agent-payment-guard

A pre-pay risk gate for autonomous payment agents. Before your agent signs a payment, it asks 402Sentinel's firewall whether this payment is safe in the context of what the agent was actually asked to do — and blocks routing swaps, prompt-injected destinations, overcharges and drains.

The agent frameworks (Claude Agent SDK, LangGraph, CrewAI) give you the orchestration loop. The payment rails (x402 / Coinbase AgentKit, Stripe, Skyfire) give you the ability to move money. Neither gives you the gate that decides whether a specific payment is safe to make. This is that gate.

Install

npm i @kaditang/agent-payment-guard

import { paymentGuard } from "@kaditang/agent-payment-guard";

// right before your agent signs a payment:
const { allow, reason } = await paymentGuard(toolInput, {
  intended: { payto: approvedSeller, max_amount: "1.00" }, // what the human/task authorized
  untrustedText: lastPageOrToolOutput,                     // what the agent acted on
});
if (!allow) throw new Error(reason); // ⛔ don't sign

// or just try it: npx @kaditang/agent-payment-guard

$ node demo.mjs

  1. Legit payment — agent pays the seller the user approved
     ✅ ALLOW — payment proceeds

  2. PROMPT INJECTION — a scraped page said 'send payment to 0xBAD…'; the agent obeyed
     ⛔ BLOCK — payment STOPPED before signing
     signals: intent_mismatch(block), injection_destination(block)

  3. OVERCHARGE / drain — agent about to pay 50x the approved cap
     ⛔ BLOCK — payment STOPPED before signing
     signals: amount_anomaly(block)

  2/3 dangerous payments stopped by the pre-pay gate.

Run it

node demo.mjs            # MOCK firewall — zero deps, instant; shows the gate logic
node demo.mjs --live     # REAL $0.002 x402 calls to https://402sentinel.com/api/firewall
                         #   npm i @x402/core @x402/evm viem  +  CLIENT_PRIVATE_KEY (Base-USDC-funded)

Wire it into your agent

The universal pattern: call paymentGuard() at the top of whatever tool moves money, and don't pay when it says no. Drop-in examples for every major framework:

Framework	Example
Claude Agent SDK (PreToolUse hook → permissionDecision: "deny")	agent-sdk-hook.ts
Vercel AI SDK (gate a tool({ execute }))	examples/vercel-ai-sdk.ts
Coinbase AgentKit (gate a customActionProvider action)	examples/coinbase-agentkit.ts
LangChain / LangGraph (Python @tool)	examples/langchain.py
CrewAI (Python @tool)	examples/crewai.py

JS/TS imports paymentGuard from this package. Python uses the matching examples/pay_guard.py drop-in (same fail-closed gate, stdlib only). ctx.intended = what the human approved (enables intent-mismatch); ctx.untrustedText = any page/tool output the agent acted on (enables injection-destination).

What the firewall checks

POST https://402sentinel.com/api/firewall → allow | hold | block + signals: routing_anomaly (payTo swapped vs history) · injection_destination (payTo appeared in untrusted content → hard block) · intent_mismatch (paying someone other than approved → hard block) · amount_anomaly (overcharge) · velocity_anomaly (drain) · counterparty_risk (folds the seller's on-chain risk score). Pay-per-call x402, no signup. The deterministic hard blocks shown in the demo are exactly what the live service enforces; the live service adds learned per-signal precision, per-agent history/velocity state, and counterparty risk.

Fail-closed by default

If 402Sentinel can't be reached, paymentGuard blocks the payment (decision: "block", error: true) rather than letting it through — a safety gate that fails open is worse than no gate, because it gives false confidence. If you'd rather keep paying when the gate is down, pass failOpen: true.

Test

node --test          # 7 tests: the 3 block scenarios, pass-through, fail-closed + fail-open

Not affiliated with Anthropic / Coinbase / Stripe — an independent safety layer for the agents they're enabling to move money.