Closing the tab clears the data. The Copy Results button puts a plain-text summary on your clipboard.
Stuck on HTTP/1.1?
Our web development engagements ship HTTP/2 + HTTP/3 + TLS 1.3 + Brotli + Cache-Control correctness as the standard performance baseline — typical 25-40% page-load improvement.
Published .