Sign-In With Agent (SIWA) lets AI agents authenticate with services using their ERC-8004 onchain identity.

Install

npm install @buildersgarden/siwa

Skills

Agent-Side (Signing)

Choose based on your wallet provider:

• Bankr — Bankr Agent API wallets
• Circle — Circle developer-controlled wallets
• Privy — Privy server wallets
• Private Key — Raw private key (viem LocalAccount)
• Keyring Proxy — Self-hosted proxy with optional 2FA

Server-Side (Verification)

• Server-Side Verification — Next.js, Express, Hono, Fastify

SDK Modules

x402 Payments (Agent-Side)

When an API requires payment, it returns HTTP 402 with a Payment-Required header. The agent decodes the payment options, constructs a signed payment, and retries with a Payment-Signature header — all while maintaining SIWA authentication.

Handling a 402 Response

import {
  encodeX402Header,
  decodeX402Header,
  type PaymentRequired,
  type PaymentPayload,
} from "@buildersgarden/siwa/x402";
import { signAuthenticatedRequest } from "@buildersgarden/siwa/erc8128";
// 1. Make initial authenticated request (may get 402)
const signedRequest = await signAuthenticatedRequest(
  new Request("https://api.example.com/premium", { method: "POST" }),
  receipt,
  signer,
  84532,
);
const res = await fetch(signedRequest);
if (res.status === 402) {
  // 2. Decode payment requirements from header
  const header = res.headers.get("Payment-Required");
  const { accepts, resource } = decodeX402Header(header!);
  // 3. Pick a payment option and construct payload
  const option = accepts[0];
  const payload: PaymentPayload = {
    signature: "0x...",  // sign the payment with your wallet
    payment: {
      scheme: option.scheme,
      network: option.network,
      amount: option.amount,
      asset: option.asset,
      payTo: option.payTo,
    },
    resource,
  };
  // 4. Retry with both SIWA auth + payment header
  const retryRequest = await signAuthenticatedRequest(
    new Request("https://api.example.com/premium", {
      method: "POST",
      headers: {
        "Payment-Signature": encodeX402Header(payload),
      },
    }),
    receipt,
    signer,
    84532,
  );
  const paidRes = await fetch(retryRequest);
  // paidRes.headers.get("Payment-Response") contains { txHash, ... }
}

x402 Headers

Pay-Once Sessions

Some endpoints use pay-once mode: the first request requires payment, subsequent requests from the same agent to the same resource pass through without payment until the session expires. If you receive a 200 on a previously-paid endpoint, the session is still active — no need to pay again.

Captcha (Reverse CAPTCHA)

SIWA includes a "reverse CAPTCHA" mechanism — inspired by MoltCaptcha — that proves an entity is an AI agent, not a human. Challenges exploit how LLMs generate text in a single autoregressive pass (satisfying multiple constraints simultaneously), while humans must iterate.

Two integration points:

• Sign-in flow — server requires captcha before issuing a nonce
• Per-request — middleware randomly challenges agents during authenticated API calls

Agent-Side: Handling a Captcha Challenge

The SDK provides two convenience wrappers for the captcha retry pattern:

Sign-In Captcha: solveCaptchaChallenge()

import { solveCaptchaChallenge } from "@buildersgarden/siwa/captcha";
// 1. Request nonce
const nonceRes = await fetch("/api/siwa/nonce", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ address, agentId, agentRegistry }),
});
const data = await nonceRes.json();
// 2. Detect + solve captcha if required
const captcha = await solveCaptchaChallenge(data, async (challenge) => {
  // LLM generates text satisfying all constraints in a single pass
  // challenge: { topic, format, lineCount, asciiTarget, wordCount?, timeLimitSeconds, ... }
  // Your LLM generates text satisfying all constraints in one pass.
  // Use any provider (Anthropic, OpenAI, etc.) — the solver just returns a string.
  return await generateText(challenge);
});
if (captcha.solved) {
  // 3. Retry with challenge response
  const retryRes = await fetch("/api/siwa/nonce", {
    method: "POST",
    headers: { "Content-Type": "application/json" },
    body: JSON.stringify({ address, agentId, agentRegistry, challengeResponse: captcha.challengeResponse }),
  });
}

Per-Request Captcha: retryWithCaptcha()

import { signAuthenticatedRequest, retryWithCaptcha } from "@buildersgarden/siwa/erc8128";
const url = "https://api.example.com/action";
const body = JSON.stringify({ key: "value" });
// 1. Sign and send
const signed = await signAuthenticatedRequest(
  new Request(url, { method: "POST", body }),
  receipt, signer, chainId,
);
const response = await fetch(signed);
// 2. Detect + solve captcha, re-sign, and get retry request
const result = await retryWithCaptcha(
  response,
  new Request(url, { method: "POST", body }), // fresh request (original body consumed)
  receipt, signer, chainId,
  async (challenge) => generateText(challenge), // your LLM solver
);
if (result.retry) {
  const retryResponse = await fetch(result.request);
}

Note: Pass a fresh, unconsumed Request to retryWithCaptcha — the original is consumed after signing/sending.

Difficulty Levels

Links

• Documentation
• ERC-8004
• ERC-8128