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

Annotations already declare the full safety profile (readOnly, idempotent, non-destructive, closed-world), so the description is not burdened with that. It adds the meaningful behavioral constraint that this only reads UCP-created sessions and returns current state, plus a pointer to the checkout guide. It stops short of noting auth nuances (e.g. the agent_key fallback for clients without an Authorization header), which the schema covers instead.

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?

Front-loaded with the core action and return in the first two sentences, then the routing exclusion, then a guide pointer. Every sentence carries distinct information with no repetition of the schema or annotations.

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?

With an output schema present, return-value explanation is unnecessary, and the annotations cover the safety semantics. The description supplies the purpose, the flow-based routing, the prerequisite, and a guide reference, which is everything an agent needs to invoke this 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 100%, so checkout_id, meta, and agent_key are already documented in the schema with their formats and constraints. The description adds no syntax or format detail beyond that, which matches the baseline 3 when the schema does the heavy lifting.

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 and resource ('Retrieve a UCP checkout session by ID') plus scope ('Returns the current state'). It explicitly distinguishes itself from the standard-flow siblings (create_cart, preview_checkout, complete_checkout) and pairs with ucp_create_checkout, so an agent can select it 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?

Gives an explicit 'Use ONLY for...' condition, names the required prerequisite (a session created with ucp_create_checkout), and states the when-not ('for the standard flow ... do not call this tool'). It even routes the agent to a guide resource for further context, leaving nothing to inference.

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