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

Annotations already declare destructiveHint=true and idempotentHint=true, so the safety profile is partly covered. The description adds valuable operational context beyond annotations: 422 for completed/canceled sessions, a required price compute credential scope, and 403 for unclaimed sandbox workspaces. It still does not say whether omitted fields are preserved or cleared on update, so it is not fully complete.

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 tightly packed sentences with no filler. The eligibility constraint is front-loaded, followed by required inputs, error behavior, and auth prerequisites.

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 complex mutation with eight nested parameters and no output schema, the description covers eligibility, required input shape, error responses, and credential/workspace prerequisites. Nothing essential for correct invocation appears missing.

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 the schema already documents individual fields and the baseline is 3. The description adds a cross-parameter rule not enforced by the schema: checkout_id is required and at least one of line_items, buyer, context, payment, signals, fulfillment, or discounts must also be supplied. It adds little about nested object syntax, but the schema covers that.

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 — change a UCP checkout session — and immediately scopes it to sessions that are still open. This clearly separates it from create_ucp_checkout, complete_ucp_checkout, and cancel_ucp_checkout without needing their schemas.

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?

Explicitly limits use to open sessions and warns that completed or canceled sessions answer 422, which is a clear when-not-to-use condition. It does not explicitly name the sibling alternatives such as complete_ucp_checkout or cancel_ucp_checkout, so it stops short of a 5.

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