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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. Because there is no output schema, the description's disclosure of return contents (lines, vendor, linked PO, and PO-line fields qty_received/qty_invoiced) adds real value, though it says nothing about not-found behavior or payload size.

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?

A single short sentence with no padding, but it is an incomplete fragment lacking a subject-verb clause, so brevity here reflects under-specification rather than tight editing.

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 does useful work by sketching the return shape, but it omits the lookup-by-ID framing, the invoice_id parameter, and error behavior — gaps that matter for a single-record fetch tool.

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?

One required parameter, invoice_id, with 0% schema description coverage, and the description never mentions it. The agent must infer from the parameter name alone that this is the invoice identifier and that it is mandatory.

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 is a noun phrase ('An invoice with its lines, vendor, and linked PO') with no verb, so the retrieval action is only implied by the name get_invoice. It conveys what the returned object contains but never states that the tool fetches a single invoice by ID, and it draws no distinction from siblings like search_invoices or get_po.

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?

There is no guidance on when to use this tool versus search_invoices (list/filter) or get_po (linked purchase order). Nothing tells the agent the invoice_id is the lookup key or what happens if the ID is unknown.

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