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Fluid publishes two REST surfaces for carts:
  • The Checkout API is for direct REST integrations.
  • The Public SDK API is the contract behind the @fluid-app FairShare SDK.
They are two versioned front doors to the same cart engine. A cart token created through either surface identifies the same cart on the other. The surfaces are not aliases, however. Their operation names and request shapes differ, and one bundle-subscription behavior differs materially. Use one surface for each cart lifecycle. Follow the generated reference links below for the current request and response contracts.

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Choose a surface

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Distinguish the two v2025-06 APIs

Two unrelated APIs carry the same version label: The Public SDK API version names the specification, not one uniform path prefix. Most operations use /api/public/v2025-06/. The same spec also owns payment callbacks under /api/v202506/carts/, several unversioned public utilities, and a cart price-override operation under /api/carts/. Use each generated operation page as the source of truth for its exact path.

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Map equivalent operations

The table contains the 21 shared integration operations. Several operations that the Checkout API combines are separate operations on the Public SDK API.

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Do not mix surfaces on a bundle cart

Pick one surface for each cart lifecycle.Unsubscribing a bundle line produces different results. The Public SDK API reprices the bundle’s child items and releases the subscription-derived price lock. The Checkout API clears the subscription plan fields but does not reprice the bundle children or release that price lock.Moving a bundle cart between surfaces can therefore leave child pricing in a state that neither surface would have produced on its own.

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Understand the stable alias

/api/public/stable/ is a live alias for the operations mounted from the same public route set as /api/public/v2025-06/. The FairShare SDK uses the alias for realtime authentication. The alias is not blanket. The video-analytics operation uses a separate versioned route and has no stable twin. The 18 operations in this specification that sit outside /api/public/v2025-06/ are also not rewritten through the alias. Use the explicit version when you need a pinned contract. Use stable only when you deliberately want to follow the current Public SDK API version.

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Next steps

Checkout API

Build a cart directly through REST.

Public SDK API

Inspect the operations used by the FairShare SDK.

Build a headless checkout

Follow a product-to-order workflow on the Checkout API.

FairShare SDK cart methods

Use the SDK methods backed by the Public SDK API.