MCP server PayStage

@paystage/mcp

PayStage umbrella MCP server: payment preflight, auth watch, HIP-3 trade preflight, and velocity checks.

Not financial advice. The trade preflight tool is a local policy gate only — it does not place orders or recommend trades.

Published on npm as @paystage/mcp — see the npm page for the current version rather than a number here that can go stale.

What a valid receipt means

paystage-verify returning exit 0 means the key it verified against signed this receipt and it has not been altered. That is all it means — and which key that was matters:

• key_source: "published" → PayStage's published key: the issuer signed it.
• key_source: "supplied" (you passed --key or --key-url) → the key you named signed it, which is a weaker claim and not evidence about PayStage at all. Exit 0 on that path says nothing more than that the holder of that key produced the receipt.

It does not mean the payment happened — settlement is a separate on-chain event. It does not mean the verdict was correct — a receipt records a decision, not the soundness of the policy behind it. And it does not identify the caller; no caller identity is bound into the payload, by design.

Two narrower points a careful reader should know:

• key_source tells you where the verifying key came from: published (PayStage's endpoint) or supplied (someone passed --key). A receipt verified against a supplied key proves only that the holder of that key signed it.
• A receipt with signed: false is still useful: the request and policy digests are checkable, so "over these inputs" survives even when provenance does not.
• --max-age is your policy, not the issuer's. Nothing in the signed payload says how long a verdict is meant to hold, so a freshness bound is set by the verifier. If "this verdict is good for 24 hours" ever becomes the issuer's promise it has to be signed, and that is a format change.

Install

npx -y -p @paystage/mcp paystage-mcp

The server waits silently for an agent app to connect. That's normal; press Ctrl+C to exit. To try the tools without an app, run npx @modelcontextprotocol/inspector npx -y -p @paystage/mcp paystage-mcp.

Or:

npm install @paystage/mcp

The package exposes a stdio MCP server (bin: paystage-mcp).

MCP configuration

{
  "mcpServers": {
    "paystage": {
      "command": "npx",
      "args": ["-y", "-p", "@paystage/mcp", "paystage-mcp"],
      "env": {
        "PAYSTAGE_POLICY_PATH": "./policy.json"
      }
    }
  }
}

Environment variables (all optional):

• PAYSTAGE_POLICY_PATH — path to policy.json spending rules
• PAYSTAGE_RPC_URL — Base RPC URL override

Tools

evaluate_mcp_payment — Available now

Evaluates a proposed x402 payment against a local policy file and returns a verdict before the agent signs anything.

list_outstanding_auths — Available now

Lists outstanding ERC-20, Permit2, and EIP-3009 authorizations.

evaluate_outstanding_auths — Available now

Flags outstanding authorizations that need attention (e.g. authorized but never settled).

evaluate_hip3_trade — Available now

Evaluates a proposed HIP-3 / trade.xyz order against a local TradePolicy and returns a decision: allow | review | do_not_trade.

Backed by @paystage/trade-preflight (published separately), which can also be run standalone:

npx -y @paystage/trade-preflight

The policy is passed as a tool argument; no extra environment variables are required for this tool.

Packages

Package	Version	Purpose
@paystage/mcp	Umbrella server (5 tools)
@paystage/preflight	x402 payment preflight
@paystage/auth-watch	Outstanding-auth tools
@paystage/trade-preflight	HIP-3 / trade.xyz trade preflight
@paystage/velocity	Rate / spend-pattern checks
@paystage/policy	Shared policy schema + loader

Verify a receipt

Every verdict can carry a signed receipt. paystage-verify checks one without trusting us — it fetches the published key and verifies the signature over the canonical payload.

npx -y -p @paystage/mcp paystage-verify receipt.json
cat receipt.json | npx -y -p @paystage/mcp paystage-verify -

Exit codes are distinct on purpose:

Code	Meaning
0	signature matches the payload
1	invalid — altered, or signed by a different key
2	unsigned — a record of the decision, with no proof of who made it
3	unverifiable — the key could not be fetched, or an input file could not be read, so nothing can be checked
4	inputs-mismatch — the receipt is genuine but does not cover the --request/--policy you supplied
5	stale — the receipt is genuine but older than --max-age allows

"Invalid" and "could not check" are different facts and must not collapse into one failure. --key <file> verifies against a local JWK for point-in-time checks.

Supply chain and access

This is the umbrella, so its install tree is wider than any single tool and Socket flags it accordingly. What it actually contains:

Flag	Where it comes from
Network access, URL strings	@paystage/auth-watch (Base RPC reads) and @paystage/trade-preflight (one Hyperliquid info query).
Filesystem access	@paystage/policy reading your policy file.
Environment variable access	PAYSTAGE_POLICY_PATH, PAYSTAGE_RPC_URL.
"AI-detected code anomaly"	The official @modelcontextprotocol/server SDK's bundled JSON-schema validator, not PayStage code.

The umbrella itself is ~16 KB with no network, no filesystem and no eval — the capability lives in the dependencies, because that is what the tools are for.

Install a narrower surface if you only need part of it: @paystage/preflight and @paystage/policy make no network calls at all.

Full table: SECURITY.md.

Links

• Verify receipts: npx -y -p @paystage/mcp paystage-verify <receipt.json>

• Website: paystage.xyz

• HIP-3 context: docs.trade.xyz

• Source: github.com/base-buzz/paystage

• X: x.com/paystage_xyz

License

MIT