@agentgate-builder/signcheck

SignCheck: is it safe to sign this?

Pre-sign safety check for AI agent wallets, paid per call over x402 (USDC on Base), with a free preview and an MCP server.

An agent with a wallet is one signature away from an unlimited approval to a drainer, a "free" NFT listing, an EIP-7702 account takeover, or an x402 payment 1,000x the price it was quoted. SignCheck decodes what the agent is about to sign into plain English and returns SAFE / CAUTION / DANGER with the reasons.

• Live API: https://api.youraitools.co/signcheck (also https://signcheck.agentgate.workers.dev)
• Price: $0.015 per full check, $0.005 per offline lite check (USDC on Base, x402 v2 + v1). Invalid input and RPC outages are never charged.
• Free: /v1/preview (offline verdict, summary and flag codes, 10/min per IP) and remote MCP at /mcp.
• npm: @agentgate-builder/signcheck (stdio MCP server, npx -y @agentgate-builder/signcheck)
• Source: https://agentgate-source.pages.dev/x402-signcheck/ (MIT)

What it checks

Input	Examples of what gets flagged
tx {to, data, value, from}	unlimited approve, approvals / Permit2 allowances to plain wallets (EOAs), setApprovalForAll, transfers to the token contract or the zero address, ownership transfers, proxy upgrades, Safe DELEGATECALL, unknown functions (with the public 4-byte name), multicall / Multicall3 / Safe inner calls, eth_call simulation with revert reason
typed_data (EIP-712)	EIP-2612 Permit, DAI permit, Permit2 PermitSingle / PermitBatch / (Witness)TransferFrom, EIP-3009 TransferWithAuthorization (x402), Seaport orders with zero price, never-expiring deadlines, missing or mismatched chainId, fake "USD Coin" domains, Permit2 lookalikes
authorization (EIP-7702)	delegation to an unknown contract, chainId 0 (valid on every chain)
payment_required + payment_payload (x402)	payTo mismatch, overpay, wrong asset or network, non-Circle "USDC", validity far beyond maxTimeoutSeconds, unusually high price

Every address involved is listed with its role, EOA / contract / 7702-delegated status, a known-protocol label, and an OFAC SDN match (bundled US Treasury list). On-chain facts and simulation cover Base, Ethereum and Base Sepolia; any other EVM chain is decoded offline.

Quick start

# free preview (offline)
curl -s -X POST https://api.youraitools.co/signcheck/v1/preview -H 'content-type: application/json' \
  -d '{"tx":{"to":"0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913","data":"0x095ea7b30000000000000000000000003c1a1e01e790cd326495631b3b38bd7068f08d6fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff"}}'

# paid: answers 402 with x402 terms; any x402 client pays it
curl -i -X POST https://api.youraitools.co/signcheck/v1/check -H 'content-type: application/json' -d @tx.json

Check an x402 payment before your client sends it:

const r = await pay('https://api.youraitools.co/signcheck/v1/check', {
  method: 'POST', headers: { 'content-type': 'application/json' },
  body: JSON.stringify({ payment_required: res.headers.get('payment-required'), payment_payload: paymentHeader }),
});
const { verdict, flags, x402 } = await r.json(); // x402.matches === true when the signature pays exactly what the 402 asked

MCP

{ "mcpServers": { "signcheck": { "type": "http", "url": "https://api.youraitools.co/signcheck/mcp" } } }

{ "mcpServers": { "signcheck": { "command": "npx", "args": ["-y", "@agentgate-builder/signcheck"] } } }

Tool check_signature. The free servers return the offline preview. Set SIGNCHECK_X402_PRIVATE_KEY (a Base wallet with a little USDC) in the stdio server's env and it pays the API per call (max $0.02 per payment, only to the SignCheck pay-to address). Paid mode needs @x402/fetch, @x402/evm and viem: npx -y -p @agentgate-builder/signcheck -p @x402/fetch -p @x402/evm -p viem signcheck.

Docker: docker build -t signcheck-mcp . && docker run -i --rm signcheck-mcp

Endpoints

Route	Price
POST /v1/check (JSON body) / GET /v1/check?to=&data=&value=&from=&chain=	$0.015	full check
POST /v1/lite / GET /v1/lite?… (same inputs)	$0.005	offline: every reason, decoded args, counterparties, x402 payment-vs-402 match, OFAC SDN; no RPC or simulation
`POST	GET /v1/preview`	free, 10/min/IP
POST /mcp	free	remote MCP (Streamable HTTP, stateless)
/openapi.json, /.well-known/x402, /v1/stats, /health	free	discovery, usage counters

HEAD /v1/check returns the same 402 + PAYMENT-REQUIRED header as GET.

Limits and honesty

• Read-only. SignCheck never asks for keys or signatures and never sends transactions.
• SAFE means no known-bad pattern was found, not that the counterparty is trustworthy. Labels are a short list of well-known protocol contracts, not an endorsement. Not financial or legal advice.
• The simulation is a single eth_call against the latest block (no state diff / balance-change tracing yet).
• The OFAC SDN list is a bundled snapshot (EVM addresses, list date in /health), rebuilt with scripts/build-sdn.py.

Development

npm test            # 47 tests, offline (fake RPC + fake facilitator)
npx wrangler deploy # Cloudflare Workers; counters in a SQLite Durable Object (no KV)
node scripts/live-check.mjs

Part of the agent trust kit by agentgate-builder: AgentGate, WebRead, InjectScan, DomainCheck, PayGuard, PkgGuard, StackDetect, TokenVerdict and SignCheck. Contact: [e-mail]