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

With readOnlyHint=true and destructiveHint=false already declaring a safe read, the description goes further by specifying the returned payload shape and the auth prerequisite. The note that the PDF 'is the same file the download icon in the overview gives' adds context, though it is unclear whether the tool actually emits a PDF, which slightly muddies the 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?

Front-loaded with the core return statement, but the second sentence redundantly re-lists fields already named in the first (lines, customer email). The trailing PDF remark is tangential and costs space without earning it.

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 usefully compensates by describing the returned fields (header, lines, totals, email, invoice vs. e-invoice). Auth requirement is covered; only the ambiguity around the PDF reference leaves a small gap.

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% and the single invoice_id parameter is fully documented in the schema. The description adds no syntax or format detail beyond what the schema provides, 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+resource ('Returns one sales invoice') and enumerates the returned content: header, lines, totals, VAT mode, customer email, and invoice type. The 'get one by ID' scope is distinguishable from the sibling list_sales_invoices and create_sales_invoice, though the description never names those alternatives explicitly.

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 by the required invoice_id (schema notes it comes from list_sales_invoices or create_sales_invoice) and the prerequisite 'Needs the user's Cotlet API key with access to this app' is stated. However there is no explicit when-to-use vs. list_sales_invoices guidance or exclusion, so it remains at the implied level.

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