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# Payments (x402)

Go Micro can require a payment before a tool runs, using [x402](https://x402.org) — the open HTTP **402 Payment Required** standard for stablecoin payments, designed for AI agents and onchain APIs. It lets every Go Micro endpoint, already exposed as an AI-callable tool, become a *paid* tool: a service answers a call with `402` and payment requirements, the client pays and retries, and the gateway verifies the payment before serving.

Payments are **opt-in** and **dependency-light**. Go Micro carries no chain or crypto code — it speaks the protocol and delegates verification and settlement to a pluggable **facilitator** (Coinbase CDP, Alchemy, or self-hosted), so Base and Solana are just different facilitators behind one interface.

## The wrapper

The core is HTTP middleware in `go-micro.dev/v6/wrapper/x402`:

```go
import "go-micro.dev/v6/wrapper/x402"

pay := x402.Middleware(x402.Config{
    PayTo:          "0xYourAddress",            // where payments go (required)
    Network:        "base",                     // or "solana", ...
    Amount:         "10000",                    // smallest units, e.g. 0.01 USDC
    FacilitatorURL: "https://facilitator.example",
})
mux.Handle("/paid", pay(handler))
```

A request with no `X-PAYMENT` header gets a `402` with the requirements; once a payment verifies through the facilitator, the request is served (with settlement details on the `X-PAYMENT-RESPONSE` header).

### Pluggable facilitator

`Config.Facilitator` is an interface; the default is an `HTTPFacilitator` pointed at `FacilitatorURL`. Implement your own to target any chain or hosted service:

```go
type Facilitator interface {
    Verify(ctx context.Context, payment string, req Requirements) (Result, error)
}
```

## At the MCP gateway

Because every endpoint is already an MCP tool, the gateway is where you charge. Payments are wired into both `micro mcp serve` and the standalone `micro-mcp-gateway`, gated on `/mcp/call` (listing tools and health stay free), and **off unless you set a pay-to address**.

```bash
micro mcp serve --address :3000 \
    --x402_pay_to 0xYourAddress \
    --x402_network solana \
    --x402_amount 10000 \
    --x402_facilitator https://facilitator.example
```

## A shoppable catalog

When payments are enabled, `/mcp/tools` advertises each priced tool's payment requirements, so an agent can see the cost before calling and choose by price — the catalog is shoppable, not just discoverable:

```json
{
  "tools": [
    { "name": "weather.Weather.Forecast", "description": "...",
      "payment": { "amount": "10000", "network": "solana", "asset": "USDC", "payTo": "0x…" } },
    { "name": "time.Time.Now", "description": "..." }
  ]
}
```

Free tools carry no `payment` block. This is the foundation for a tool marketplace: offering a tool is registering a priced service; using it is list → choose → call → pay.

## Per-tool amounts

Different tools can cost different amounts. Pricing is an **operator** concern — the payTo address is the operator's, and amounts change without redeploying anyone's service — so it's configured at the gateway with a file, the same way per-tool scopes and rate limits are. Point the gateway at an x402 config:

```bash
micro mcp serve --address :3000 --x402_config x402.json
```

```json
{
  "payTo": "0xYourAddress",
  "network": "solana",
  "asset": "USDC",
  "amount": "0",
  "amounts": {
    "weather.Weather.Forecast": "10000",
    "search.Search.Query": "5000"
  }
}
```

`amount` is the default (here `"0"` — free unless priced), and `amounts` sets per-tool overrides keyed by tool name. There is no "pricing" abstraction; it's the x402 `amount`, resolved per tool, in the protocol's own vocabulary. `micro mcp serve` accepts the file via `--x402_config`; the standalone gateway accepts the same file via `--x402-config` or the `X402_CONFIG` environment variable.

## Paying for tools (the consumer side)

The counterpart to the server middleware is `x402.Client` — an HTTP client that settles 402 challenges automatically, up to a **spend budget**. This is the safety piece for an autonomous caller: it pays what a tool requires, but refuses (before paying) once a call would exceed the budget.

```go
c := &x402.Client{
    Payer:  myWallet,   // constructs the payment payload (signs with a wallet)
    Budget: 1_000_000,  // max total spend in the asset's smallest unit (0 = unlimited)
}

resp, err := c.Do(req) // a 402 is paid and retried; over-budget calls error instead
```

`Payer` is an interface (`Pay(ctx, Requirements) (payment string, error)`) — the consumer counterpart to `Facilitator`. The budget accumulates across calls, so a long-running agent can be handed a fixed allowance for a task. Budget is reserved before payment is created, which means parallel paid calls cannot race past the cap; if payment creation or verification fails, the reservation is released.

## Agent-level spend guardrail

For unattended agents, set the same cap at the agent tool-execution layer so paid tools are refused before their handler — and therefore before a payer — can run:

```go
agent := micro.NewAgent("buyer",
    micro.AgentMaxSteps(8),
    micro.AgentMaxSpend(20_000), // per Ask, smallest units
    micro.AgentToolSpend("weather.Weather.Forecast", 10_000),
)
```

`AgentMaxSpend` is disabled by default (`0`). `AgentToolSpend` records the price discovered from your shoppable MCP/x402 catalog for the tools this agent may call. When a call would exceed the per-run allowance, the result is a normal structured guardrail refusal with `Refused: "spend_budget"` and an explanatory error in the run timeline/inspect output, distinct from provider/model failures.

### Live facilitator conformance

The regular test suite uses in-process facilitators and does not need network credentials. To smoke-test a hosted facilitator, run the opt-in live conformance test with a real payment payload and matching requirements:

```sh
GO_MICRO_X402_LIVE_FACILITATOR_URL=https://facilitator.example \
GO_MICRO_X402_LIVE_PAYMENT='...' \
GO_MICRO_X402_LIVE_PAY_TO=0xYourAddress \
GO_MICRO_X402_LIVE_NETWORK=base \
GO_MICRO_X402_LIVE_AMOUNT=1 \
go test ./wrapper/x402 -run TestLiveFacilitatorConformance -count=1
```

Leave those variables unset in normal CI; the live test skips unless the facilitator URL, payment payload, and pay-to address are all provided.

## Notes

- **Opt-in.** No pay-to address (and no config), no payments — nothing changes.
- **No crypto in the framework.** The facilitator does verification and settlement on-chain; Go Micro speaks HTTP.
- **A paying agent needs a budget.** Use `AgentMaxSpend` plus `AgentToolSpend` next to `MaxSteps` and `ApproveTool` so a run has an explicit allowance before any paid tool can execute.

## See also

- [Building Effective Agents — Agents and Workflows](agents-and-workflows.html)
- [MCP & AI Agents](../mcp.html)
- [x402 — Coinbase Developer Docs](https://docs.cdp.coinbase.com/x402/welcome) · [x402 on Solana](https://solana.com/x402/what-is-x402)

## AP2 payment mandates

AP2 can authorize an x402 payment without making A2A carry settlement state. A
payment mandate records the buyer intent and names an `x402` rail reference; the
existing x402 facilitator remains responsible for payment verification and
settlement. This keeps AP2 as the signed mandate/audit layer while x402 stays the
pluggable payment rail.