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

The description goes well beyond the annotations by disclosing a charging side effect, instant delivery, escalation to requires_escalation with continue_url, and the no-charge case for a report included in the plan. It also calls out the required idempotency-key metadata. This is rich, honest behavioral disclosure.

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?

The description is three dense sentences with no filler. The main purpose is front-loaded, followed by the conditional payment behavior and metadata requirement. Every sentence contributes useful 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?

Given the tool's complexity and the presence of an output schema, the description covers the essential behavioral paths, the free-report edge case, and the idempotency requirement. Nothing critical is missing for an agent to select and invoke it correctly.

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 add parameter meaning, and it does: it explains the payment.instruments structure, the important handler_id and credential.token fields, and the need for meta['idempotency-key'] plus meta['ucp-agent']. It does not discuss the id parameter or checkout.signals, but these are less critical for invoking the tool correctly.

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 ('Completa una sesión de checkout UCP') that clearly distinguishes the tool from siblings like create_checkout, update_checkout, and cancel_checkout. It also states the main behavioral outcomes, so an agent can immediately tell what this tool is for.

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 clear operational context: when a Stripe Shared Payment Token is present, the session is charged and delivered; otherwise it escalates. It does not explicitly name alternatives or exclusions, but the first sentence combined with the sibling list makes the intended use clear enough.

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