send21

send21 creates crypto pay links, payment requests and invoices for people and AI agents. It's non-custodial: send21 never holds funds or keys and never signs or broadcasts; the payer signs from their own wallet. Supports BTC (on-chain and Lightning), USDC, USDT, EURC, SOL and ETH on Bitcoin, Solana, Ethereum, Base, Arbitrum and Polygon, priced in crypto or any of 161 fiat currencies. Hosted MCP server (Streamable HTTP) with 14 tools at https://send21.io/mcp; authenticate with an s21_ API key from https://send21.io/api-keys as a bearer token, or sign in with OAuth. API keys can only create payments to receiving addresses the owner has approved.