> For the complete documentation index, see [llms.txt](https://docs.fast.xyz/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.fast.xyz/sdk-references/payment-protocol-adapters.md).

# Payment Protocol Adapters

Payment Protocol Adapters define *how* payments are requested and executed between entities — whether that's an agent paying for an API call, a service charging for compute, or two agents transacting with each other.\
\
These adapters sit above the core SDKs and handle the negotiation layer: "I want this resource, here's my payment, verify and deliver."

### x402-sdk

Packages:\
\
• `@fastxyz/x402-client` — For buyers/agents making payments\
• `@fastxyz/x402-server` — For merchants/services receiving payments\
• `@fastxyz/x402-facilitator` — For verifying and settling payments

HTTP-native payment protocol based on the `402 Payment Required` status code. Your agent requests a resource, the server responds with payment requirements, the agent pays, and gets access — all in a single HTTP flow.

Use cases:\
\
• Pay-per-API-call (micropayments for AI services)\
• Gated content or compute access\
• Agent-to-agent commerce

> *More protocol adapters coming soon: ACP (Agent Communication Protocol), Stripe integration, and others.*


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://docs.fast.xyz/sdk-references/payment-protocol-adapters.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.