Agent payments are not one protocol but a stack. Settlement rails — x402 on stablecoins, L402 on Bitcoin Lightning — move the actual money in a single HTTP round-trip. Coordination layers — MPP, A2A and AP2 — decide how agents discover prices, authorize spending and delegate mandates. Commerce layers — ACP — orchestrate the full buy flow, and MCP is the tool-access surface where much of this monetization actually happens.
Each standard below has its own live listing of the open-source projects implementing it, ranked by GitHub stars and updated continuously. If you are evaluating which layer to build on, start with the protocol, then pick an SDK or facilitator from the infrastructure categories.
