Charge per message on a tab

Seller: charge per send

Protect your send route with tabOrExactMiddleware. Set a flat perUnit price in human-units USDC per message. The middleware handles the 402 handshake; your delivery handler only runs after payment settles for each message. The buyer's tab caps cumulative spend across all sends.

import express from 'express';
import { Connection } from '@solana/web3.js';
import { tabOrExactMiddleware } from '@dexterai/x402/tab/seller';

const app = express();
const connection = new Connection(process.env.SOLANA_RPC_URL!);

// Each message costs perUnit USDC. The buyer's tab caps cumulative spend
// across all sends and reserves what's accrued on-chain — no runaway billing.
app.post('/send',
  tabOrExactMiddleware({
    connection,
    sellerPubkey: process.env.SELLER_SOLANA_ADDRESS!,
    network: 'solana:mainnet',
    perUnit: '0.0004', // USDC per message
  }),
  async (req, res) => {
    const receipt = await sendMessage(req.body);
    res.json(receipt);
  },
);

Buyer: send within the cap

Call the send endpoint once per message with payAndFetch. Each send meters one unit into the tab. When the cap is reached,result.ok is false — the loop stops cleanly instead of overrunning your budget.

import { payAndFetch, createKeypairWallet } from '@dexterai/x402/client';

const solana = await createKeypairWallet(process.env.AGENT_KEY!);

// Send a batch — each call meters one message into the tab.
const messages = ['Hello, Alice', 'Hello, Bob', 'Hello, Carol'];

for (const text of messages) {
  const result = await payAndFetch('https://push.example/send', {
    method: 'POST',
    body: JSON.stringify({ to: '+1555...', text }),
  }, { solana });
  if (!result.ok) break; // tab cap reached — stop cleanly
}

Why tabs fit high-volume micro-spend

Push notifications and messages are the canonical high-volume tab use case. Each individual send is cheap — a fraction of a cent — but volume accumulates fast. A tab puts a hard ceiling on total spend before the first message is sent, so a runaway notification loop or an unbounded retry can't drain you.

From the seller's side, every delivery is guaranteed payment before the message goes out. No chargebacks, no net-30 invoices, no over-billing disputes. What's delivered is paid; what's paid is reserved on-chain.

Open a tab at dexter.cash/tabs with a per-campaign USDC cap. Hand the spending authorization to your notification service. Each send meters in real time — the cap enforces itself.