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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=true, so safety is covered. The description adds genuine context beyond them by naming the auth requirement ('Needs the user's Cotlet API key with access to this app') and the field-level content of the return, but omits edge behavior such as what happens on an unknown customer number.

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?

Short and front-loaded, with the usage tip and auth note following the core statement. Mild redundancy between 'contact data, terms and balance' and the subsequent field list, but no wasted filler.

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 one-parameter read tool with no output schema, the description covers what the record contains and the auth requirement, which is enough to invoke it correctly. Missing only the failure/not-found behavior.

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% and the single parameter's description already tells the agent where to source the number ('Customer number from list_customers or create_customer'). The description adds no meaning beyond the schema, so the baseline of 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+resource ('One customer with contact data, terms and balance') and enumerates the returned fields, so an agent knows exactly what it retrieves. The singular 'one customer' implicitly separates it from the plural list_customers sibling, but no sibling is named explicitly.

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?

Gives a concrete usage context: 'Use it before drafting an invoice to see the terms the draft will carry.' Combined with the schema note that the customer number comes from list_customers or create_customer, the workflow is clear, though no explicit when-not-to-use or alternative is stated.

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