> ## Documentation Index
> Fetch the complete documentation index at: https://docs.crossmint.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Create Checkouts Server-Side

> Create checkouts from your backend with a server key — no user JWT — for an agent that buys for itself.

The [quickstart](/agents/agent-checkouts-quickstart) runs in the browser with a **client** key and a user's JWT. If you're building a skill for **your own agent** — buying for the agent itself, not on behalf of signed-in end users — skip the JWT and call the API **server-side** with a **server key** (`sk_...`).

* Send only `X-API-KEY: sk_...` (no `Authorization` header). The key needs the same `agent-checkouts.*` scopes. Keep it secret — unlike the `ck_` key, it is not safe in the browser.
* Checkouts then run under a fixed **service subject** for your project — ideal for one agent that buys for itself.
* To create a checkout **on behalf of a specific user**, add the **`x-crossmint-user-id`** header. The subject is namespaced under your project (a server key can only address its own project's subjects); omit it and it falls back to the shared service subject.

```typescript theme={null}
const res = await fetch("https://www.crossmint.com/api/unstable/agent-checkouts", {
    method: "POST",
    headers: {
        "Content-Type": "application/json",
        "X-API-KEY": CROSSMINT_SERVER_API_KEY, // sk_... — no JWT needed
        "x-crossmint-user-id": "user_123", // optional — act on behalf of a user; omit for the shared service subject
    },
    body: JSON.stringify({
        target: { kind: "direct_url", url: "https://merchant.example/products/classic-tee" },
        constraints: { maxCost: { amount: "100.00", currency: "USD" } },
    }),
});
```

Checkouts and buyer profiles are isolated per `(project, subject)`, so resources created under one `x-crossmint-user-id` are only visible to that subject. Everything else — polling, responding to actions, reading the receipt — works exactly as in the [quickstart](/agents/agent-checkouts-quickstart).