Checkout is the point where agent-ready commerce stops being mostly read-only.
Discovery can be a projection. Comparison can be a query. Policy quotation can be a controlled answer backed by source evidence. Checkout is different. Checkout mutates commercial state.
A cart changes. A price may be selected or locked. Inventory may be revalidated or reserved. Shipping options may be calculated. Policy coverage may become a precondition. A checkout snapshot may be created. Payment authority may become relevant. An order may eventually be committed.
That is why checkout should not be modeled as a form endpoint in an agent-ready platform.
It should be modeled as a state machine.
