# MCP 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
### Agent
Call a tool
```json
{
"jsonrpc": "2.0",
"id": 1,
"method": "tools/call",
"params": {
"name": "web-search",
"arguments": {"query": "MCP payments"}
}
}
```
### Server
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", ... }]
}
}
}
```
### Agent
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": { ... }
}
}
}
```
### Server
Return result with receipt
```json
{
"jsonrpc": "2.0",
"id": 2,
"result": {
"content": [{ "type": "text", "text": "Results..." }],
"_meta": {
"org.paymentauth/receipt": { "status": "success", ... }
}
}
}
```
:::
## Full specification