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

Annotations already cover the safety profile (readOnlyHint=true), so the description's added value is the returned field inventory and the underlying API mapping (GET /Invoice/{invoiceId}), which helps an agent ground the call. It does not disclose auth requirements or error behavior for a missing id, keeping it below a 5.

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 tight sentence with the primary purpose front-loaded, followed by a compact API reference clause. Nearly zero waste, though the raw endpoint string is marginally redundant for an agent that never calls the HTTP API directly.

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 two-parameter read tool whose annotations declare safety, this covers purpose, the id requirement, and the shape of the returned data, which compensates for the absent output schema. It stops short of clarifying how embed changes the response or what happens on an unknown id.

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 both invoice_id and the embed expansion parameter are already documented in the schema. The description names the id but adds no semantics for what values are valid or what embed expansion returns, matching the baseline 3 when the schema does the work.

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 ('Fetch a single invoice by id') and enumerates what the response contains (totals, status, dates, contact, contact person). It implicitly separates itself from list_invoices and get_invoice_positions via 'single invoice', but no sibling is named explicitly, so it falls short of a 5.

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?

The phrase 'by id' implies retrieval of a specific known invoice, which is adequate usage context, but there is no explicit when-to-use guidance, no exclusions, and no pointer to list_invoices for browsing or get_invoice_positions for line items.

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