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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds meaningful context beyond them: ownership scoping to the authenticated customer, the error-on-missing/unauthorized behavior, and the USD currency normalization for all amounts.

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 action and scope, then a compact enumeration of returned fields. The field list is somewhat long, but it earns its place because there is no output schema, so it is not wasted text.

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 usefully spells out the returned fields (id, name, billing period, subtotal/VAT/total, currency, status) and the currency unit, plus the error case. An agent has enough to call and interpret the result, though it does not describe the error shape or id format beyond the schema.

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?

With a single parameter and 100% schema description coverage ('The invoice id (uuid)'), the schema already carries the parameter semantics. The description reinforces the ownership scoping of the id but adds no format or lookup detail beyond what the schema states, 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 and resource ('Get the details of a single invoice') plus the scoping dimension (belonging to the authenticated customer, by its id). This clearly distinguishes it from the sibling list_invoices.

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 clear context for use: retrieve one invoice by id for the authenticated customer, with an explicit failure condition ('returns an error if no invoice with that id belongs to the customer'). It does not, however, explicitly name list_invoices as the alternative for browsing multiple invoices, so the routing guidance is implied rather than stated.

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