"connection is not using a post-quantum key exchange algorithm"

Newer OpenSSH clients print "connection is not using a post-quantum key exchange algorithm" when the server they connect to can't agree on a post-quantum key exchange. The session still works, but someone recording it today could decrypt it with a future quantum computer. (check: the exact wording and the first OpenSSH version that prints it.)

Why you see it

• The server runs OpenSSH older than 9.0 (no post-quantum key exchange), or
• the server's KexAlgorithms list leaves out mlkem768x25519-sha256 and sntrup761x25519-sha512.

Fix it on the server

1. Check the version: ssh -V on the server.
2. Upgrade OpenSSH to 9.9 or later (10.0 or later makes mlkem768x25519-sha256 the default).
3. If sshd_config sets KexAlgorithms, put the post-quantum ones first:

KexAlgorithms mlkem768x25519-sha256,sntrup761x25519-sha512,curve25519-sha256

4. Check and restart: sshd -t && systemctl restart sshd.

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