Payment Rails¶

A PaymentRail describes one payment method the server accepts. Rails are ranked by priority (lower is preferred). A manifest's accepts field lists all available rails.

Supported Rails¶

Rail	Network	Status
`x402`	Base, Solana	Implemented
`mpp`	Tempo	Planned
`lightning`	Bitcoin	Planned
`card`	Stripe	Planned

The RailType enum also includes ach and custom for ACH bank transfers and vendor-specific rails.

PaymentRail Fields¶

Field	Type	Required	Notes
`rail`	string	Yes	`x402`, `mpp`, `lightning`, `card`, `ach`, `custom`
`network`	string	No	CAIP-2 chain ID or network name
`asset`	string	No	Token/asset (e.g. `"USDC"`)
`contract`	string	No	Token contract address
`pay_to`	string	No	Recipient address
`facilitator`	string	No	Facilitator URL (x402 / MPP)
`provider`	string	No	Payment provider for card rails
`lnurl`	string	No	LNURL for Lightning
`checkout_url`	string	No	Checkout URL for card
`priority`	integer	No	Rail preference (lower preferred)

Convenience Constructors¶

mcp-pay-schema provides ready-made constructors for the common cases.

x402 on Base with USDC¶

PaymentRail::x402_base_usdc("0x1234...");

Produces:

Field	Value
`rail`	`x402`
`network`	`eip155:8453`
`asset`	`USDC`
`contract`	`0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913`
`facilitator`	`https://x402.org/facilitator`
`priority`	`0`

MPP on Tempo¶

PaymentRail::mpp_tempo("0x1234...");

Defaults to network: "tempo", asset: "USDC", priority: 1.

Lightning¶

PaymentRail::lightning("lnurl1dp68gurn8ghj7...");

Defaults to network: "bitcoin", priority: 2.

Card via Stripe¶

PaymentRail::stripe("https://checkout.example.com");

Defaults to provider: "stripe", priority: 3.

Builder Methods¶

let rail = PaymentRail::x402_base_usdc("0x1234...")
    .with_priority(0)
    .with_facilitator("https://x402.org/facilitator");

Choosing a Rail¶

The specification calls out four reasons agents need rail awareness:

Discover which MCP servers require payment.
Compare pricing across equivalent services.
Choose appropriate payment rails based on cost, speed, and budget.
Verify payment-specific SLAs (settlement time, refund policy).

payment_sla.settlement_time_seconds is the natural place to publish expected settlement latency per rail, e.g. { "x402": 2, "lightning": 5 }.