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

# API Introduction

> Build billing, metering, entitlement, credit, and margin workflows with Nozle

Nozle provides two complementary HTTP API surfaces:

| Base URL                        | Use it for                                                                                                   |
| ------------------------------- | ------------------------------------------------------------------------------------------------------------ |
| `https://core.nozle.app/api/v1` | Customers, plans, subscriptions, usage events, invoices, wallets, payments, taxes, and billing configuration |
| `https://api.nozle.app/api/v1`  | Checkout, product credits, entitlements, margin intelligence, and other Nozle extensions                     |

The complete Core API reference is generated from Nozle's OpenAPI contract. Nozle-specific workflow endpoints are documented alongside it.

## Typical integration

1. Create or update a customer using your application's stable external identifier.
2. Define typed Features and plans in the dashboard or Core API.
3. Create a subscription or start a payment-gated checkout.
4. Send idempotent usage events from your backend.
5. Process signed webhooks and reconcile subscription, invoice, and payment state.

## Authentication

Server endpoints require a secret key in the Bearer authorization header:

```http theme={null}
Authorization: Bearer sk_nozle_your_secret_key
```

Publishable keys are limited to the public plan catalog. Never expose a secret key in browser or mobile code. See [Authentication](/api/authentication) for the full trust-boundary model.

## Identifiers

Use `external_id`, `external_customer_id`, and `external_subscription_id` for identifiers owned by your application. Treat Nozle-generated UUIDs as opaque values and preserve exact compatibility field names shown in each endpoint schema.

## Next steps

* Review [API Standards](/api/api-standards) for requests, pagination, errors, and idempotency.
* Start with [Customers](/api/create-customer), [Plans](/api/list-plans), or [Usage Events](/api/events).
* Use the [Node.js SDK](/sdks/node/installation) or [Python SDK](/sdks/python/installation) for typed server integrations.