Agentic payments let software discover a price, authorize a payment, and receive a resource in the same HTTP workflow. An agent can pay for a data query, model inference, or tool call without first creating a billing account or copying an API key.
MPP
Use HTTP authentication for one-time charges and metered payment sessions.
x402
Add x402 V2 payment requirements, signatures, and settlement to an HTTP resource.
The common payment flow
MPP and x402 use different headers and data models, but their basic HTTP flow is similar:
MPP and x402
Both protocols can settle stablecoin payments on Solana. Choose based on the
payment model and interoperability your service needs, not only the shared 402
status code.
| MPP | x402 | |
|---|---|---|
| Challenge | WWW-Authenticate: Payment | PAYMENT-REQUIRED |
| Client authorization | Authorization: Payment | PAYMENT-SIGNATURE |
| Receipt | Payment-Receipt | PAYMENT-RESPONSE |
| Solana payment models | One-time charge and capped, metered session intents | Schemes such as exact and upto; support varies by network and client |
| Settlement architecture | Server validation, or an optional relay or gateway | Resource server with local verification or a facilitator |
| Good starting point | APIs that need HTTP authentication semantics or repeated metering | Pay-per-request resources and existing x402 clients |
MPP is the Machine Payments Protocol. Do not confuse it with the Model Context Protocol (MCP). MCP exposes tools and resources to agents; MPP or x402 can require payment when an agent invokes one of those tools.
A service may advertise more than one protocol. The client selects a payment option it supports, then uses the matching challenge, authorization, and receipt headers for the entire exchange.
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