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

Annotations already establish readOnlyHint=true, openWorldHint=true, and destructiveHint=false, so the safety profile is covered. The description adds genuine context beyond that: the draft-has-no-PDF constraint, the XML variant for XRechnung e-invoices, and the auth requirement ('user's Cotlet API key with access to this app'). It stops short of noting rate limits or link expiry, hence a 4 rather than 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?

Front-loaded with the core action and return value, then layered with the draft caveat, the XRechnung variant, and the auth note. Every sentence carries information, though the opening fragment 'The PDF of a finalised invoice or quote' is slightly awkward as a lead.

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?

No output schema exists, so the description must explain the return; it does so clearly ('Returns a download link for the PDF'). Combined with the edge cases, the XML variant, and the auth prerequisite, an agent has everything needed to invoke this correctly.

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?

With a single parameter at 100% schema description coverage, the schema fully documents document_id (including its source in list_documents or create_invoice). The description adds no syntax or format detail about the identifier, so the baseline 3 applies when structured fields do the heavy lifting.

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 ('Download document PDF') and clarifies the exact artifact returned: a download link for the PDF generated when a document was finalised. It explicitly distinguishes itself from the sibling get_document by naming it as the tool for drafts. An agent can identify the operation 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 an explicit when-not condition and names the alternative: 'A draft has no PDF; use get_document to read it instead.' It also flags the XRechnung e-invoice case where the returned link points to an XML file rather than a PDF, so the caller knows what to expect for that document class.

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