HTTP/2, HTTP/3 and ALPN checker
Check HTTP/2, HTTP/3 and ALPN online
Enter a domain to test HTTPS protocol negotiation: ALPN, HTTP/2, HTTP/1.1 fallback, HTTP/3 probing and Alt-Svc headers.
The report helps spot server configuration gaps such as missing h2 in ALPN, missing HTTP/3 advertisement or a broken fallback path for older clients.
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The tool runs directly in the browser and is ready for quick checks without installing software.
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Input can be cleared and recalculated immediately, which is useful for repeated small tasks.
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Use fullscreen mode when you need more space for long text, structured data or large result blocks.
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Copy buttons are available on generated output so results can be reused in code, documents or support chats.
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Most operations are deterministic: the same input and options produce the same output.
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For sensitive content, prefer local browser tools and review the result before sharing it further.
