# Financial Comptroller — Machine Payments Protocol (MPP)

> Agentic KG Holdings → https://agent-kg.ai. This surface implements the rail it is named after. Nothing here is endorsed by any protocol's owner. Prices are placeholders set by the office.

The Machine Payments Protocol (MPP) is an open standard for machine-to-machine payments over HTTP 402. A resource that costs money answers 402 with a WWW-Authenticate: Payment challenge naming an id, a realm, a payment method, an intent and a base64url-encoded request; the client answers with a credential in the Authorization: Payment header; a successful payment returns a Payment-Receipt header. This surface implements the challenge, credential and receipt shapes as the specification read on 2026-09-19.

## Doors

- https://agent-mpp.ai/api — the paid door (x402 v2: 402 without payment, the office door's statement and a signed receipt with PAYMENT-SIGNATURE)
- https://agent-mpp.ai/offers.json — the two offers (placeholder prices)
- https://agent-mpp.ai/x402/receipt/{nonce} — resolve a receipt
- https://agent-mpp.ai/x402/settled.json — this host's register, four-host total beside it
- https://agent-mpp.ai/x402/jwks.json — receipt key (kid fc-x402-2026-09)
- https://agent-kg.ai/mcp — the office door (Model Context Protocol (MCP)) — the product

## Rails on every host

- x402 — live, USDC on Base mainnet
- Machine Payments Protocol (MPP) — challenge, credential, receipt as the spec reads; settles in test mode once the office's Stripe test keys are placed
- Universal Commerce Protocol (UCP) — /checkout-sessions REST; /.well-known/ucp published when a handler can settle
- Agentic Commerce Protocol (ACP) — /checkout_sessions at API-Version 2025-09-12; /acp/feed.jsonl served when the rail can settle

## The four

- https://agent-x402.ai/
- https://agent-mpp.ai/
- https://agent-ucp.ai/
- https://agent-acp.ai/

## Kit

- https://agent-mpp.ai/.well-known/agent-card.json · https://agent-mpp.ai/.well-known/jwks.json · https://agent-mpp.ai/.well-known/mcp.json · https://agent-mpp.ai/.well-known/ai-catalog.json · https://agent-mpp.ai/.well-known/security.txt · https://agent-mpp.ai/facts.json · https://agent-mpp.ai/health.json · https://agent-mpp.ai/index.md (Markdown twin, also on Accept: text/markdown)

Surface 0.1.0 · as of 2026-09-19. No cookies. No tracking.