Merge pull request #893 from up2itnow0822/feat/agent-payment-x402-skill

feat: add agent-payment-x402 skill for autonomous agent payments

skills/agent-payment-x402/SKILL.md

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+---
+name: agent-payment-x402
+description: Add x402 payment execution to AI agents — per-task budgets, spending controls, and non-custodial wallets via MCP tools. Use when agents need to pay for APIs, services, or other agents.
+origin: community
+---
+
+# 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.
+
+```json
+{
+  "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`
+
+```typescript
+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`](https://www.npmjs.com/package/agentwallet-sdk)
+- **Merged into NVIDIA NeMo Agent Toolkit**: [PR #17](https://github.com/NVIDIA/NeMo-Agent-Toolkit-Examples/pull/17) — x402 payment tool for NVIDIA's agent examples
+- **Protocol spec**: [x402.org](https://x402.org)