MCP server PayStage Preflight

@paystage/preflight

Evaluate an x402 PaymentRequired against a local spending policy before your agent pays. Returns one of three verdicts: pay, review, or do_not_pay.

• Local-first — policy lives in a JSON file on your machine. No secrets, no network calls to decide.
• MCP-native — runs as a stdio MCP server so Claude Desktop, Cursor, and any MCP client can call it as a tool.
• Deterministic — same input + same policy ⇒ same verdict.

Install

Run directly with npx (no global install):

npx -y @paystage/preflight

Or add it to your MCP client config.

Claude Desktop

Add to claude_desktop_config.json:

{
  "mcpServers": {
    "paystage-preflight": {
      "command": "npx",
      "args": ["-y", "@paystage/preflight"],
      "env": {
        "PAYSTAGE_POLICY_PATH": "/absolute/path/to/policy.json"
      }
    }
  }
}

Cursor

Add to .cursor/mcp.json:

{
  "mcpServers": {
    "paystage-preflight": {
      "command": "npx",
      "args": ["-y", "@paystage/preflight"],
      "env": {
        "PAYSTAGE_POLICY_PATH": "/absolute/path/to/policy.json"
      }
    }
  }
}

Example policy.json

{
  "version": 1,
  "name": "standard",
  "allowed_networks": ["eip155:8453", "eip155:84532"],
  "allowed_assets": [
    "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
    "0x036CbD53842c5426634e7929541eC2318f3dCF7e"
  ],
  "allowed_pay_to": ["0x209693Bc6afc0C5328bA36FaF03C514EF312287C"],
  "max_amount_per_call": "1.00",
  "max_amount_per_session": "10.00",
  "require_human_above": "0.50",
  "max_timeout_seconds": 300,
  "default_verdict": "review"
}

Amounts are human-readable USDC (e.g. "0.50" = 0.50 USDC). Networks use CAIP-2 identifiers.

Verdicts

Verdict	Meaning
pay	Within policy — safe to proceed automatically.
review	Allowed but above a human threshold — ask a human before paying.
do_not_pay	Violates policy (network, asset, payTo, amount, timeout, or malformed).

Supply chain and access

Socket.dev scores this package on what it can do, not what it does with it, so the flags it raises are worth explaining rather than removing:

Flag	Why it is there
Filesystem access	Reads the policy JSON you point it at (policy_path / PAYSTAGE_POLICY_PATH). Nothing is written.
Environment variable access	PAYSTAGE_POLICY_PATH only (and PAYSTAGE_RPC_URL via auth-watch).
Network access, URL strings	None in this package. The RPC endpoints you may see in the tree belong to @paystage/auth-watch.

No shell, no eval, no telemetry, no key custody. The full table is in SECURITY.md; the deny list is published with a citation per entry in KNOWN_DRAINERS.md.

Links

• Website: https://paystage.xyz
• X: https://x.com/paystage_xyz
• Source: https://github.com/base-buzz/paystage
• License: MIT