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

Annotations already declare readOnlyHint/openWorldHint/destructiveHint, so safety is covered. The description adds real value beyond them: the auth requirement (Cotlet API key with access to this app) and the refund-aware use case for interpreting the returned line items.

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?

Front-loaded with the core purpose and efficient overall. There is mild redundancy between 'One invoice with its line items, state and totals' and 'Returns one invoice by invoice_number: ...', which keeps it from a 5.

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?

With no output schema, the description carries the burden of describing return values, and it does so (line items, state, totals, subscription). Auth needs and the refund workflow context are included. Complete for an agent to call it correctly, though it does not mention pagination or error 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%, so the single invoice_number parameter is already documented ('Invoice number from list_invoices'). The description restates 'by invoice_number' without adding format or syntax detail beyond the schema, so baseline 3 is appropriate.

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 (get one invoice) and elaborates on what it returns: line items with ids/quantities/amounts, state, totals, and the linked subscription. This clearly distinguishes it from the sibling list_invoices and refund_invoice tools.

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 routes the agent: 'Read it before refund_invoice to pick the right line item_id and to see what was already refunded.' This names the downstream tool and the reason to call this one first. It stops short of a full when-not-to-use statement, so it is not quite a 5.

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