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 valuable non-schema context: the auth requirement ('Needs the user's Cotlet API key with access to this app') and the shape of the returned record, including derived KSeF status.

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 purpose and efficient overall. Minor redundancy: 'One invoice with its lines, buyer, payments and KSeF status' is largely restated by 'Returns one invoice as the app shows it: number, buyer, lines...'.

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, describing the returned fields is an appropriate completeness contribution, and the KSeF post-send workflow plus auth note cover the remaining gaps. An agent has enough to call it correctly, though identifier sourcing could be reinforced.

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 already documented as coming 'from list_invoices.' The description adds no syntax or format detail beyond the schema, 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 (retrieve one invoice) and enumerates its contents (number, buyer, lines, net/VAT/gross totals, payments, reminders, KSeF number/status). The phrase 'One invoice' clearly separates it from the sibling list_invoices, and the KSeF focus differentiates it from get_invoice_pdf.

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 usage scenario: use it after send_invoice_to_ksef to check whether KSeF accepted the invoice, naming a concrete sibling tool as the trigger. No explicit when-not-to-use or alternative paths are stated, so it stops short of a 5.

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