Quickstart

The fastest path from zero to your first signed inference call. Five minutes once devnet USDC is in your hands.

1. Install

npm install @vdm-nexus/x402
# or
pnpm add @vdm-nexus/x402

2. Generate an agent + grab its pubkey

import { X402Agent } from "@vdm-nexus/x402";

const agent = X402Agent.generate();
console.log("pubkey:", agent.pubkey);
console.log("secret (save somewhere safe):", agent.secretKeyBase58);

The pubkey is what you fund with USDC. The secret is the 64-byte Ed25519 keypair encoded in base58 — losing it means losing the agent.

3. Get devnet USDC

Go to faucet.circle.com, select Solana Devnet, paste your agent's pubkey, request 5+ USDC. It arrives in seconds and auto-creates the USDC associated token account.

No SOL needed — the facilitator pays the transaction fees.

4. Make the call

const reply = await agent.payAndInfer("https://nexus.vdmnexus.com/api/v1", {
  model: "openai/gpt-4o-mini",
  messages: [
    { role: "user", content: "Explain signed inference in one sentence." }
  ],
});

console.log("Model said:", reply.openai.choices[0].message.content);
console.log("Tx:", reply.payment?.txSignature);
console.log("Cost:", reply.receipt?.cost_usdc, "USDC");

That's a real signed inference. Check the tx_signature on Solana explorer — it's a real on-chain transaction.

What just happened

1. The SDK POSTed to /chat/completions with no payment header
2. Got back a 402 with the x402 challenge (amount, recipient, network)
3. Built a signed Solana SPL USDC transfer matching the challenge
4. POSTed again with the signed transfer in the X-Payment header
5. Nexus's facilitator verified, co-signed, broadcast to Solana, ran OpenRouter inference, returned OpenAI body + signed receipt

The full sequence, diagrammed.

Next

• Verify a receipt — what the receipt contains and how to validate it
• @vdm-nexus/x402 API reference
• Signed inference concept — the longer rationale