# Usage Guide

Everything you need to call the OmniCart API — authentication, request and response
formats, common patterns, and error handling. This guide is split into focused pages:

- **[Authentication](/usage/authentication)** — the four auth patterns (admin JWT, secret
  API keys, partner, and customer/publishable-key), how to obtain tokens, and how auth is
  enforced per route category.
- **[Core Commerce Endpoints](/usage/core-endpoints)** — the standard commerce surface
  (products, orders, customers) beyond the routes catalogued in the reference.
- **[Common Patterns](/usage/common-patterns)** — pagination, filtering, searching,
  sorting, and selecting relations & fields.
- **[Request & Response Formats](/usage/formats)** — headers, request bodies, query and
  path parameters, and the shape of success, paginated, created, and deleted responses.
- **[Error Handling](/usage/error-handling)** — the error response format, common HTTP
  status codes, and error types.
- **[Best Practices & Examples](/usage/best-practices)** — recommendations plus complete,
  end-to-end request examples.

> **New here?** Start with [Authentication](/usage/authentication) — every request needs
> the right credentials for its route category.

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Looking for endpoint-by-endpoint details? See the [Admin API](/admin/fulfillment-orders)
reference. Running the store from the dashboard instead of the API? See the
[Operator Guides](/guides/).