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# MCP and JSON-RPC transport \[Payment flows for AI tool calls]

The [Model Context Protocol](https://modelcontextprotocol.io) (MCP) transport enables payments for AI tool calls using JSON-RPC.

## Overview

AI agents use MCP to call tools on remote servers. The MCP transport allows these tool calls to require payment, enabling autonomous agent-to-service payments.

| MPP Concept | MCP Encoding |
|-------------|--------------|
| Challenge | JSON-RPC error code `-32042` |
| Credential | `_meta.org.paymentauth/credential` |
| Receipt | `_meta.org.paymentauth/receipt` |

## Challenge

Payment requirements are signaled using JSON-RPC error code `-32042`:

```json
{
  "jsonrpc": "2.0",
  "id": 1,
  "error": {
    "code": -32042, // [!code highlight]
    "message": "Payment Required",
    "data": {
      "httpStatus": 402,
      "challenges": [{ // [!code highlight]
        "id": "ch_abc123",
        "realm": "search.example.com",
        "method": "tempo",
        "intent": "charge",
        "request": {
          "amount": "10",
          "currency": "usd",
          "recipient": "0xa726a1..."
        }
      }]
    }
  }
}
```

## Credential

Credentials are passed in the `_meta` field of the tool call:

```json
{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "tools/call",
  "params": {
    "name": "web-search",
    "arguments": {"query": "MCP protocol"},
    "_meta": { // [!code highlight]
      "org.paymentauth/credential": { // [!code highlight]
        "challenge": { ... },
        "source": "0x1234...",
        "payload": { "signature": "0xabc..." }
      }
    }
  }
}
```

## Receipt

Receipts are returned in the result `_meta`:

```json
{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "content": [{ "type": "text", "text": "Search results..." }],
    "_meta": { // [!code highlight]
      "org.paymentauth/receipt": { // [!code highlight]
        "status": "success",
        "challengeId": "ch_abc123",
        "method": "tempo"
      }
    }
  }
}
```

## Comparison with HTTP

| Aspect | HTTP | MCP |
|--------|------|-----|
| Challenge delivery | `WWW-Authenticate` header | JSON-RPC error `-32042` |
| Credential delivery | `Authorization` header | `_meta.org.paymentauth/credential` |
| Receipt delivery | `Payment-Receipt` header | `_meta.org.paymentauth/receipt` |
| Encoding | Base64url in headers | Native JSON in body |

## Example flow

:::steps
### <div className="mr-2">Agent</div> Call a tool

```json
{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "web-search",
    "arguments": {"query": "MCP payments"}
  }
}
```

### <div className="mr-2">Server</div> Return payment Challenge

Respond with error code `-32042`:

```json
{
  "jsonrpc": "2.0",
  "id": 1,
  "error": {
    "code": -32042,
    "message": "Payment Required",
    "data": {
      "challenges": [{ "id": "ch_abc", "method": "tempo", ... }]
    }
  }
}
```

### <div className="mr-2">Agent</div> Retry with Credential

Include the Credential in `_meta`:

```json
{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "tools/call",
  "params": {
    "name": "web-search",
    "arguments": {"query": "MCP payments"},
    "_meta": {
      "org.paymentauth/credential": { ... }
    }
  }
}
```

### <div className="mr-2">Server</div> Return result with Receipt

```json
{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "content": [{ "type": "text", "text": "Results..." }],
    "_meta": {
      "org.paymentauth/receipt": { "status": "success", ... }
    }
  }
}
```
:::

## Full specification

[IETF Specification](https://paymentauth.org) — Read the full specification