@steamlink/server

x402 payment middleware for Nexus game endpoints — monetize HTTP routes, settle on Base.

What it is

@steamlink/server puts an x402 paywall in front of your HTTP endpoints. Use it to charge entry fees, paid actions, or any monetized route: a request with no payment gets a 402 challenge; a request carrying a valid redemption is verified on-chain and allowed through.

The verification work sits behind a FacilitatorAdapter port, so your route code never touches a concrete provider. The default DelegationFacilitator is delegation-aware: instead of demanding a fresh payment signature, it redeems the player's existing session delegation — the single ERC-7710 grant they signed once at joinRoom(), bounded by their on-chain budget caveats. Payments settle in USDC on Base, and the token address is resolved from relayer capabilities (never hardcoded).

The middleware is framework-agnostic at its core, with thin adapters for Express and Hono.

Install

npm install @steamlink/server

Key exports

Monetize middleware

• monetize — the canonical middleware factory (defaults to the Express adapter).
• monetizeExpress(opts, runtime?) — Express middleware: 402 on missing/invalid payment, attaches req.settlement and calls next() on success.
• monetizeHono(opts, runtime?) — Hono middleware: returns the 402 JSON, or stashes the settlement via c.set("settlement", …) and calls next() on success.
• createMonetizeHandler(opts, runtime?) — the framework-agnostic handler the adapters wrap; returns (req) => Promise<MonetizeResult>.
• statusForError(err) — maps a NexusError to the HTTP status the middleware uses.
• PAYMENT_HEADER ("x-payment"), PAYER_HEADER ("x-payer") — header names by convention.

Types: MonetizeOptions, MonetizeRuntime, MonetizeRequest, MonetizeResult (Challenge402Result | RejectResult | PassResult), and the per-framework types ExpressMiddleware / ExpressRequestLike / ExpressResponseLike / ExpressNext and HonoMiddleware / HonoContextLike / HonoNext.

Facilitator (the x402 seller side)

• FacilitatorAdapter — the port: challenge(req) builds the 402 body and mints a single-use nonce; verify(redemption) confirms settlement on Base (idempotent on the nonce).
• DelegationFacilitator — the default delegation-aware adapter.
• DelegationFacilitatorConfig — its config (capabilities resolver, Base public client, nonce store, TTL, min confirmations, recipient authorization).
• DEFAULT_MIN_CONFIRMATIONS.
• Port data types: PaymentRequest, Challenge402, Redemption, Settlement.

Settlement verification

• verifyTransferOnChain(params) — reads the receipt and confirms the ERC-20 transfer.
• Types: ReceiptReaderClient, TransactionReceiptLike, LogLike, VerifyTransferParams.

Nonce store (replay protection)

• InMemoryNonceStore, randomNonce, DEFAULT_NONCE_TTL_MS.
• Types: NonceStore, NonceRecord.

Usage

monetize() returns Express middleware. The runtime supplies the default facilitator selected by facilitator: "nexus". The price/asset config is set per-route:

import express from "express";
import { monetize, DelegationFacilitator } from "@steamlink/server";

const facilitator = new DelegationFacilitator({
  // capabilities are the source of truth for the token address + targetAddress
  capabilities: () => relayer.getCapabilities(),
  publicClient, // a viem public client on Base
});

const app = express();
app.use(express.json());

app.post(
  "/rooms/:id/join",
  monetize(
    {
      price: "5", // human units, e.g. 5 USDC
      token: "USDC", // resolved to an address from capabilities
      chain: "base", // Base only
      recipient: "0xPotOrSeller", // must be in the payer's budget caveat
      facilitator: "nexus", // use the default DelegationFacilitator from runtime
      reason: "Room entry fee",
    },
    { defaultFacilitator: facilitator },
  ),
  (req, res) => {
    // Payment verified — req.settlement holds the on-chain Settlement.
    res.json({ joined: true, txHash: req.settlement!.txHash });
  },
);

For Hono, use monetizeHono(...) with the same MonetizeOptions; the verified settlement is read with c.get("settlement"). You can also pass a concrete FacilitatorAdapter directly as facilitator instead of the "nexus" literal.

The middleware expects an authenticated payer (the player's smart account), resolved from req.payer or the x-payer header set by the gateway's auth layer. A redemption is bound to that payer before any settlement is accepted — it is not a bearer token.

Part of Nexus

This is the library for putting x402 in front of your endpoints. It pairs with:

• @steamlink/core — game definition, ECS, client, and the delegation engine.
• @steamlink/relayer — the 1Shot relayer client whose capabilities supply the payment token address and targetAddress.

Base only. chain is strictly "base" and settlement happens in USDC on Base.