Model Substitution Window

Model Substitution Window is the elapsed time an agency needs to move a live client workload from one model provider to another without breaking delivery. It is a delivery metric, not a procurement one: it measures how many places in your stack hard-code a vendor's endpoint, prompt format, safety filter, or token accounting. A short window (days) lets an agency reprice and re-route when a provider changes terms or pricing; a long window (months) converts every provider announcement into a margin event. The window is set by architecture, not by contract: a gateway layer such as Portkey or OpenRouter keeps routing logic in one place, while a direct SDK integration scattered across client repos stretches the window with every deployment. The practical test is a fire drill: pick one production client workflow and time how long a full provider swap takes, including prompt retesting and eval reruns. That number belongs in the account plan next to the retainer renewal date.