I shipped coinopai-mcp two days ago. It's a local MCP server that gives any AI agent access to crypto intelligence — preflight checks, trade signals, risk state — for USDC micropayments on Base. What I didn't expect was how much the payment layer would teach me about where agent commerce is actually heading. The Stack Nobody Asked For (But Everyone Needs) If you're building agents in 2026, you're suddenly dealing with a protocol stack that didn't exist 12 months ago: MCP — how your agent talks to tools A2A — how agents talk to each other x402 — how agents pay each other ACP / UCP / AP2 — how agents pay merchants That's four layers, three competing standards, and zero consensus on which one to use when. What x402 Gets Right x402 is cleverly simple: HTTP 402 response → payment header → USDC on Base → retry with receipt. Round-trip under 2 seconds. Fees under $0.001. No intermediary, no account signup, no API key management. For a solo builder, this is the obvious choice. It's live on mainnet.…