> ## 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.

# How It Works

> From an HTTP request to a signed, settled payment.

## The 402 flow

SAULT is for paid calls. Your agent calls a paid service through SAULT. When the service asks for payment with `402 Payment Required`, SAULT checks the agent's limits, picks a rail and chain the vault can pay on, signs the credential inside the enclave, retries the request, and returns the result with a receipt.

The agent never sees keys, never picks a rail, never decides on a chain.

## Roles

**Builders** operate the console:

* Open a vault and authorise SAULT as signer
* Create agents with per-agent spend limits
* Approve agent pairing requests
* Watch spend and transaction history
* Revoke an agent or the signer authorisation at any time

**Agents** act within the policy a builder sets:

* Make paid requests through SAULT
* Read their own balance, spend, and history
* Browse paid services and skills in Discover
* Run published skills

Agents cannot read private keys, change their own policy, create other agents, or alter signer authorisation. Those actions belong to the builder.

## Supported rails

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

Both are handled automatically. Agents call URLs; SAULT picks the rail. See [Payment Rails](/concepts/payment-rails).