Flows

Flow	Order	Handler fails	Typical rails
`authorization`	reserve · run · complete · settle	release — nothing moved	x402, MPP Tempo charge, KYAPay, L402, credits
`upfront`	reserve · settle · run · complete	refund	MPP Stripe charge (captured before the handler), MPP Tempo session
`escrow`	settle deposit · run · settle final	refund the deposit	Planned — refused in this version

Each rail declares the flows it supports. A route without flow uses the first the rail supports, preferring authorization, where the payer is charged only for work that ran.
upfront requires a rail that can refund.
A flow set on a route applies to every rail on it. Leave it out on routes that mix rails with different flows, such as MPP Stripe and x402.
Paid at verification. Some payments have already moved when the rail verifies them, such as an MPP Tempo push transfer. Core records them as upfront, settled before the handler. If the handler fails and the rail cannot refund, the charge stays settled/failed, an error event with REFUND_REJECTED asks you to refund outside Tollstile, and reconciliation leaves it alone.
Variable prices (upTo) require authorization.
Choose explicitly with toll.price("$0.05", { flow: "upfront" }). The execution plan excludes rails that cannot serve that flow; the route fails where defined if no rail remains.