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

No annotations are provided, so the description carries the full behavioral burden. It discloses that it creates a checkout session, attaches attribution data, and returns merchant requirements, offering transparency beyond a simple mutation. It does not cover auth needs or reversibility, but for a checkout start this is acceptable.

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?

The description is front-loaded with a one-sentence purpose, followed by two concise sentences explaining the process and return value. Every sentence adds value without redundancy, making it well-structured and efficient.

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?

Given the tool's moderate complexity and the presence of an output schema, the description sufficiently covers purpose, process, and expected outcomes. It mentions the type of information returned (email, shipping address), exceeding the minimum requirement and providing a complete picture.

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?

The input schema already documents all three parameters (100% coverage), so the baseline is 3. The description adds indirect context about session_id's role in linking attribution data but does not provide parameter-level detail beyond what the schema offers.

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 clearly states the tool's function: 'Start a UCP checkout session for a product,' and explains the steps (discover capabilities, create session, attach attribution data). This distinguishes it from siblings like complete_purchase or update_checkout_info.

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?

The description provides clear context for when to use the tool—initiating a checkout session—but does not explicitly mention alternatives or exclusions. The purpose is distinct enough from sibling tools to infer appropriate usage, meeting the 'clear context' criterion.

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