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

Discloses the replacement behavior (destructive in effect, though annotated destructiveHint=false) and the price/status recalculation side-effect. It adds state restrictions and the required ucp-agent identification beyond what annotations convey. No contradiction with annotations exists.

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 sentences with no fluff: the core action is front-loaded, followed by constraints and the authentication requirement. Every clause is informative and earns its place.

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 mutation tool with a rich schema and an output schema, the description covers the essential behavior (replacement, recalculation), state limitations, and required metadata. Nothing critical for calling the tool correctly appears to be 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 baseline is 3. The description adds little beyond what the schema already documents: the 'checkout' parameter is described in the schema as 'Nuevo contenido del pedido, que sustituye al de la sesión abierta', which mirrors the description's replacement language. The meta ucp-agent requirement is also already in the schema.

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 ('Reemplaza') and a specific resource ('contenido de una sesión de checkout UCP'), and clearly identifies what is replaced (line_items y buyer) plus the recalculation of price and state. This distinguishes it from the siblings create_checkout, complete_checkout, cancel_checkout, and get_checkout without relying on the title.

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?

Provides clear context: the tool updates an existing UCP checkout session, and it explicitly excludes sessions in complete_in_progress or terminated states. It also states the required meta['ucp-agent'] prerequisite. However, it does not explicitly name the alternative for creating new sessions (create_checkout), so the 'when vs alternatives' guidance is not fully explicit.

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