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Specification

The authoritative mcp-pay specification lives in specs/mcp-pay-specification.md in the repository. This page summarises the key points and links to the JSON Schema.

  • Version: 0.1 (Draft)
  • Status: Proto-SEP for the MCP Server Card Working Group
  • Authors: Neul Labs

Abstract

MCP-Pay extends the Model Context Protocol (MCP) ecosystem with payment awareness. It defines a standardised JSON manifest (pay.json) that MCP servers can expose to declare pricing, accepted payment rails, and payment-specific SLAs. This specification complements SEP-2127 (MCP Server Cards) by adding payment capabilities without duplicating general server metadata.

Namespace Alignment

mcp-pay uses the /.well-known/mcp/ namespace being registered with IANA per SEP-2127:

/.well-known/mcp/server-card   ← identity, remotes, capabilities (SEP-2127)
/.well-known/mcp/pay.json      ← pricing, payment rails, SLA (this spec)

Motivation

The MCP Registry provides a standard for agent tool discovery, but has no payment awareness. Payment protocols like x402 and MPP each have their own discovery mechanisms that don't integrate with MCP. Agents need a way to:

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

Manifest Schema

See Payment Manifest for the field-level reference. The machine-readable JSON Schema lives at specs/schema/pay.schema.json and is published as https://mcp-pay.io/schema/v0.1/pay.schema.json (via the SCHEMA_URL constant in mcp-pay-schema).

Relationship to MCP Server Card

mcp-pay is designed to complement, not replace, SEP-2127:

MCP Server Card MCP-Pay
Server identity Payment pricing
Capabilities Payment rails
Authentication Payment SLA
Transport Payment stats

Servers SHOULD link to their Server Card via the server_card field.

Roadmap

  • [x] Phase 1 — Schema + Reference Server
  • [ ] Phase 2 — Registry Crawler
  • [ ] Phase 3 — Rail Router (cost optimization)
  • [ ] Phase 4 — Submit as SEP to the MCP Working Group

References

Changelog

  • 0.1 — Initial draft specification.