The cost of leaving an API belongs in the decision before its shutdown notice arrives. Useful features on day one do not pay for rebuilding an integration. When the provider's deadline forces the switch, whoever depends on the integration still has ongoing work to deliver.
The Assistants API shut down on August 26, 2026. OpenAI directed developers to Responses and Conversations. That deadline should sit right beside the capability list, because a dependency can still serve a product while its provider forces developers to change how they use it.
An integration accumulates decisions outside the model selector. How is a session identified? Where does an interrupted execution resume? What needs to survive from a conversation to serve the user? If this information only makes sense inside the provider's interface, leaving means first working out what the stored data stands for.
I prefer to separate early what belongs to the product from what is an API convenience. A task's objective and artifacts need to remain understandable. An enormous abstraction written before any real work exists also costs maintenance. I would start with the data that actually needs to survive the switch and a way to retrieve it in a useful form.
Getting the migration to compile settles only part of the verification. An agent can keep answering while changing how it works. To check that, I want representative tasks with expected results recorded before the change. Wording can vary; the behavior satisfying the request needs to remain verifiable.
An announced deadline helps reserve time, but a reminder does none of the migration work. When I assess a dependency, I require a readable route for carrying an ongoing task across: its objective, artifacts, and enough information to continue. I will include that effort in the integration cost. If leaving requires explaining everything from scratch, the entry price was hiding a good part of the work.