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

Annotations already declare readOnlyHint=true, openWorldHint=true and destructiveHint=false, so the safety profile is covered. The description adds genuinely new context beyond annotations: the conditional KSeF behavior (gov_status and KSeF number only returned when KSeF is on for the account) and the API-key authorization requirement.

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?

The opening two sentences largely restate each other: 'One invoice with its line items, buyer, payments and KSeF fields' is immediately followed by 'Returns one invoice with its positions, buyer and seller data, status and the payments linked to it.' One of the two is redundant, costing efficiency even though the auth note is a worthwhile closer.

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 and only one parameter, the description carries the return-value burden and does so competently, listing the salient fields and noting the conditional KSeF fields. Safety and auth are covered; the main omission is when to prefer this tool over its siblings.

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 invoice_id already carries 'Invoice ID from list_invoices.' The description adds no syntax, format or sourcing detail beyond that, so the baseline 3 for a schema-covered parameter 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?

The description states a specific verb (get) and resource (one invoice) and enumerates the returned contents: line items, buyer, seller, status, payments and KSeF fields. It is clear what the tool does, though it never explicitly names the sibling it differs from (e.g., list_invoices or get_invoice_pdf), so sibling differentiation is only implicit.

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?

It states a prerequisite ('Needs the user's Cotlet API key with access to this app'), which is useful context. However, it gives no guidance on when to choose this single-invoice fetch over fakturownia_list_invoices, get_invoice_pdf or get_ksef_status. Usage is implied rather than stated.

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