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

Beyond the annotations it discloses real behavioral traits an agent could not infer: the 501 payment-provider precondition, that an already-completed session is returned unchanged, the required Last Price credential with price compute scope, and the 403 workspace-claim gate. That is exactly the extra 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 front-loaded sentences: action first, then failure modes, then credential/workspace requirements. Every sentence carries operative information, though the credential and workspace prerequisites are packed densely into the closing lines rather than split cleanly.

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?

Auth requirements and error codes are well covered, which is most of what a one-parameter call needs. However, with destructiveHint=true and no output schema, the description never says what completion actually does or destroys (payment capture, order placement, irreversibility), leaving a material gap.

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 coverage is 100% and the schema already explains that checkout_id is the id shown by create_ucp_checkout and list_ucp_checkouts. The description's "pass checkout_id" adds no format or sourcing detail beyond that, 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?

States a specific verb and resource ("Complete a UCP checkout session") plus the single required input, which cleanly separates it from the create/update/get/cancel siblings by verb. It does not explicitly name which sibling to use instead, but the action is unambiguous.

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?

It gives concrete when-not conditions: open sessions answer 501 until a payment provider is configured, and unclaimed workspaces or sandbox self-registration keys are refused with 403. It stops short of naming an alternative tool to use in those failure cases, so it is clear context without full routing guidance.

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