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@overdraft-protocol/mpx

A transport-safe, in-band payment extension for MCP servers.

Standard MCP payment approaches using HTTP headers (e.g. x402's X-PAYMENT-REQUIRED) are invisible to agents: the MCP client transport swallows non-2xx HTTP responses before the JSON-RPC layer and never exposes arbitrary response headers to the model. This package implements the MCP Payments Extension (MPX) — a payment handshake that lives entirely inside JSON-RPC message bodies, works identically over stdio and Streamable HTTP, and is visible to any MCP-capable agent.

The protocol

All payment signaling travels in _meta fields on JSON-RPC messages, using the reserved namespace mpx/v1.*. No HTTP headers or status codes are used.

1. Challenge (server → agent)

When a tool is called without a valid payment authorization, the server returns an isError result:

{
  "isError": true,
  "content": [{ "type": "text", "text": "payment_required: ... (1.50 USDC). Retry with _meta authorization." }],
  "_meta": {
    "mpx/v1.challenge": {
      "mpxVersion": 1,
      "paymentRequestId": "<uuid>",
      "expiresAt": "<ISO-8601>",
      "reason": { "tool": "<tool-name>", "description": "<human-readable purpose>" },
      "amount": { "value": "1.50", "currency": "USDC", "decimals": 6 },
      "accepts": [
        {
          "rail": "x402-evm-exact",
          "payTo": "<payee address>",
          "requirements": { "...": "rail-specific requirements" }
        }
      ]
    }
  }
}

The content text duplicates the key facts so agents that don't read _meta still see a meaningful error. Machine clients read _meta for the structured challenge.

2. Authorization (agent → server)

The agent re-issues the same tools/call (identical arguments) and adds the signed authorization to params._meta:

{
  "method": "tools/call",
  "params": {
    "name": "<tool>",
    "arguments": { "...": "unchanged" },
    "_meta": {
      "mpx/v1.authorization": {
        "mpxVersion": 1,
        "paymentRequestId": "<uuid from challenge>",
        "rail": "x402-evm-exact",
        "payload": { "...": "rail-specific signed payload" }
      }
    }
  }
}

3. Receipt (server → agent)

On success, the result carries a receipt in result._meta:

{
  "_meta": {
    "mpx/v1.receipt": {
      "mpxVersion": 1,
      "paymentRequestId": "<uuid>",
      "rail": "x402-evm-exact",
      "settlementRef": "<rail-specific reference>",
      "amount": { "value": "1.50", "currency": "USDC", "decimals": 6 },
      "settledAt": "<ISO-8601>"
    }
  }
}

Security

  • Single-use on success — each paymentRequestId is consumed only after the handler completes successfully. Verification and handler failures release the challenge so the agent can retry with the same signed authorization until expiresAt. A replay after success is rejected even though the authorization still verifies cryptographically.
  • Expiry — challenges expire (default 300 seconds). The server rejects authorizations after expiresAt.
  • Verify before settle — the package verifies the authorization before calling the tool handler. The handler's settle() callback moves funds after any application-level validation (e.g. content signatures). Money never moves on an invalid request.
  • Conditional gating — intent() can return null to skip the challenge entirely for calls that don't require payment.

Why isError: true and not a proper JSON-RPC error?

A payment challenge semantically isn't a tool failure — it's a mid-execution pause requesting input. A JSON-RPC error object with a fixed code and structured data would be the right shape, but the installed MCP SDK (@modelcontextprotocol/sdk v1.29.0, current latest) catches all McpError throws from tool handlers and flattens them to a plain isError text result, dropping data entirely — except for the single special code UrlElicitationRequired.

The isError: true + _meta approach is therefore the only channel that reliably carries structured challenge data to the caller today. It is also consistent with the MCP spec, which says tool-originated errors should live in isError results so the LLM can see and react to them.

The long-term path is MCP elicitation (elicitation/create). When a PaymentAuthorizationRequired error code is added to the SDK (following the same carve-out as UrlElicitationRequired), withPayment can be updated to throw it instead of returning an isError result. No tool handler or marketplace wiring changes — the switch is entirely inside the wrapper. See the protocol design notes for a detailed analysis of why elicitation is not yet feasible.

Installation

npm install @overdraft-protocol/mpx

The x402-evm rail additionally requires x402 and viem as peer dependencies:

npm install x402 viem

Usage

1. Create the extension

import { createPaymentExtension, InMemoryChallengeStore } from '@overdraft-protocol/mpx';

import { consolePaymentLogger } from '@overdraft-protocol/mpx';

const withPayment = createPaymentExtension({
  rails: [myRail],           // PaymentRail[] — see Implementing a rail below
  store: new InMemoryChallengeStore(),  // or your durable ChallengeStore
  settlement: mySettlement,  // SettlementStrategy — what "settle" does in your app
  challengeTtlSeconds: 300,  // optional, default 300
  logger: consolePaymentLogger, // optional, default no-op — see Logging below
});

2. Wrap tool handlers

import { McpServer } from '@modelcontextprotocol/sdk/server/mcp.js';

const server = new McpServer({ name: 'my-server', version: '1.0.0' });

server.registerTool(
  'my_paid_tool',
  { inputSchema: { amount_usdc: z.number() } },
  withPayment(
    {
      tool: 'my_paid_tool',
      description: 'Service description shown in the challenge',
      intent(args) {
        return {
          amount: { value: String(args.amount_usdc), currency: 'USDC', decimals: 6 },
          payTo: '0x...',
          binding: { myAppData: 'for settlement' },
        };
      },
    },
    async (args, extra) => {
      // 1. Do your application validation here (content sig, nonce, etc.)
      // 2. Call settle() after validation passes — this is when money moves.
      const receipt = await extra.settle();
      // extra.verifiedPayment is also available for accessing the raw rail payload.
      return { content: [{ type: 'text', text: JSON.stringify({ ok: true }) }] };
    },
  ),
);

3. Argument fallback for agents that cannot set params._meta

Standard LLM harnesses only let the model control arguments — params._meta is populated by the client host. withPayment automatically checks args.payment_authorization if _meta does not contain an authorization. The value must be a JSON string of the authorization object, or the object itself:

{ "arguments": { "..": "..", "payment_authorization": "{\"mpxVersion\":1,\"paymentRequestId\":\"...\",\"rail\":\"x402-evm-exact\",\"payload\":{...}}" } }

To make payment_authorization reachable in the handler (Zod strips unknown fields), declare it in the tool's inputSchema:

inputSchema: {
  amount_usdc: z.number(),
  payment_authorization: z.string().optional().describe(
    'JSON-encoded MPX authorization. Alternative to params._meta["mpx/v1.authorization"].'
  ),
}

params._meta always takes priority if both are present.

4. Conditional gating

Return null from intent() to skip payment for calls that don't require it:

{
  tool: 'file_dispute',
  description: 'Buyer dispute stake',
  async intent(args) {
    const needsPayment = await checkIfPaymentRequired(args.bid_id);
    if (!needsPayment) return null;  // no challenge issued, handler called directly
    return { amount: ..., payTo: ..., binding: ... };
  },
}

intent() may be synchronous or async.

Implementing a rail

A PaymentRail has two required responsibilities: building the offer shown in the challenge, and verifying a signed authorization. It never settles — settlement is an injected SettlementStrategy. The core is fully rail-agnostic: it knows nothing about x402, EVM, cards, or any specific scheme.

import type { PaymentRail, PaymentIntent, VerifiedAuthorization } from '@overdraft-protocol/mpx';
import type { RailOffer } from '@overdraft-protocol/mpx';

const myRail: PaymentRail = {
  id: 'my-rail',

  buildOffer(intent: PaymentIntent): RailOffer {
    return {
      rail: 'my-rail',
      payTo: intent.payTo,
      requirements: {
        // rail-specific fields the payer needs to sign
        amount: intent.amount.value,
        currency: intent.amount.currency,
      },
    };
  },

  async verify(payload: unknown, offer: RailOffer): Promise<VerifiedAuthorization> {
    // verify the signed payload against the offer — throw if invalid
    const verified = await myVerifyFn(payload, offer);
    return {
      rail: 'my-rail',
      amount: { value: verified.amount, currency: 'USDC', decimals: 6 },
      raw: verified,  // passed to SettlementStrategy.settle()
    };
  },
};

Optional rail hooks (agent ergonomics)

A rail can own its agent-facing details so they never leak into the core. All are optional — a rail that omits them still works, falling back to generic behaviour.

Hook Purpose
coerceAuthorization(raw, hints) Normalize loosely-shaped agent input (the common case: a JSON blob in a payment_authorization argument because the harness can't write params._meta) into a schema-valid MPX authorization. The core tries each rail's coercer in order and keeps the first result that validates.
retryInstructions(challenge) Rail-specific "how to pay" text appended to the challenge content.
describePayload(payload) Redact a signed payload to a safe summary for the structured logger (never log secrets/signatures in full).
authorizationArgDescription Description for the payment_authorization tool argument, surfaced by apps in their inputSchema.

The cleanest reference implementation of all of these is the dev-signature rail (src/rails/dev-signature/index.ts) — it's zero-dependency and self-contained; copy it as a starting point.

Bundled rails

Three rails ship with the package: dev-signature (zero-dependency, for dev/CI/reference), x402-evm-exact (EVM stablecoin payments), and stripe-card (card payments via Stripe) — proof the core is rail-agnostic across both crypto and traditional rails.

dev-signature (reference / dev / CI)

A zero-dependency rail at @overdraft-protocol/mpx/rails/dev-signature. "Authorization" is an HMAC-SHA256 over the offer terms with a shared secret, standing in for a wallet signature. It moves no real funds — use it for local development, demos, CI, and as the template for a real rail.

import { createDevSignatureRail, signDevAuthorization } from '@overdraft-protocol/mpx/rails/dev-signature';

const rail = createDevSignatureRail({ secret: process.env.DEV_PAY_SECRET! });
// A payer signs an offer with: signDevAuthorization(secret, challenge.accepts[0])

Implementing a SettlementStrategy

import type { SettlementStrategy, VerifiedAuthorization, SettlementRef } from '@overdraft-protocol/mpx';

const mySettlement: SettlementStrategy = {
  async settle(verified: VerifiedAuthorization, binding: unknown): Promise<SettlementRef> {
    const b = binding as MyAppBinding;  // narrow the opaque binding here
    const ref = await myOnChainDeposit(verified.raw, b.orderId);
    return { ref };
  },
};

The binding parameter is exactly what you returned in intent(). The package treats it as opaque (unknown) so it never leaks application concerns into the generic layer.

Implementing a ChallengeStore

The default InMemoryChallengeStore is suitable for single-process servers and tests. For production, implement ChallengeStore backed by a database:

import type { ChallengeStore, ChallengeRecord } from '@overdraft-protocol/mpx';

class MyDurableChallengeStore implements ChallengeStore {
  async save(record: ChallengeRecord): Promise<void> {
    await db.insert('payment_challenges', {
      id: record.challenge.paymentRequestId,
      expires_at: record.challenge.expiresAt,
      data: JSON.stringify(record),
    });
  }

  async get(paymentRequestId: string): Promise<ChallengeRecord | undefined> {
    const row = await db.get('payment_challenges', { id: paymentRequestId, consumed: false });
    if (!row || new Date(row.expires_at) < new Date()) return undefined;
    return JSON.parse(row.data);
  }

  async consume(paymentRequestId: string): Promise<ChallengeRecord | undefined> {
    // Must be atomic — only one caller should succeed
    const record = await this.get(paymentRequestId);
    if (!record) return undefined;
    await db.update('payment_challenges', { consumed: true }, { id: paymentRequestId });
    return record;
  }

  async release(record: ChallengeRecord): Promise<void> {
    if (new Date(record.challenge.expiresAt) < new Date()) return;
    await db.update('payment_challenges', { consumed: false, data: JSON.stringify(record) }, {
      id: record.challenge.paymentRequestId,
    });
  }
}

x402-evm-exact (production EVM stablecoins)

A generic x402 EVM rail at the @overdraft-protocol/mpx/rails/x402-evm subpath. It requires x402 and viem as peer dependencies, imported dynamically inside verify() so the core compiles and runs without them when this rail isn't used.

import { createX402EvmRail } from '@overdraft-protocol/mpx/rails/x402-evm';
import { createPublicClient, http } from 'viem';
import { base } from 'viem/chains';

const rail = createX402EvmRail({
  publicClient: createPublicClient({ chain: base, transport: http() }),
  assetAddress: '0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913',  // USDC on Base
  network: 'base',
  chainId: 8453,        // optional — included in offers for EIP-712 wallets
  currencySymbol: 'USDC',
  decimals: 6,
  // Optional EIP-712 token metadata (defaults shown — for USDC):
  assetName: 'USD Coin',
  assetVersion: '2',
});

This rail handles buildOffer (constructs x402 PaymentRequirements) and verify (exact.evm.verify — no on-chain action). It also wires the optional hooks (coerceAuthorization, retryInstructions, describePayload, authorizationArgDescription), exported individually as coerceX402Authorization, x402RetryInstructions, and X402_AUTHORIZATION_ARG_DESCRIPTION. Settlement is always injected: the rail has no depositBid, no Escrow.sol, no application knowledge.

stripe-card (production cards)

A card rail at @overdraft-protocol/mpx/rails/stripe, proving the core works for traditional payments, not just crypto. stripe is an optional peer dependency, imported dynamically only when you pass a secretKey (inject a stripe client directly for tests).

import { createStripeRail } from '@overdraft-protocol/mpx/rails/stripe';

const rail = createStripeRail({
  secretKey: process.env.STRIPE_SECRET_KEY!,  // or: stripe: new Stripe(key)
  currency: 'usd',
  publishableKey: process.env.STRIPE_PUBLISHABLE_KEY, // optional, for client confirmation
});

It maps cleanly onto the small rail interface, and shows the two enhancements that make non-crypto rails possible:

  1. buildOffer(intent) is async — it creates a Stripe PaymentIntent (capture_method: 'manual') and returns its id + client_secret in requirements. (buildOffer may return a promise; the core awaits it.)
  2. verify(payload, offer) — payload carries the confirmed PaymentIntent id; the rail retrieves it and asserts status === 'requires_capture' and that amount/currency match the offer. No money moves — an uncaptured authorization is just a hold.
  3. Settlement stays injected — your SettlementStrategy.settle() calls stripe.paymentIntents.capture(id). This is what preserves verify-before-settle: the hold is verified before the handler runs, capture happens only after the handler validates and calls extra.settle().
  4. Optional hooks are wired: coerceAuthorization accepts { paymentRequestId, paymentIntentId } shorthand; retryInstructions tells the agent how to confirm the PaymentIntent; describePayload logs only the id.

The single invariant every rail must uphold: verify() proves funds are committed but moves nothing; only the injected settle() moves money. Everything else is rail-specific.

Try it against Stripe test mode — examples/stripe-integration.ts runs the whole loop (create → confirm with a test card → verify → capture) against real Stripe test APIs:

npm i stripe
export STRIPE_SECRET_KEY=sk_test_...   # test mode only; the script refuses live keys
npx tsx examples/stripe-integration.ts

Confirmation & settlement helpers

Settlement stays injected into createPaymentExtension — the core never moves money itself, and apps with bespoke settlement (escrow, ledgers, the marketplace) implement SettlementStrategy directly. But for the common cases you don't have to write it: each rail ships an opt-in settlement strategy and a payer-side helper (signing/confirming an offer), so both ends are batteries-included.

Rail Payer helper (client side) Settlement strategy (server side)
dev-signature signDevAuthorization(secret, offer) devSignatureSettlement (no-op)
x402-evm-exact signX402Authorization({ account, offer }) createX402TransferSettlement({ wallet }) — bare USDC transfer via exact.evm.settle
stripe-card confirmStripePaymentIntent(stripe, offer, { paymentMethod }) createStripeCaptureSettlement(stripe) — captures the hold 
import { createStripeRail, createStripeCaptureSettlement } from '@overdraft-protocol/mpx/rails/stripe';

const stripe = new Stripe(key);
const withPayment = createPaymentExtension({
  rails: [createStripeRail({ stripe, currency: 'usd' })],
  store,
  settlement: createStripeCaptureSettlement(stripe),  // ← shipped, no custom code
});

Settlement strategies that need the original offer (e.g. x402 needs the verified PaymentRequirements) receive it via the third settle(verified, binding, context) argument — context.offer. The argument is additive: existing two-parameter strategies keep working unchanged.

Example server

examples/stdio-server.ts is a minimal MCP server with one paid tool, using the zero-dependency dev-signature rail — no chain, keys, or network. Run it as a real stdio server, or self-contained:

# real stdio MCP server (connect any MCP client / inspector):
npx tsx examples/stdio-server.ts

# self-contained demo — drives itself and prints challenge → sign → pay → receipt:
npx tsx examples/stdio-server.ts --demo

Swapping in a real rail is the same wiring: replace the rail + settlement with createX402EvmRail/createX402TransferSettlement or createStripeRail/createStripeCaptureSettlement.

Logging

The core never writes to console — it emits structured PaymentLogEvents to an injected PaymentLogger:

import { createPaymentExtension, consolePaymentLogger, type PaymentLogger } from '@overdraft-protocol/mpx';

// Built-ins: noopPaymentLogger (default), consolePaymentLogger.
// Or forward to your own structured logger:
const logger: PaymentLogger = {
  log(event) { myLogger.info({ mcpPayment: event }); },
};

const withPayment = createPaymentExtension({ rails, store, settlement, logger });

Event types: challenge_issued, authorization_received, authorization_parse_failed, verify_started, verify_succeeded, verify_failed, challenge_not_found, settled. Payload fields are redacted by the rail's describePayload — the core never logs raw signatures.

API

createPaymentExtension(config)

Returns a withPayment(spec, handler) function. Config:

Field Type Description
rails PaymentRail[] Supported payment rails, in preference order
store ChallengeStore Challenge persistence (use InMemoryChallengeStore for dev/tests)
settlement SettlementStrategy What settle() does — injected by the application
challengeTtlSeconds number? Challenge TTL in seconds (default: 300)
logger PaymentLogger? Structured event sink (default: no-op). See Logging

withPayment(spec, handler)

Returns an AnyToolHandler suitable for passing to server.registerTool().

The handler receives (args, extra) where extra is augmented with:

Field Type Description
extra.verifiedPayment VerifiedAuthorization | undefined Verified rail payload; undefined when call is not gated
extra.settle() () => Promise<SettlementRef | undefined> Call after validation to move funds; idempotent; returns undefined when not gated

PaymentSpec

Field Type Description
tool string Tool name (shown in challenge reason)
description string Human-readable payment purpose
intent(args) PaymentIntent | null | Promise<...> Return a PaymentIntent to gate the call; null to skip payment

PaymentIntent

Field Type Description
amount MpxAmount { value, currency, decimals }
payTo string Payee address/account
binding unknown App-specific data passed unchanged to SettlementStrategy.settle()
railHints Record<string, unknown>? Opaque hints forwarded to rail.buildOffer()

Boundary rules

This package has a hard dependency boundary: it must never import from application code. It depends only on @modelcontextprotocol/sdk and zod (plus x402/viem inside the optional x402 rail subpath). All application concerns — persistence, settlement, gating logic, chain access — are injected through interfaces.

The package tsconfig.json has no baseUrl/paths aliases so it is structurally impossible to import application modules. The eslint.config.js adds a no-restricted-imports rule as a second line of defense.

License

MIT

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Transport-safe, in-band payment extension for MCP servers (MCP Payments Extension / MPX). Rail-agnostic core with pluggable settlement rails.

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