WebTransport tester

What this tool does

This page uses the browser’s built-in WebTransport API. You enter a URL such as https://example.com:4433/echo, press Connect, and the browser opens a WebTransport session to that server. You can then send a text message on a new bidirectional stream, or as an unreliable datagram, and watch the reply arrive in the log.

It is a client only. It does not run a server, it cannot make a server that is down respond, and it is not the old public echo service that used to live on this domain. To find a server to point it at, see the list of public echo servers or run one yourself with the local testing guide.

Requirements for the server

• It must implement WebTransport over HTTP/3 and be reachable over UDP on the port in your URL.
• The URL must use https://; WebTransport has no plain-text mode.
• The server must accept a request whose Origin header is this site’s origin. A server may reject other origins.
• It needs a certificate your browser trusts, or a self-signed certificate that satisfies the serverCertificateHashes rules described above.
• It must send a reply for the tester to display. Echo servers do; servers that only count bytes will show nothing.

How do I use it?

1. Enter a https:// URL for a WebTransport endpoint, including the port and path your server uses, for example https://example.com:4433/echo (a placeholder, not a live server).
2. If the server uses a self-signed certificate, open the optional section and paste the SHA-256 hash of its certificate.
3. Press Connect. The page reports connecting, then connected or failed.
4. Type a message, then send it on a bidirectional stream or as a datagram. Replies and errors appear in the log.
5. Press Disconnect when you finish.

What does it check?

• Browser support. It feature-detects the WebTransport class and shows a clear message if it is missing.
• URL validity. It rejects non-https URLs and URLs with a fragment before opening a connection, as the specification requires.
• Session establishment. It reports whether the browser could open a session and shows the error the browser returns.
• Echo behaviour. It shows whatever the server sends back on a stream or as a datagram.

What are its limits?

• Browser support. The API must exist in your browser. MDN lists Chrome 97, Firefox 114 and Safari 26.4 as the first versions; options such as serverCertificateHashes arrived in some browsers later.
• Secure context. WebTransport only works on HTTPS pages (or localhost), which this site is.
• Certificates. A normal connection needs a certificate your browser trusts. A self-signed one works only through serverCertificateHashes: ECDSA P-256 key, total validity under two weeks.
• HTTP/3. The server must speak HTTP/3 over QUIC and be reachable over UDP. The tester cannot tell you why a handshake failed; browsers withhold that detail on purpose.
• One message per stream. Each stream send opens a new bidirectional stream and writes one message. It is not a load-testing or benchmarking tool.

Privacy

The tester sends nothing to this site. Your URL, messages and replies stay in the page and are discarded when you close it. There is no analytics on payloads.

If the connection fails

Read debugging WebTransport handshake errors. Browsers report very little detail for a failed handshake by design, so the checklist matters more than the error text.