send21 — Non-Custodial Crypto Payment Drafts & Pay Links (BTC, USDC, USDT, EURC) for Humans & AI Agents

# send21 > send21 is a non-custodial payment-draft platform for BTC, USDC, USDT, and EURC (on Bitcoin, Solana, > Ethereum, and Base) with a first-class REST API for AI agents. Agents create payment drafts (fixed > amount or fiat-pegged with a locked exchange rate); the sender pays from their own wallet > (PSBT-enabled for multi-output drafts: Bitcoin Core, Sparrow, Electrum). send21 is software > that prepares payment instructions only — it is not a custodian, exchange, broker, or payment > service provider. It never holds private keys or funds, never signs, and never broadcasts or > executes transactions; signing and broadcasting always happen in the sender's wallet. Key facts for agents: - Base URL: `https://send21.io/api/v1` - OpenAPI spec: `https://send21.io/swagger/v1/swagger.json` (UI at `/swagger`; aliases `/openapi.json`, `/swagger.json`) - MCP server: `https://send21.io/mcp` — Streamable HTTP, stateless; authenticate with the same `Authorization: Bearer s21_<key>` header. Tools: create_draft, get_draft, get_draft_status, get_transaction_template, create_payment_request, get_payment_request, list_address_book, get_rate, get_fee_schedule. tools/list works unauthenticated; tool calls require a key. - Supported assets: BTC native on Bitcoin (on-chain PSBT, or Lightning against a receiver-supplied BOLT11 invoice); USDC, USDT, and EURC as tokens on BOTH Solana and Ethereum; USDC and EURC also on Base (Coinbase's Ethereum L2, EIP-681 chain id 8453); wrapped BTC on Solana (Portal) and Ethereum (WBTC). When a checkout offers the same currency on several networks, pass `network` in `POST /checkout/{token}/select`. - Auth: `Authorization: Bearer s21_<key>` — scoped API keys created by a human account owner - Scopes: `drafts:read`, `drafts:write`, `addressbook:read`, `addressbook:write`, `webhooks:manage` - Idempotency: send an `Idempotency-Key` header on `POST /drafts`; identical keys return the same draft - Batch: `POS

{ "openapi": "3.0.4", "info": { "title": "send21 API", "description": "Non-custodial Bitcoin payment drafts for humans and AI agents. send21 never holds keys or funds; it prepares multi-output payment templates that the sender's own PSBT-enabled wallet (Bitcoin Core, Sparrow, Electrum) funds, reviews, and signs. AI agents authenticate with scoped API keys (Authorization: Bearer s21_...), get idempotent draft creation, batch operations, recurring templates, and HMAC-signed webhooks. M