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 tangible context beyond that: the full shape of the returned document and the auth requirement ('Needs the user's Cotlet API key with access to this app'). It stops short of notes on pagination or rate limits, which is why it is not a 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 and reasonably tight, but the opening fragment ('One invoice with buyer, items, VAT, totals and recorded payments') overlaps heavily with the following 'Returns one issued invoice: ...' sentence, duplicating the field list.

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, the description carries the full burden of documenting return values, and it does so field by field (buyer, tax number, line items, VAT, totals, dates, payments) while also stating the auth prerequisite and the sequencing with related tools. Nothing essential for calling it correctly 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 description coverage is 100% and the single parameter already documents its format ('Invoice number as shown in Outgoing invoices, from list_invoices'). The description adds no syntax or format detail for invoice_number, 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 ('Get invoice' / returns one issued invoice) and enumerates the exact payload: buyer with tax number, line items with quantity, unit net price, VAT rate and gross, totals per VAT rate, payment method, dates and recorded payments. This distinguishes it clearly from list_invoices, create_invoice, and the PDF sibling.

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?

Explicitly says to use it 'before cancel_invoice or record_payment so the agent works on the right document,' naming alternatives and the condition that selects this tool. That is a real routing instruction rather than implied usage.

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