Trusteed Demo Store

ucp_update_checkout

Idempotent

Update a UCP checkout session. Send line_items to replace all lines (each line may name a variant_id), and/or fulfillment to set the delivery address (fulfillment.destination, which returns the shipping options) and choose one (fulfillment.selected_option_id, which adds shipping to the totals). Only allowed when status is 'incomplete'. Use ONLY for sessions created with ucp_create_checkout; for the standard flow use create_cart and select_shipping_option. Checkout guide: read the MCP resource trusteed://guides/checkout.

Input Schema

TableJSON Schema

Name	Required	Description
`meta`	No	UCP request metadata. meta['ucp-agent'].profile = URL of the platform profile; it must be an https URL without credentials (at most 2048 characters).
`currency`	No	ISO 4217 currency code (e.g. USD). Optional: defaults to the checkout's currency.
`agent_key`	No	Sandbox credential from `create_sandbox_key`. Use this when your client cannot set an `Authorization: Bearer` header.
`line_items`	No	Replacement line items (replaces all lines). Omit to leave the lines as they are.
`checkout_id`	Yes	UCP checkout session ID (valid UUID)
`fulfillment`	No	Delivery. Send destination first, then selected_option_id (or both in one call).
`idempotency_key`	No	REQUIRED for this write unless sent as the `Idempotency-Key` header or `params._meta["trusteed/idempotency-key"]`. 1-128 chars from [A-Za-z0-9._:/-]. Reuse the same value when retrying the same operation.

Output Schema

TableJSON Schema

Name	Required	Description
`id`	Yes
`label`	No
`intent`	No
`status`	Yes
`totals`	Yes
`currency`	Yes
`next_step`	No	Next-action code, not a tool name: provide_shipping_destination, refresh_shipping_rates, choose_shipping_option, ask_user_payment_method.
`created_at`	Yes
`expires_at`	No	ISO-8601 UTC instant at which this checkout SESSION stops being usable. It is not a stock reservation and not a price guarantee.
`extensions`	No	`warnings[]` with `code`, `message`, `severity`.
`line_items`	Yes
`updated_at`	Yes
`fulfillment`	No
`next_action`	No	Only when a person must approve or pay: `url`, `kind`, `retry`.
`approval_url`	No
`idempotency_key`	No
`payment_options`	No	When present, ask the person which option they want; never choose for them.
`next_step_message`	No
`expires_in_seconds`	No	Whole seconds left on this checkout SESSION at response time (0 when already past). It is not a stock reservation and not a price guarantee.

Schema Changelog

Changes observed during successful MCP inspections.

Changed5 schema fields changedOct 15, 2026

changedInput schema / properties / idempotency_key / description

Previous value: -"REQUIRED for this write unless sent as the `Idempotency-Key` header or `_meta`. Reuse the same value when retrying the same operation."New value: +"REQUIRED for this write unless sent as the `Idempotency-Key` header or `params._meta[\"trusteed/idempotency-key\"]`. 1-128 chars from [A-Za-z0-9._:/-]. Reuse the same value when retrying the same operation."

changedInput schema / properties / idempotency_key / maxLength
```
Previous value: -255New value: +128
```
removedInput schema / properties / idempotency_key / minLength
```
Removed value: -1
```
addedInput schema / properties / idempotency_key / pattern
```
Added value: +"^[A-Za-z0-9._:/-]{1,128}$"
```

addedOutput schema / properties / next_step / description

Added value: +"Next-action code, not a tool name: provide_shipping_destination, refresh_shipping_rates, choose_shipping_option, ask_user_payment_method."

Changed1 schema field changedOct 15, 2026

addedInput schema / properties / idempotency_key

Added value: +{
+  "description": "REQUIRED for this write unless sent as the `Idempotency-Key` header or `_meta`. Reuse the same value when retrying the same operation.",
+  "maxLength": 255,
+  "minLength": 1,
+  "type": "string"
+}

First observedOct 15, 2026

TDQS

A4.5/5.0

Behavior4/5

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

Annotations cover the safety profile (readOnlyHint=false, destructiveHint=false, idempotentHint=true), and the description adds real behavior: line_items fully replaces lines, sending destination returns shipping options, and selecting an option mutates totals. It stops short of describing failure modes or what an invalid-status call returns, so not a 5.

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 tight sentences plus a guide pointer; the core mutation semantics come first, then the precondition, then the alternatives. No filler and nothing repeated.

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?

Covers what the tool does, its precondition, its sibling alternatives, and side effects on totals, while an output schema exists so return values need no explanation. Given a nested 7-parameter write tool, this is complete enough to call correctly.

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 the schema already documents the replace-all semantics of line_items and the destination/selected_option_id sequencing, so the description largely restates structured data. It does clarify the workflow relationship (destination first, then option), but adds little beyond the schema's own field text.

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 ('Update a UCP checkout session') and immediately distinguishes itself from siblings by restricting scope to sessions created with ucp_create_checkout versus the standard create_cart/select_shipping_option flow. An agent can route between this and the standard checkout tools without opening a schema.

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

Usage Guidelines5/5

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

Explicit when-to-use ('ONLY for sessions created with ucp_create_checkout'), when-not ('for the standard flow use create_cart and select_shipping_option'), and a hard precondition ('Only allowed when status is incomplete'). It also points to the checkout guide resource for deeper context.

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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