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

Beyond the annotations (readOnly/idempotent/non-destructive), the description discloses non-obvious behaviour: only paid orders yield a result, the return is a link rather than the document, the customer must sign in to open it, and the invoice never passes through the assistant. These are exactly the operational facts an agent needs and cannot get from structured fields.

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?

Three tight sentences, front-loaded with the usage condition followed by return behaviour and the key limitation. No filler and nothing restated from the schema.

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, yet the description still clarifies the practical return (a PDF link requiring sign-in) and the assistant's non-involvement. Combined with the annotation safety profile and the single well-documented parameter, nothing needed to call this correctly is 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 coverage is 100% and the single order_number parameter is documented with a format example, so the schema carries the load. The description adds no syntax or constraint detail beyond what is already structured.

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 names a specific verb (retrieve the invoice) and resource (invoice for a teas.co.uk order), plus the actor precondition (signed-in customer). It is unambiguous among the siblings, though it does not explicitly contrast itself with near-neighbours like my_orders or track_order.

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?

"Use this when a signed in customer wants the invoice for a teas.co.uk order" gives a clear triggering condition. There is no when-not guidance (e.g. unpaid orders) and no named alternative, but the intended context is explicit.

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