Skip to main content

Documentation Index

Fetch the complete documentation index at: https://docs.stable.xyz/llms.txt

Use this file to discover all available pages before exploring further.

Agent settlement is Stable’s rail for machine payments. An agent holds a USDT0 balance, pays for a resource over HTTP, and the payment settles on-chain in the same request cycle. The agent spends down from one balance for both the payment and the network fee. There is no separate gas token, no sign-up, and no API key rotation.

​
Why this matters for agents

Agents transact differently from humans. They run continuously, make many small payments, and cannot complete sign-up flows or rotate API keys. Settlement on Stable matches that workload:
  • USDT0 is the gas token and the payment token. An agent wallet holds a single asset and spends it down for both fees and payments.
  • Sub-cent, predictable fees. Fees are denominated in dollars, so agents can budget cost per action without converting from a volatile gas asset.
  • Sub-second finality. A paid HTTP call settles inside the request lifecycle (~700 ms block time), which makes high-frequency machine traffic viable.
  • USDT distribution. USDT is the most widely held stablecoin; Stable is the venue purpose-built for it.

​
How the layers fit

Two layers do different jobs and are complementary, not alternatives:
  • x402 is the payment standard. It is an HTTP-native protocol where a server responds with 402 Payment Required, a client signs an authorization, and a facilitator submits it.
  • MPP (Machine Payments Protocol) is the IETF-track standard that supersedes x402 with broader intents and multi-rail support; x402 is the backward-compatible subset Stable supports today. See MPP.
  • Stable is the settlement layer. It is where the on-chain transfer of USDT0 actually happens.
A facilitator sits between the two: it verifies the signed payment and submits the on-chain call so the developer does not run settlement infrastructure. See Facilitators for the providers that support Stable today. 
agent (client)  ──HTTP──▶  resource server  ──signed payment──▶  facilitator  ──tx──▶  Stable
                          (returns 402)                          (verify + submit)     (USDT0 settles)

​
What you can build

  • Pay-per-call APIs priced per request in USDT0, settled via x402 or MPP.
  • Agent-to-agent commerce where one agent pays another for a service over HTTP.
  • Paid MCP tools that wrap x402 endpoints so an AI client calls and pays for them through prompts.
  • Autonomous procurement against a budgeted USDT0 balance.
  • Usage-based billing that settles per request instead of per invoice.
  • Agent wallets funded with USDT0 only, no custodial middleware.

​
Start here

Build a pay-per-call API

Stand up an x402-gated endpoint and settle a real USDT0 payment in the request.

Build an MPP endpoint on Stable

Write the three MPP custom-method hooks for USDT0 and settle on Stable.

Develop with AI

Wire Docs MCP and Runtime MCP into your AI editor and paste the Stable context block.

Pay with an MCP server

Expose x402-paid APIs as MCP tools an agent can call through natural-language prompts.

x402 in depth

Read how the HTTP payment protocol works end to end on Stable.

MPP

The broader IETF-track standard that x402 belongs to.

Facilitators

See which facilitators already settle USDT0 payments on Stable.
Last modified on June 2, 2026