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 value beyond that by disclosing the returned fields and the authentication requirement ('Needs the user's Cotlet API key with access to this app'), which annotations cannot express.

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?

Three front-loaded sentences that put the resource and payload first. There is mild redundancy between sentence one ('rows, totals, booked state and payments') and sentence two ('every row, net, VAT and total... booked... payments'), which slightly overlaps but does not bloat the text.

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 burden of explaining return contents, and it does so thoroughly (rows, net, VAT, total, booked/saved state, sent status, payments). Combined with usage and auth notes, an agent has everything needed to call it 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 100% schema description coverage, the single invoice_number parameter is already documented ('Invoice number from list_invoices or create_invoice'). The description adds no format or sourcing 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?

The description names a specific verb and resource ('Get invoice' / 'one invoice') and enumerates the payload: rows, net, VAT, total, booked state, sent flag and registered payments. The singular 'one invoice' cleanly distinguishes it from the sibling fortnox_list_invoices.

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 gives an explicit usage trigger: 'Read it before register_payment so a short payment is registered as partial.' This links the read to a downstream sibling tool with a concrete reason, though it does not state when not to use it or point to list_invoices for enumerating.

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