> For the complete documentation index, see [llms.txt](https://docs.usexpay.xyz/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.usexpay.xyz/learn-more/glossary.md).

# Glossary

**x402**\
An HTTP-friendly standard for per-request payments. Calls either include valid proof of payment or receive `402 Payment Required`.

**Faremeter**\
Middleware that enforces x402 on Solana. Verifies payment before a request is forwarded to your original API.

**MCP (Model Context Protocol)**\
A protocol that lets AI agents discover and call “tools” (functions/APIs) in a consistent way. xPay auto-publishes your endpoints as MCP tools.

**Agent**\
Software that plans, decides, and calls APIs autonomously using AI. Agents prefer protocols (like MCP and x402) over dashboards and keys.

**SPL Token**\
Token standard on Solana. USDC is an SPL token frequently used for stable pricing.

**Mint Address**\
The on-chain address identifying an SPL token. You can use any SPL token by its mint address.

**Proxy URL**\
The xPay-wrapped endpoint (e.g., `https://api.usexpay.xyz/:username/:endpointName`) that enforces x402 payments and forwards to your original API.

**Sample Body / Sample Response**\
Example request/response payloads you provide when registering an endpoint. Used to improve docs and auto-generate MCP tool schemas, as well as inform agents about the expected payload.


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