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

Annotations already cover safety (readOnly, idempotent, non-destructive), yet the description adds substance beyond them: required credential scope, the workspace-claim requirement, and the specific 403 refusal path for unclaimed sandbox keys. That is exactly the extra operational context annotations cannot carry.

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?

Three short sentences, front-loaded with the action and parameter, then the two hard prerequisites. No filler and nothing repeated from 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?

For a single-parameter read tool, auth requirements and the workspace gate are fully spelled out, which is what an agent needs to avoid a 403. Absent an output schema, it could say slightly more about what a checkout session contains, but the intent is clear enough.

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?

Only one parameter, already at 100% schema coverage with a description pointing at create_ucp_checkout and list_ucp_checkouts as the source of ids. The description's "pass checkout_id" adds nothing beyond the schema, so baseline 3 applies.

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?

"Read one UCP checkout session" names a specific verb (read) and resource, and "one" implicitly contrasts with list_ucp_checkouts. It does not explicitly name the sibling tools it supersedes or complements (list_ucp_checkouts, get_ucp_quote), so an agent must infer the boundary.

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 concrete preconditions for invocation — a Last Price credential with price compute scope and a claimed workspace — and states the failure mode (403) for sandbox keys from self-registration. It never says when to prefer this over get_ucp_quote or list_ucp_checkouts, but the prerequisites are unusually clear.

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