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

Annotations already declare readOnlyHint=true and destructiveHint=false, so safety is covered; the description adds real value beyond them by disclosing the return shape (base64 content plus file name), the not-found failure mode, and the API-key access requirement. It stops short of describing size limits or format-specific caveats.

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 front-loaded sentences: what it returns, the failure mode, then the auth prerequisite. Every sentence carries information, though the opening repeats the product name from the title.

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 still explains the return format (base64 + file name), the failure condition, and the auth requirement, and both parameters are fully documented in the schema. Nothing needed to invoke it correctly is missing.

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%, so baseline is 3, but the description adds meaning by clarifying that the XML option is the legal CFDI while the PDF is the rendered document, which helps an agent choose the right enum 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 (download) and resource (XML or PDF of one invoice or credit note), and the 'as a file' / base64 framing clearly separates it from a sibling like facturama_get_invoice that returns invoice data rather than a document file. An agent can identify the action and output type 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?

Usage is implied (fetch the file for a given invoice/credit note) and the ID source is noted, but there is no explicit when-to-use vs facturama_get_invoice guidance and no exclusions. Adequate but with a clear gap.

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