Agent Payment Execution (x402)

Agent Payment Execution (x402)

Enable AI agents to make autonomous payments with built-in spending controls. Uses the x402 HTTP payment protocol and MCP tools so agents can pay for external services, APIs, or other agents without custodial risk.

When to Use

Use when: your agent needs to pay for an API call, purchase a service, settle with another agent, enforce per-task spending limits, or manage a non-custodial wallet. Pairs naturally with cost-aware-llm-pipeline and security-review skills.

How It Works

x402 Protocol

x402 extends HTTP 402 (Payment Required) into a machine-negotiable flow. When a server returns 402, the agent's payment tool automatically negotiates price, checks budget, signs a transaction, and retries — no human in the loop.

Spending Controls

Every payment tool call enforces a SpendingPolicy:

• Per-task budget — max spend for a single agent action
• Per-session budget — cumulative limit across an entire session
• Allowlisted recipients — restrict which addresses/services the agent can pay
• Rate limits — max transactions per minute/hour

Non-Custodial Wallets

Agents hold their own keys via ERC-4337 smart accounts. The orchestrator sets policy before delegation; the agent can only spend within bounds. No pooled funds, no custodial risk.

MCP Integration

The payment layer exposes standard MCP tools that slot into any Claude Code or agent harness setup.

Security note: Always pin the package version. This tool manages private keys — unpinned npx installs introduce supply-chain risk.

{
  "mcpServers": {
    "agentpay": {
      "command": "npx",
      "args": ["agentwallet-sdk@6.0.0"]
    }
  }
}

Available Tools (agent-callable)

Tool	Purpose
get_balance	Check agent wallet balance
send_payment	Send payment to address or ENS
check_spending	Query remaining budget
list_transactions	Audit trail of all payments

Note: Spending policy is set by the orchestrator before delegating to the agent — not by the agent itself. This prevents agents from escalating their own spending limits. Configure policy via set_policy in your orchestration layer or pre-task hook, never as an agent-callable tool.

Examples

Budget enforcement in an MCP client

When building an orchestrator that calls the agentpay MCP server, enforce budgets before dispatching paid tool calls.

Prerequisites: Install the package before adding the MCP config — npx without -y will prompt for confirmation in non-interactive environments, causing the server to hang: npm install -g agentwallet-sdk@6.0.0

import { Client } from "@modelcontextprotocol/sdk/client/index.js";
import { StdioClientTransport } from "@modelcontextprotocol/sdk/client/stdio.js";

async function main() {
  // 1. Validate credentials before constructing the transport.
  //    A missing key must fail immediately — never let the subprocess start without auth.
  const walletKey = process.env.WALLET_PRIVATE_KEY;
  if (!walletKey) {
    throw new Error("WALLET_PRIVATE_KEY is not set — refusing to start payment server");
  }

  // Connect to the agentpay MCP server via stdio transport.
  // Whitelist only the env vars the server needs — never forward all of process.env
  // to a third-party subprocess that manages private keys.
  const transport = new StdioClientTransport({
    command: "npx",
    args: ["agentwallet-sdk@6.0.0"],
    env: {
      PATH: process.env.PATH ?? "",
      NODE_ENV: process.env.NODE_ENV ?? "production",
      WALLET_PRIVATE_KEY: walletKey,
    },
  });
  const agentpay = new Client({ name: "orchestrator", version: "1.0.0" });
  await agentpay.connect(transport);

  // 2. Set spending policy before delegating to the agent.
  //    Always verify success — a silent failure means no controls are active.
  const policyResult = await agentpay.callTool({
    name: "set_policy",
    arguments: {
      per_task_budget: 0.50,
      per_session_budget: 5.00,
      allowlisted_recipients: ["api.example.com"],
    },
  });
  if (policyResult.isError) {
    throw new Error(
      `Failed to set spending policy — do not delegate: ${JSON.stringify(policyResult.content)}`
    );
  }

  // 3. Use preToolCheck before any paid action
  await preToolCheck(agentpay, 0.01);
}

// Pre-tool hook: fail-closed budget enforcement with four distinct error paths.
async function preToolCheck(agentpay: Client, apiCost: number): Promise<void> {
  // Path 1: Reject invalid input (NaN/Infinity bypass the < comparison)
  if (!Number.isFinite(apiCost) || apiCost < 0) {
    throw new Error(`Invalid apiCost: ${apiCost} — action blocked`);
  }

  // Path 2: Transport/connectivity failure
  let result;
  try {
    result = await agentpay.callTool({ name: "check_spending" });
  } catch (err) {
    throw new Error(`Payment service unreachable — action blocked: ${err}`);
  }

  // Path 3: Tool returned an error (e.g., auth failure, wallet not initialised)
  if (result.isError) {
    throw new Error(
      `check_spending failed — action blocked: ${JSON.stringify(result.content)}`
    );
  }

  // Path 4: Parse and validate the response shape
  let remaining: number;
  try {
    const parsed = JSON.parse(
      (result.content as Array<{ text: string }>)[0].text
    );
    if (!Number.isFinite(parsed?.remaining)) {
      throw new TypeError("missing or non-finite 'remaining' field");
    }
    remaining = parsed.remaining;
  } catch (err) {
    throw new Error(
      `check_spending returned unexpected format — action blocked: ${err}`
    );
  }

  // Path 5: Budget exceeded
  if (remaining < apiCost) {
    throw new Error(
      `Budget exceeded: need $${apiCost} but only $${remaining} remaining`
    );
  }
}

main().catch((err) => {
  console.error(err);
  process.exitCode = 1;
});

Best Practices

• Set budgets before delegation: When spawning sub-agents, attach a SpendingPolicy via your orchestration layer. Never give an agent unlimited spend.
• Pin your dependencies: Always specify an exact version in your MCP config (e.g., agentwallet-sdk@6.0.0). Verify package integrity before deploying to production.
• Audit trails: Use list_transactions in post-task hooks to log what was spent and why.
• Fail closed: If the payment tool is unreachable, block the paid action — don't fall back to unmetered access.
• Pair with security-review: Payment tools are high-privilege. Apply the same scrutiny as shell access.
• Test with testnets first: Use Base Sepolia for development; switch to Base mainnet for production.

Production Reference

• npm: agentwallet-sdk
• Merged into NVIDIA NeMo Agent Toolkit: PR #17 — x402 payment tool for NVIDIA's agent examples
• Protocol spec: x402.org