Payment drafts & pay links.
Zero custody. Built for humans and AI agents.

Non-custodial by structure, not by promise

Payment drafts AI agents can actually use

send21 is agent-first: everything a human does in the UI, an agent does over REST. Scoped API keys mean an agent can create and track drafts but can never move funds — nobody can, that's the point.

Create a draft in one call

curl -X POST https://send21.io/api/v1/drafts \
  -H "Authorization: Bearer s21_yourkey" \
  -H "Idempotency-Key: invoice-2026-001" \
  -H "Content-Type: application/json" \
  -d '{
    "receiverAddress": "bc1q...",
    "network": "Mainnet",
    "paymentMethod": "MultiOutput",
    "fiatCurrency": "USD",
    "fiatAmount": 50.00,
    "memo": "Invoice 2026-001"
  }'

The full agent payment flow

agent → create draft → fetch tx template → fund & sign in own wallet → broadcast → webhook: draft.confirmed

1. Authenticate

   Send a scoped API key as Authorization: Bearer s21_…. Grant only the scopes the agent needs (drafts:write, drafts:read, webhooks:manage). No scope can ever move funds.

2. Create the draft

   POST /api/v1/drafts with the receiver address and either an exactamountSats or a fiat-pegged fiatCurrency + fiatAmount. Include an Idempotency-Key header — retries are safe and return the same draft.

3. Read the rate lock & fee

   The response contains the exact receiver amount (amountSats), the send21 fee as a separate transparent output (platformFeeSats, feeOutputs), and for fiat-pegged drafts a locked exchange rate (quote.rate, valid until quote.expiresAt — call POST /drafts/{id}/requote if it lapses).

4. Fetch the transaction template, fund & sign locally

   GET /api/v1/drafts/{id}/tx-template returns the exact output list, a ready bitcoin-cli walletcreatefundedpsbt example, and a pay-to-many CSV. The agent funds and signs it with its own node, wallet, or HSM — private keys never touch send21, and send21 contains no signing code.

   curl https://send21.io/api/v1/drafts/{id}/tx-template \
     -H "Authorization: Bearer s21_yourkey"

5. Broadcast from your own node

   Broadcast the signed transaction yourself. There is nothing to report back — send21 watches the receiver address on-chain and matches the exact amount automatically.

6. Get confirmation — webhook-first

   Register a webhook (POST /api/v1/webhooks) and receive HMAC-SHA256-signed events instead of polling. For quick checks, GET /drafts/{id}/status returns status, txid, and confirmation count.

Machine-readable overview at /llms.txt · full OpenAPI spec at /swagger · MCP endpoint at https://send21.io/mcp (Streamable HTTP, Authorization: Bearer s21_…)

Transparent fees. No custody, no surprises.

The send21 fee is priced in USD when the payment is created (same price feed as fiat conversion) and shown in your selected fiat currency too. On multi-output drafts it rides as a separate transparent output you can see and verify in your wallet. On single-output payments (checkout pay links, QR drafts) nothing extra rides along — the fee accrues to the creator's account and is settled with one payment (BTC, USDC, USDT, or EURC) once the balance reaches the cap. Either way, the receiver always gets the exact billed amount.

Full details on the pricing page · machine-readable: /api/v1/fees/schedule

How a payment flows

Supported currencies & networks

Works with the wallets people already use

There are two ways to pay through send21. Payment drafts (Bitcoin) are multi-output PSBT templates and need a PSBT-capable wallet — Bitcoin Core, Sparrow, Electrum — with hardware wallets signing through them. Checkout pay links are single transfers with a standard QR code (bitcoin: / solana: / ethereum: URI), so any wallet that can scan a QR and send an exact amount works — no fee output rides along; the send21 fee accrues to the account that created the payment and is settled separately.

"Top 5" per 2026 wallet roundups (Ledger, Coincub, bitcoinfoundation.org). Compatibility reflects each wallet's published feature set as of August 2026 — always verify the address and amount in your own wallet before signing.

Run a shop? Accept BTC, USDC, USDT & EURC at the counter

Price in your own currency — any of 165 world currencies — show one QR code, and your customer pays in the crypto of their choice — straight to your wallet, rate locked at checkout, accounting-ready webhooks with the exact conversion rate.