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

With no annotations at all, the description carries the burden well by disclosing the key non-obvious semantic: line_items is replaced while omitted fields are preserved (PATCH-like), and 'Options are listed in the returned checkout' implies a checkout object is returned after mutation. It omits error behavior (invalid coupon code), inventory/price re-validation, and permission requirements, which keeps it below 5.

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

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

Two sentences, no filler, and the mutation scope is front-loaded before the merge-semantics caveat. Every clause conveys distinct information.

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

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

For a nested mutation tool with zero annotations and no output schema, the description answers what can be changed but not the full picture: no statement of the return payload shape, no failure modes for expired checkouts or invalid coupon codes, no indication of the required 'id' source. Adequate to start, but an agent would still guess on error handling.

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

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

Schema coverage is low (33%), but the prose compensates by naming the exact nested paths for coupon codes (discounts.codes), the fulfillment selection shape, and what line_items.item.id means (a variant id from search_catalog). It says nothing about the top-level 'id' parameter (where to obtain the checkout id) or the 'meta'/'context' fields, leaving a residual gap.

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

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

States a specific verb ('Change a checkout') plus the concrete mutable aspects: line_items, buyer, discount codes, delivery option. This unambiguously separates it from create_checkout, get_checkout, complete_checkout and cancel_checkout, which an agent can infer 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.

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

Usage is implied by the enumeration of what can be changed, and 'Fields you omit stay as they are' signals it is for incremental modification of an existing checkout. However, it never states prerequisites (an existing checkout id, likely from get_checkout) nor when to prefer create_checkout or complete_checkout instead.

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