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

With no annotations, the description does real work: it discloses the response contents, and critically that a cs_... id is accepted and returns the session with the invoice expanded — behavior an agent could not infer from the schema. It omits error/not-found handling and any rate-limit guidance for the polling use case it recommends.

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 short sentences, front-loaded with what is returned and back-loaded with the dual-id behavior. No filler or restatement of the tool name.

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 and no annotations, the description adequately conveys both input modes and the shape of the result. Missing pieces are failure behavior and pagination/refresh characteristics, which matter for the polling use it recommends.

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 schema already notes "inv_... or cs_... id", so the baseline is 3. The description earns an extra point by explaining the consequence of passing a cs_ id (session returned with invoice expanded), which the schema does not convey.

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 (retrieve an invoice) and enumerates the returned payload: status, payment_options with address/amount/payment_uri/qr_payload, and paid summary. That payload detail plus the single-id form makes it trivially distinguishable from list_invoices and the rest of the sibling set.

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 for polling or reconciliation" gives an explicit usage context rather than leaving it implied. It stops short of naming when-not-to-use or pointing at an alternative (e.g. list_invoices for enumeration), so it falls just short of a 5.

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