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

Goes well beyond the readOnly/idempotent annotations to disclose non-obvious behavior: the invoice_id functions as a bearer secret, the download URL is unsigned, and `download_url_expires_at` is informational only today. These are exactly the operational traits an agent needs before calling and before echoing the response anywhere.

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-loads the action and return value in the first sentence, then layers security, URL caveats, and 404 behavior. Every sentence carries distinct, non-redundant information with no filler.

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?

There is no output schema, so the description supplies the return shape itself (metadata plus download URL, expired-at field) and pairs it with security and error semantics. Nothing material is missing for a single-parameter read 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?

Schema description coverage is 0%, so the description must carry the parameter, and it does: invoice_id is a UUID that acts as an access credential and must be treated as a secret. That is meaning the schema cannot express and directly affects how the agent handles the value.

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 an existing invoice by invoice_id) and the return payload (metadata plus download URL). It also distinguishes itself from create_invoice and explains the 404 boundary cases, so an agent can tell exactly what this tool does without opening the schema.

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 a clear when-to-use: "Use this after `create_invoice` to re-download bytes from a different device," plus explicit failure conditions (unknown IDs and Causa Prima invoices return 404). It stops short of naming an alternative read path, but the sibling set contains no competing fetch/list-invoice tool, so the guidance is effectively complete.

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