Guardian — Transaction Firewall for AI Agents

An OKX.AI A2MCP service. Every agent that can sign can also be tricked. Guardian is the check that runs first.

The agent economy has a hole in the middle of it. We are handing autonomous agents the ability to move money, and the only thing standing between a prompt-injected agent and an empty wallet is the agent's own judgment. That is not a security model.

Guardian is the layer that says no. An agent submits the transaction it intends to sign; Guardian returns ALLOW, WARN or DENY, with every reason. It never sees a private key, and it never signs anything.

Agent  →  "I want to sign this"  →  Guardian  →  ALLOW / WARN / DENY  →  Wallet
                                        │
                              decode · policy · on-chain scan · simulate

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What it actually catches

Not a risk score handed down from a black box — deterministic rules, each with a stated reason.

Rule	What it stops
unlimited_approval	approve(spender, MAX_UINT256) — the single most exploited pattern in crypto. One compromised contract, and the whole balance is gone years later.
set_approval_for_all	A blanket grant over an entire NFT collection.
approval_to_eoa	Spending power granted to a personal wallet instead of a contract. Real protocols are contracts. This is a drainer signature.
fresh_recipient	Recipient with zero balance, zero nonce and no code — a freshly generated address, i.e. the wallet an attacker made ten minutes ago.
zero_address	Funds sent into the void and destroyed.
max_spend_exceeded	The agent exceeding its budget.
unknown_selector	Calldata the agent cannot prove the meaning of.
simulation_revert	A transaction that reverts on-chain.

Tools

Tool	Purpose
guardian_assess_transaction	The main event. Full verdict: decode + policy + on-chain scan + simulation.
guardian_decode_calldata	Raw hex → plain language. "Grant 0xabc… UNLIMITED permission to spend your USDC."
guardian_scan_address	Contract or wallet? Funded? Ever used? How much bytecode?
guardian_simulate_transaction	Dry-run: does it revert, what does it cost?
guardian_supported_chains	Chain list.

Chains

X Layer (196, default) · X Layer Testnet (1952) · Ethereum · Base · BNB Chain · Arbitrum · Polygon

Each chain carries several RPC endpoints and fails over between them. A protocol error ("execution reverted") is a real answer from a healthy node, so it propagates immediately; only transport failures (timeout, 5xx, dead origin) move to the next endpoint. npm run test:live health-checks every configured URL, so a rotting provider surfaces in CI rather than in front of a user.

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The design decision that matters

A security tool that guesses when it cannot see is worse than no security tool, because it is trusted.

An early version of Guardian used soft RPC fallbacks: if a call failed, it substituted an empty value and carried on. The result was a system that, whenever the network hiccuped, confidently reported that Vitalik's wallet was a freshly generated drainer address and that the Uniswap router was a personal wallet — because "no code" and "could not read the code" had collapsed into the same value.

Guardian now separates the chain says no from the chain did not answer:

• DENY is deterministic. Policy checks and hard security rules are computed offline, so a flaky RPC can never soften a DENY.
• On-chain enrichment may only add risk. A failed lookup never manufactures a finding.
• No blind ALLOW. If the chain is unreachable, the response carries degraded: true and the verdict is capped at WARN. Guardian will tell you it does not know.

This is enforced by tests, not by convention. See suite 4, "an unreachable chain must never be faked."

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Run it

npm test        # 37 assertions: protocol, decoder, policy, degraded mode
npm run test:live   # real attack scenarios against live chain state
npm run dev     # http://localhost:8787/mcp

No dependencies. No build step. Node 18+.

Deploy

vercel --prod

Your MCP endpoint is https://<your-domain>/api/mcp. That HTTPS URL is what you register with OKX.AI.

Call it

curl -X POST https://<your-domain>/api/mcp \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/list"}'

A worked example — an agent about to grant an unlimited USDC allowance:

{
  "verdict": "DENY",
  "riskScore": 80,
  "degraded": false,
  "summary": "Grant 0x1111…1111 UNLIMITED permission to spend your USDC",
  "findings": [
    { "severity": "critical", "rule": "unlimited_approval",
      "message": "Unlimited spend approval… the entire USDC balance can be drained — today or years from now." },
    { "severity": "high", "rule": "approval_to_eoa",
      "message": "Spending power is being granted to an EOA, not a contract. Legitimate protocols are contracts." }
  ],
  "recommendation": "DO NOT SIGN. Fix: request an exact-amount approval instead of an unlimited one. Fix: never grant token allowances to a personal wallet address."
}

Pricing

Free. Guardian is infrastructure: the more agents that check before they sign, the safer the marketplace everyone is building on.

MIT.