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

Annotations already indicate a mutating, idempotent, non-destructive operation. The description adds meaningful behavioral detail beyond that: content is replaced, price and status are recalculated, and certain session states are rejected. This is useful context that the annotations do not provide, though it does not cover failure modes or partial-update semantics.

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 dense sentences carry all the essential information: the action, the exact scope, the recalculation side effect, the forbidden states, and the required authentication header equivalent. No filler or redundant schema repetition.

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?

Given the nested schema and an output schema, the description covers the core behavior, the session-state limitations, and the platform identification requirement. It is complete enough for an agent to call the tool correctly in most cases, though it would benefit from explicitly routing to create_checkout when no session exists.

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 description coverage is 0%, so the description must compensate. It clarifies that checkout consists of line_items and buyer, and it explains the required meta['ucp-agent'] profile structure. However, it does not explain the id parameter or optional fields like payment, signals, and attribution, so the compensation is only partial.

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?

The description opens with a specific verb and resource: 'Reemplaza el contenido de una sesión de checkout UCP' and scopes exactly what is replaced (line_items y buyer). It also states the side effect of recalculating price and status, which clearly separates it from create, complete, cancel, and get checkout siblings.

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?

The description gives explicit exclusions: it does not support sessions in complete_in_progress or finished states, and it states the required meta['ucp-agent'] profile prerequisite. It does not explicitly name alternative tools like create_checkout or complete_checkout for when those states apply, so it stops just short of full routing guidance.

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