What are the main AI agent payment protocols?
The emerging agentic-payments stack includes x402 (per-request HTTP 402 payments in USDC), MPP (Machine Payments Protocol for agent-to-agent settlement), A2A (agent-to-agent coordination), AP2 (Agent Payments Protocol authorization layer), ACP (Agentic Commerce Protocol), L402 (Lightning-native HTTP 402), and MCP (Model Context Protocol, whose paid servers charge per tool call). They operate at different layers and are often combined.
Which agent payment protocol should I use?
For charging agents per API request or MCP tool call, x402 is the most mature. For autonomous agents paying each other, look at MPP and A2A. For delegated authorization and spending mandates, AP2. Many real systems combine them — e.g., MPP or AP2 for authorization with x402 for settlement.
How do these protocols relate to each other?
They stack rather than compete. Higher-level protocols (ACP, AP2, MPP, A2A) handle discovery, authorization and coordination, while payment rails (x402, L402) handle settlement. MCP is the tool-access layer where much agent monetization happens.