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

Beyond the readOnlyHint/openWorldHint annotations, it discloses the 4 MB decoded size cap, that PDFs are not accepted, the specific failure modes (non-invoice XML, malformed XML, DOCTYPE, engine_busy, engine_timeout), and the 1-credit cost per call. These are substantial operational details the annotations do not carry.

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-loaded with the core action, then inputs, then returns, then error modes, then cost. Every sentence carries distinct information and none is 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?

For a tool with an output schema and annotations already present, the description still covers inputs, return shape, failure modes and pricing. An agent has everything needed to call it and interpret the result; nothing material 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 real meaning: the accepted XML flavors, the mutually exclusive file-vs-base64 input path, the label-language semantics of lang, and the PDF rejection constraint not stated in the schema. It does not need to re-explain file_id/download_url, which the schema documents.

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 (render) and resource (XML e-invoice) and names the accepted formats (UBL, CII, XRechnung) and output (HTML page). This clearly distinguishes it from parse_invoice, validate_invoice, convert_invoice and generate_invoice.

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 it to show a person what an e-invoice contains" gives a clear context for when to reach for this tool. It does not explicitly name the alternatives (parse_invoice, convert_invoice) or state when NOT to use it, so it stops just short of the top score.

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