> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sault.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Payment Rails

> SAULT handles the payment standards so your agent doesn't have to.

A "rail" is the standard a paid service uses to ask for and accept payment. SAULT supports the common HTTP 402 rails, and more over time. Your agent never has to know which one a service uses — it calls the URL, SAULT handles the rest.

## What SAULT supports today

| Rail     | Chains        |
| -------- | ------------- |
| **x402** | Base, Solana  |
| **MPP**  | Tempo, Solana |

Both work the same way from your side: the service asks for payment, SAULT pays from the vault within the agent's limits, and your agent gets the result with a receipt.

## When a service supports more than one

SAULT prefers the chain where your vault has the most available balance. You can pin a preference per agent if you need predictable routing.

## New rails later

The agent's contract — call a URL, get a result — doesn't change when SAULT adds a rail. Existing agents pick up new rails automatically.