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

Annotations already declare readOnlyHint, openWorldHint and destructiveHint=false, so the safety profile is covered. The description goes further by disclosing the auth requirement ('Cotlet API key with access to this app') and the shape of each returned item, which is exactly the kind of extra context annotations cannot carry. No rate-limit or freshness detail, so 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?

One compact paragraph that front-loads the purpose, then covers selection, return shape, paging and auth in order of usefulness. Every sentence works, though it is dense enough that the paging and auth notes could be split more clearly.

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 compensates by naming the returned fields (invoice number, relation, open amount) and the auth prerequisite, which is enough for a read-only list tool with four optional params. Nothing critical is missing, though date filtering or ordering behavior is unstated.

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%, so the baseline is 3 and the schema already documents all four parameters. The description still adds meaning by glossing the enum values ('debtors (customers who owe you)' vs 'creditors (suppliers you owe)'), which the terse schema labels do not explain.

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 ('Open invoices of debtors ... or creditors ... Returns invoices that are not fully paid') and pins the scope with 'with the open amount', which cleanly separates it from the sibling e-boekhouden_list_invoices. An agent can pick this tool without opening the schema.

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 tells the agent how to steer the call (choose D or C, narrow to one invoice number, page with limit/offset) and implies the condition that selects it ('not fully paid'). It never names an alternative tool or an explicit when-not-to-use case, so it falls 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.