MCP
386Model Context Protocol · Anthropic, Agentic AI Foundation
JSON-RPC protocol for exposing tools, resources and prompts to an agent, over stdio or streamable HTTP
protocols
Agent protocols
The standards agents use to call a service, prove who sent them and pay. These are specs, not services, so the directory doesn’t list them — it lists the services that implement them. The counts are listings that support each one today.
Calling a service
How an agent finds out what a service can do and calls it.
OpenAPI
84OpenAPI Specification · OpenAPI Initiative
Machine-readable description of an HTTP API that an agent can read and call without a hand-written integration
A2A
Agent2Agent · Google, Linux Foundation
Open protocol for agent-to-agent discovery, task delegation and streaming updates across vendors
Identity and authorization
How an agent proves who sent it and how far it may go: which apps it may reach, and how much it may spend.
XAA
Cross App Access · Okta
Lets an enterprise identity provider broker an agent's access between apps, so the organisation sees and controls which apps an agent may reach
AP2
2Agent Payments Protocol · Google
Cryptographic mandates signed by the buyer, proving an agent had authority to spend within set limits
TAP
3Trusted Agent Protocol · Visa
HTTP Message Signatures (RFC 9421) that let a merchant recognise a legitimate agent before it transacts
Agent Pay
2Mastercard Agent Pay · Mastercard
Agentic tokens and verifiable intent so a registered agent can initiate card payments for a consumer
ACE
1Agentic Commerce Experiences · American Express
Amex developer kit for cardholder-scoped authorization of agent-initiated payments
Paying per request
Payment carried on the HTTP request itself, so an agent can pay a service it has never met.
x402
13x402 · Coinbase, x402 Foundation
HTTP 402 standard for per-request stablecoin payments between an agent and an API
MPP
15Machine Payments Protocol · Stripe, Tempo
HTTP-native standard for one-shot, recurring, and streaming machine payments across cards, stablecoins, and Lightning
L402
3L402 · Lightning Labs
HTTP 402 payments over Lightning using macaroon credentials for pay-per-request access
Checkout
Agent-initiated purchases from a merchant that stays the merchant of record.
ACP
9Agentic Commerce Protocol · OpenAI, Stripe
Agent-initiated checkout against a merchant using scoped delegated payment tokens, merchant stays merchant of record
UCP
8Universal Commerce Protocol · Google, Shopify
Open standard for agentic shopping journeys across retailers, pairing with AP2 for the payment leg
ACTP
0Agentic Commerce Trust Protocol · Alibaba, Ant International
Trust protocol for agent commerce across Asian marketplaces
Spec ↗No listings yet
Bridges
Endpoints that translate between the other protocols, so a merchant integrates once.
ICC
1Intelligent Commerce Connect · Visa
Single merchant endpoint that translates between TAP, MPP, ACP, and UCP
As JSON
The same registry with adoption counts: GET /api/v1/protocols. Filter the directory by payment protocol with ?protocol=x402.