Tag every ciphertext, token, and handshake with an explicit algorithm or suite identifier, and have receivers dispatch on it from a server-controlled allowlist. Keep the list of accepted suites short and configurable, so retiring one is a configuration change rather than a code change. Test the removal path: disable your current primary suite in staging and confirm clients fall forward, not back.

Negotiation itself is an attack surface — downgrade attacks exist because of it, so never let the attacker choose and never accept a suite you would not have offered. For closed, simultaneously-updated systems with one client and one server, a single version counter is enough; full suite negotiation is overhead you do not need.