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HTTP protocol checker

Show which HTTP version a probe negotiates for a URL and whether Alt-Svc advertises HTTP/3, with QUIC marked not tested.

Press the button to check the protocol
Nothing leaves this page until you do. Type the full address, scheme included.

Honest limits, stated up front. The probe lanes do not surface the connection's negotiated protocol (ALPN), so this tool reports the negotiated version as not surfaced rather than guessing from headers. QUIC is marked Not Tested: no lane here dials UDP, so an h3 advertisement is evidence of advertising, never proof of a working HTTP/3 connection. For the version your own browser negotiates, open the site's DevTools protocol column.
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More network tools

- HTTP timing probe (/network/http-timing)
All network tools (/network)

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Canonical HTML: https://nutter.tools/network/http-protocol-checker
Markdown version: https://nutter.tools/network/http-protocol-checker.md
Plain-text version: https://nutter.tools/network/http-protocol-checker.txt
Agent index: https://nutter.tools/llms.txt