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

Annotations already declare readOnlyHint=false, idempotentHint=true, destructiveHint=false, and openWorldHint=false, so the safety profile is covered. The description adds the idempotency-key requirement, which is useful operational context, but says nothing about what happens to the session or associated cart/order after cancellation, or whether the operation is reversible.

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?

Two short sentences, zero padding, with the core action front-loaded and the hard requirement immediately after. Nothing could be removed without losing information.

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?

An output schema exists, so return values need not be explained, and the annotations cover the mutation/idempotency profile. Still missing for a state-changing checkout operation is any statement about preconditions (which checkout states are cancellable) or side effects on related cart and order resources.

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%, so the schema carries the parameter documentation and the baseline is 3. The description adds one genuinely new fact — that meta.idempotency-key is required — but this is not reflected in the schema's required array (which lists only 'id'), and it says nothing about the ucp-agent profile parameter.

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?

The description states a specific verb and resource ('Cancel a checkout session'), which is unambiguous on its own. It does not, however, differentiate itself from the sibling cancel_cart or explain the relationship between a checkout and a cart, leaving the agent to infer that distinction.

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?

There is no when-to-use guidance: nothing says whether this should be called before or after payment, what checkout states permit cancellation, or why one would choose cancel_checkout over cancel_cart. The only stated condition is the idempotency-key requirement, which is an invocation detail rather than routing guidance.

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