What is x402?

x402 is a protocol for HTTP-native micropayments. When a server returns HTTP status 402 Payment Required, it includes payment details in a PAYMENT-REQUIRED header. The client signs a payment transaction and retries the request with a PAYMENT-SIGNATURE header. The server verifies and settles the payment, then returns the protected content.

This SDK handles the entire flow automatically—you just call fetch() and payments happen transparently.

Why This SDK?

Monetize any API in minutes. Add payments to your server in ~10 lines. Clients pay automatically—no checkout pages, no subscriptions, no invoices. Just HTTP.

Dynamic pricing. Charge based on usage: characters, tokens, records, pixels, API calls—whatever makes sense. Price scales with input, not fixed rates.

Token-accurate LLM pricing. Built-in tiktoken support prices AI requests by actual token count. Works with OpenAI models out of the box, or bring your own rates for Anthropic, Gemini, Mistral, or local models.

Full-stack. Client SDK for browsers, server SDK for backends. React hooks, Express middleware patterns, facilitator client—everything you need.

Multi-chain. Solana and Base (Ethereum L2) with the same API. Add wallets for both and the SDK picks the right one automatically.

Works out of the box. Built-in RPC proxy, pre-flight balance checks, automatic retry on 402. Uses the Dexter facilitator by default—the only x402 facilitator with full Phantom wallet support on Solana mainnet.

Quick Start

Install

npm install @dexterai/x402

Client (Browser)

import { createX402Client } from '@dexterai/x402/client';

const client = createX402Client({
  wallets: {
    solana: solanaWallet,
    evm: evmWallet,
  },
});

// That's it. 402 responses are handled automatically.
const response = await client.fetch('https://api.example.com/protected');

RPC URLs are optional—the SDK uses Dexter's RPC proxy by default. Override if needed:

const client = createX402Client({
  wallets: { solana: solanaWallet },
  rpcUrls: {
    'solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp': 'https://your-rpc.com',
  },
});

React

import { useX402Payment } from '@dexterai/x402/react';

function PayButton() {
  const { fetch, isLoading, balances, transactionUrl } = useX402Payment({
    wallets: { solana: solanaWallet, evm: evmWallet },
  });

  return (
    <div>
      <p>Balance: ${balances[0]?.balance.toFixed(2)}</p>
      <button onClick={() => fetch(url)} disabled={isLoading}>
        {isLoading ? 'Paying...' : 'Pay'}
      </button>
      {transactionUrl && <a href={transactionUrl}>View Transaction</a>}
    </div>
  );
}

Supported Networks

| Network | Identifier | Status | |---------|------------|--------| | Solana Mainnet | solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp | Verified | | Base Mainnet | eip155:8453 | Verified |

All networks use USDC.

Package Exports

// Client - browser & Node.js
import { createX402Client } from '@dexterai/x402/client';

// React hook
import { useX402Payment } from '@dexterai/x402/react';

// Server helpers
import { createX402Server } from '@dexterai/x402/server';

// Chain adapters (advanced)
import { createSolanaAdapter, createEvmAdapter } from '@dexterai/x402/adapters';

// Utilities
import { toAtomicUnits, fromAtomicUnits } from '@dexterai/x402/utils';

Utilities

import { toAtomicUnits, fromAtomicUnits } from '@dexterai/x402/utils';

// Convert dollars to atomic units (for API calls)
toAtomicUnits(0.05, 6);  // '50000'
toAtomicUnits(1.50, 6);  // '1500000'

// Convert atomic units back to dollars (for display)
fromAtomicUnits('50000', 6);   // 0.05
fromAtomicUnits(1500000n, 6);  // 1.5

Server SDK

import { createX402Server } from '@dexterai/x402/server';

const server = createX402Server({
  payTo: 'YourAddress...',
  network: 'solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp',
});

// In your route handler
app.post('/protected', async (req, res) => {
  const paymentSig = req.headers['payment-signature'];

  if (!paymentSig) {
    const requirements = await server.buildRequirements({
      amountAtomic: '50000',  // $0.05 USDC
      resourceUrl: req.originalUrl,
    });
    res.setHeader('PAYMENT-REQUIRED', server.encodeRequirements(requirements));
    return res.status(402).json({});
  }

  const result = await server.settlePayment(paymentSig);
  if (!result.success) {
    return res.status(402).json({ error: result.errorReason });
  }

  res.json({ data: 'Your protected content' });
});

Client SDK, React hook, and pricing utilities are production-verified at dexter.cash/sdk. createX402Server is a convenience wrapper not yet used in production.

Dynamic Pricing

Generic pricing for any use case - charge by characters, bytes, API calls, or any unit you define. No external dependencies.

Works for:

• LLM/AI endpoints (by character count)
• Image processing (by pixel count or file size)
• Data APIs (by record count)
• Any service where cost scales with input

import { createX402Server, createDynamicPricing } from '@dexterai/x402/server';

const server = createX402Server({ payTo: '...', network: '...' });
const pricing = createDynamicPricing({
  unitSize: 1000,      // chars per unit
  ratePerUnit: 0.01,   // $0.01 per unit
  minUsd: 0.01,        // floor
  maxUsd: 10.00,       // ceiling
});

app.post('/api/llm', async (req, res) => {
  const { prompt } = req.body;
  const paymentSig = req.headers['payment-signature'];

  if (!paymentSig) {
    const quote = pricing.calculate(prompt);
    const requirements = await server.buildRequirements({
      amountAtomic: quote.amountAtomic,
      resourceUrl: req.originalUrl,
    });
    res.setHeader('PAYMENT-REQUIRED', server.encodeRequirements(requirements));
    res.setHeader('X-Quote-Hash', quote.quoteHash);
    return res.status(402).json({ usdAmount: quote.usdAmount });
  }

  // Validate quote hasn't changed (prevents prompt manipulation)
  const quoteHash = req.headers['x-quote-hash'];
  if (!pricing.validateQuote(prompt, quoteHash)) {
    return res.status(400).json({ error: 'Prompt changed, re-quote required' });
  }

  const result = await server.settlePayment(paymentSig);
  if (!result.success) return res.status(402).json({ error: result.errorReason });

  const response = await runLLM(prompt);
  res.json(response);
});

The client SDK automatically forwards X-Quote-Hash on retry.

Token Pricing (LLM-Accurate)

Accurate token-based pricing for LLMs. Uses tiktoken for token counting. Supports OpenAI models out of the box, plus custom rates for Anthropic, Gemini, Mistral, or any model.

import { createX402Server, createTokenPricing, MODEL_PRICING } from '@dexterai/x402/server';

const server = createX402Server({ payTo: '...', network: '...' });
const pricing = createTokenPricing({
  model: 'gpt-4o-mini',  // Uses real OpenAI rates
  // minUsd: 0.001,      // Optional floor
  // maxUsd: 50.0,       // Optional ceiling
});

app.post('/api/chat', async (req, res) => {
  const { prompt, systemPrompt } = req.body;
  const paymentSig = req.headers['payment-signature'];

  if (!paymentSig) {
    const quote = pricing.calculate(prompt, systemPrompt);
    const requirements = await server.buildRequirements({
      amountAtomic: quote.amountAtomic,
      resourceUrl: req.originalUrl,
      description: `${quote.model}: ${quote.inputTokens.toLocaleString()} tokens`,
    });
    res.setHeader('PAYMENT-REQUIRED', server.encodeRequirements(requirements));
    res.setHeader('X-Quote-Hash', quote.quoteHash);
    return res.status(402).json({
      inputTokens: quote.inputTokens,
      usdAmount: quote.usdAmount,
      model: quote.model,
      tier: quote.tier,
    });
  }

  // Validate quote hasn't changed
  const quoteHash = req.headers['x-quote-hash'];
  if (!pricing.validateQuote(prompt, quoteHash)) {
    return res.status(400).json({ error: 'Prompt changed, re-quote required' });
  }

  const result = await server.settlePayment(paymentSig);
  if (!result.success) return res.status(402).json({ error: result.errorReason });

  const response = await openai.chat.completions.create({
    model: pricing.config.model,
    messages: [{ role: 'user', content: prompt }],
    max_completion_tokens: pricing.modelInfo.maxTokens,
  });

  res.json({ 
    response: response.choices[0].message.content,
    transaction: result.transaction,
  });
});

Available Models

import { MODEL_PRICING, getAvailableModels } from '@dexterai/x402/server';

// Get all models sorted by tier and price
const models = getAvailableModels();
// → [{ model: 'gpt-5-nano', inputRate: 0.05, tier: 'fast' }, ...]

// Check pricing for a specific model
MODEL_PRICING['gpt-4o-mini'];
// → { input: 0.15, output: 0.6, maxTokens: 4096, tier: 'fast' }

Supported tiers: fast, standard, reasoning, premium, custom

Custom Models (Anthropic, Gemini, etc.)

Not using OpenAI? Pass your own rates:

// Anthropic Claude
const pricing = createTokenPricing({
  model: 'claude-3-sonnet',
  inputRate: 3.0,    // $3.00 per 1M input tokens
  outputRate: 15.0,  // $15.00 per 1M output tokens
  maxTokens: 4096,
});

// Google Gemini
const pricing = createTokenPricing({
  model: 'gemini-1.5-pro',
  inputRate: 1.25,
  outputRate: 5.0,
});

// Custom/local model with custom tokenizer
const pricing = createTokenPricing({
  model: 'llama-3-70b',
  inputRate: 0.50,
  tokenizer: (text) => llamaTokenizer.encode(text).length,
});

tiktoken's default encoding works well for most transformer models. Only use a custom tokenizer if your model has significantly different tokenization.

API Reference

createX402Client(options)

| Option | Type | Required | Description | |--------|------|----------|-------------| | wallets | { solana?, evm? } | Yes | Multi-chain wallets | | wallet | SolanaWallet | No | Single Solana wallet (legacy) | | preferredNetwork | string | No | Prefer this network when multiple options available | | rpcUrls | Record<string, string> | No | RPC endpoints per network (defaults to Dexter proxy) | | maxAmountAtomic | string | No | Maximum payment cap | | verbose | boolean | No | Enable debug logging |

useX402Payment(options)

Returns:

| Property | Type | Description | |----------|------|-------------| | fetch | function | Payment-aware fetch | | isLoading | boolean | Payment in progress | | status | string | 'idle' | 'pending' | 'success' | 'error' | | error | X402Error? | Error details if failed | | transactionId | string? | Transaction signature | | transactionUrl | string? | Block explorer link | | balances | Balance[] | Token balances per chain | | refreshBalances | function | Manual refresh | | reset | function | Clear state |

Development

npm run build      # Build ESM + CJS
npm run dev        # Watch mode
npm run typecheck  # TypeScript checks

License

MIT — see LICENSE