Quickstart in 5 minutes

Pick your surface

• API key

  OpenAI/Anthropic SDK, Claude Code or MCP: developer guide.

• x402 with your own wallet

  Follow the four steps below.

• Free try

  Use free/deepseek-v4.1-flash on the same endpoint: no wallet, 20 answers/day per network (IPv4 /24, IPv6 /64).

Step 1: see the 402 (free)

Send this unpaid request. Expect HTTP/2 402 and a PAYMENT-REQUIRED header containing base64 JSON.

Verified in the historical checks listed below: unpaid-curl.

curl -i https://ask.singitai.app/v1/chat/completions \
  -H 'Content-Type: application/json' \
  -d '{"model":"deepseek-v4.1-flash","max_tokens":200,"messages":[{"role":"user","content":"Say hello in five words."}]}'

Decode the header with Node:

Verified in the historical checks listed below: decode-offers.

curl -s -o /dev/null -D - https://ask.singitai.app/v1/chat/completions \
  -H 'Content-Type: application/json' \
  -d '{"model":"deepseek-v4.1-flash","max_tokens":200,"messages":[{"role":"user","content":"Say hello in five words."}]}' \
  | awk 'tolower($1)=="payment-required:"{print $2}' | tr -d '\r' \
  | node -e 'let s="";process.stdin.on("data",d=>s+=d).on("end",()=>{const r=JSON.parse(Buffer.from(s.trim(),"base64"));
      console.log("x402Version", r.x402Version);
      for (const a of r.accepts) console.log(a.scheme.padEnd(6), a.network.padEnd(40), a.amount, "atomic USDC")})'

Expected offer order: upto Base, upto Solana, exact Base, exact Solana. x402Version is 2. The Base upto offer for this request is 1500 atomic USDC (0.0015 USDC). Other amounts come from the current offer.

HTTP/2 402
payment-required: <base64 JSON>

x402Version 2
upto   Base     1500 atomic USDC
upto   Solana   <amount from offer>
exact  Base     <amount from offer>
exact  Solana   <amount from offer>

Live 31.10.2026: the 402 body also carries x402Version and accepts, matching the decoded header. upto authorizes a ceiling and settles actual usage; exact pays a flat price.

Step 2: put USDC on Base

You need Node 20+, a wallet private key you control for the agent, and a small USDC balance on the Base network. No ETH is needed for this upto path: the facilitator pays the gas through Permit2/EIP-2612 gas sponsoring.

Withdraw USDC to Base from an exchange that supports Base withdrawals, or bridge USDC to Base. Check the destination network before sending. Use a fresh wallet with a small balance for agents; keep its private key out of source files and logs.

At the 0.0015 USDC minimum ceiling, one USDC covers hundreds of short answers on cheap models: floor(1 / 0.0015) = 666 calls at that ceiling. Longer answers or other models can require more; read each offer.

Step 3: the paid call — Variant A

Run end to end once on 28.10.2026. Node with @x402/fetch and your own key. Set AGENT_KEY privately in your environment and save the script as first-paid-call.mjs. The wrapper signs the offer and retries with PAYMENT-SIGNATURE.

Verified in the historical checks listed below: install-a.

npm install @x402/[email protected] @x402/[email protected] viem

Verified in the historical checks listed below: paid-a.

// first-paid-call.mjs — run with: AGENT_KEY=0x... node first-paid-call.mjs
import { wrapFetchWithPayment, x402Client } from "@x402/fetch";
import { UptoEvmScheme } from "@x402/evm/upto/client";
import { createPublicClient, http } from "viem";
import { privateKeyToAccount } from "viem/accounts";
import { base } from "viem/chains";

// A wallet holding a little USDC on Base. No ETH: the facilitator pays the gas.
const account = privateKeyToAccount(process.env.AGENT_KEY);
const chain = createPublicClient({ chain: base, transport: http() });
const client = x402Client.fromConfig({ schemes: [{ network: "eip155:8453",
  client: new UptoEvmScheme({ address: account.address, signTypedData: account.signTypedData,
                              readContract: chain.readContract.bind(chain) }) }] });
const pay = wrapFetchWithPayment(fetch, client);

const res = await pay("https://ask.singitai.app/v1/chat/completions", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ model: "deepseek-v4.1-flash", max_tokens: 200,
                         messages: [{ role: "user", content: "Say hello in five words." }] }),
});
const answer = await res.json();
console.log(res.status, answer.choices?.[0]?.message?.content);
console.log("charged (atomic USDC):", answer.billing?.totalAtomic);
console.log("PAYMENT-RESPONSE:", res.headers.get("payment-response"));

Step 4: read the result

A successful paid answer returns HTTP 200. Read choices[0].message.content for the answer and billing.totalAtomic for the settled amount in USDC micro-units. For upto, it is at most the ceiling you signed.

PAYMENT-RESPONSE is base64 JSON describing the facilitator settlement (transaction, network, payer). X-SingIt-Receipt is JSON with SingIt’s reference, transaction, network, payer and amount charged.

If you get 402 again

Variant B: Coinbase Bazaar MCP

npm install @coinbase/cdp-sdk @modelcontextprotocol/sdk @x402/core @x402/evm @x402/mcp @x402/extensions @x402/svm

Variant C: Coinbase agentic wallet CLI

npx [email protected] --version
npx [email protected] x402 details https://ask.singitai.app/v1/chat/completions
npx [email protected] x402 bazaar search "SingIt Ask"

npx [email protected] x402 pay https://ask.singitai.app/v1/chat/completions -X POST \
  -d '{"model":"deepseek-v4.1-flash","max_tokens":200,"messages":[{"role":"user","content":"Say hello in five words."}]}' \
  --max-amount 2000

Next steps

Developers · Models · Pricing · Agent spending limits · OpenAPI · llms.txt