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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=true, so the safety profile is covered. The description adds genuinely useful behavioral context beyond that: the return payload is base64 with a file name, KSeF invoices carry the KSeF number/verification link/QR code, and it states the Cotlet API key requirement. It stops short of mentioning size limits or rate behavior.

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?

Four sentences, and the first two are largely redundant — 'The invoice as a PDF file, base64 encoded' is immediately restated as 'Returns the PDF of one invoice as base64 with its file name.' The KSeF and auth-requirement sentences do earn their place, but the leading restatement of the title wastes space.

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 carries the burden of describing the return value, which it does (base64 PDF plus file name). Combined with the documented KSeF annotations in the file and the API-key requirement, an agent has enough to call it correctly, though nothing is said about response size or how to handle the base64 payload.

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% and the single invoice_id parameter is already documented as coming from list_invoices. The description adds no syntax, format or constraint detail about the parameter, so the baseline 3 applies.

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: returns the PDF of one invoice as base64 with its file name, which clearly separates it from fakturownia_get_invoice (JSON data) by output format. It does not explicitly name or contrast the sibling tools, so it stops 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 description implies when to call it (you need the rendered PDF rather than the invoice record) but never states when to prefer it over fakturownia_get_invoice or how it relates to the KSeF flow. The KSeF sentence is a content note, not usage guidance, and no exclusions or prerequisites are given.

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