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

With no annotations, the description carries the full burden. It usefully discloses the state machine (incomplete -> ready_for_complete -> completed) and that no auth key is required, which is real behavioral context. It does not state read-only behavior explicitly, error behavior for unknown ids, or rate limits, so disclosure remains partial.

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?

One front-loaded sentence with the resource and identifier first, then a compact parenthetical for the lifecycle and a short clause for auth/cloud. No filler sentences.

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 two-parameter fetch tool with no output schema, no annotations, and 0% schema coverage, the description covers identity, lifecycle, and auth context but omits what the response contains, error/not-found behavior, and the concrete value expected for 'cloud'. Adequate but with clear gaps.

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 explains both parameters at a high level: 'id' identifies the session and 'cloud' is passed for a hosted merchant. It still does not clarify expected formats (e.g., id shape, valid cloud values), leaving gaps the schema also fails to cover.

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 ('Fetch a UCP checkout session by id') and adds the lifecycle states, so an agent knows it retrieves an existing session rather than creating or completing one. It does not explicitly name a sibling, but the get/create/cancel/complete verb distinction makes it separable from uxc_create_checkout and uxc_complete_checkout.

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?

'No key; pass cloud for a hosted merchant' gives a usage condition for the cloud parameter, and the status chain implies the tool is used to poll session progress. However, there is no explicit when-to-use vs. alternatives guidance (e.g., call this after uxc_create_checkout, before uxc_complete_checkout), so usage is only implied.

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