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Facet UCP Shopping

by llc.facet

create_checkout

Open a checkout session for one or more catalog line items (or promote a cart with cart_id). Returns the session plus server-resolved payment_handlers (rail, pay_to, exact amount) to satisfy: the price and pay_to are the MERCHANT's, sealed here, never read from your request. Physical goods REQUIRE a fulfillment ship-to (else FULFILLMENT_REQUIRED), sealed at create. REQUIRES IDENTITY on a signature-required Terminal: a Facet KYA as Authorization: Bearer <kya>.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
cart_idNoPromote an existing cart: its stored lines drive the checkout and any line_items are ignored (the business must use the cart contents).
line_itemsNoItems to buy, each `{ item: { id }, quantity }`. Omit when using cart_id.
fulfillmentNoShip-to destination. REQUIRED for physical products; sealed at create.

TDQS

A4.7/5.0
Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations, the description carries the full burden. It discloses critical behaviors: server-resolved payment_handlers, merchant-sealed price/pay_to, FULFILLMENT_REQUIRED for physical goods, and identity/auth requirements. This goes far beyond basic 'create' wording.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Three dense sentences, each delivering unique information. Front-loaded with the core action, followed by high-value behavioral details. No wasted words.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given no annotations and no output schema, the description covers the essential selection and invocation context: what it creates, key return value (payment_handlers), constraints, and auth. Complexity is moderate, and the description addresses it well.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so baseline is 3. The description adds meaning by explaining cart_id promotion semantics, line_items structure, and the requirement for fulfillment on physical goods, enhancing what the schema already provides.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific action ('Open a checkout session') on a specific resource, and distinguishes it from related tools like create_cart ('or promote a cart with cart_id') and get_checkout. The verb+resource combination is clear and unambiguous.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

It explains when to use this tool versus alternatives (line items vs cart promotion) and provides context for physical goods requiring fulfillment. It doesn't explicitly name sibling tools or exclude scenarios, but the usage context is clearly implied.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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TDQS

A3.6/5.0
Disambiguation3/5

Most tools map to a distinct lifecycle step, and the detailed descriptions separate discovery, cart, checkout, payment, order, and wishlist concerns well. However, get_product and lookup_catalog are largely the same lookup by identifier, discover_products and search_catalog overlap across scope, and update_checkout is actually a read-only no-op.

Naming Consistency4/5

The set overwhelmingly follows a verb_noun pattern: create_cart, get_checkout, cancel_checkout, complete_checkout, search_catalog. Minor deviations are the wishlist_* resource-first subgroup and the misleading update_checkout, which does not perform an update.

Tool Count3/5

At 23 tools, this sits at the heavy end of the range, though the broad commerce lifecycle does justify substantial surface area. The count is inflated by redundant product lookup tools and a no-op update_checkout, so a more trimmed set in the low-to-mid teens would be healthier.

Completeness4/5

The server covers the full shopper lifecycle well: discovery, product lookup, cart, checkout, payment, order placement, order read, refunds, and wishlist. Gaps are minor and workable, such as update_checkout not actually supporting updates and no explicit order-cancellation path once a checkout is complete.

Resources