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Create cart

create_cart

Create a cart (UCP cart). Not an order; nothing is charged. At most 3 per line item and 5 items per cart (quantity_limit_exceeded).

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "cart": "<cart>"
    },
    "name": "create_cart"
  }
}

Input Schema

Name	Required	Description	Default
`cart`	Yes	UCP cart payload, shape per https://ucp.dev/2026-08-25/schemas/shopping/cart.json
`meta`	No	UCP request metadata: {"ucp-agent": {"profile": "<platform profile URL>"}, "idempotency-key": "<uuid>"}.

Output Schema

Name	Required	Description	Default
`ucp`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

First observedNov 14, 2026

TDQS

A4.2/5.0

Behavior4/5

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

Annotations only cover the safety profile (readOnlyHint false, destructiveHint false, idempotentHint false); the description adds genuinely new behavior: hard limits of 3 per line item and 5 items per cart, plus the specific error code quantity_limit_exceeded the caller must handle. It does not mention cart expiry or whether creation requires an authenticated agent, which the schema's ucp-agent profile hints at but never explains.

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?

Two short sentences, front-loaded with the definition followed by the disambiguation and the failure constraint. Nothing is repeated from the schema or annotations and no sentence is wasted.

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

Completeness4/5

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

With a full output schema and annotations covering safety, the description only needs to convey constraints and framing, which it does via the order/charge disambiguation and the quantity-limit error. Missing only secondary context such as cart lifetime and auth expectations for a nested, open-ended UCP payload.

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

Parameters3/5

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

Schema description coverage is 100% and points at the external UCP cart schema plus the meta idempotency-key/profile fields, so the heavy lifting is already done structurally. The description adds no syntax or field-level meaning beyond the quantity caps, which is the expected baseline when the schema is fully documented.

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?

States a specific verb and resource ("Create a cart") and immediately scopes it against the most likely confusion with "Not an order; nothing is charged." An agent can distinguish this from create_checkout/create_order without opening any schema.

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?

The negative scoping ("not an order, nothing is charged") gives clear context for choosing cart creation over checkout/order paths, and the quantity caps signal preconditions. It stops short of explicitly naming a sibling to use instead once the user wants to pay, so it is strong but not a full when/when-not routing statement.

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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