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

Annotations already declare readOnlyHint=true, and the description merely repeats 'Read-only' rather than adding new behavioral information. It does not describe what the aggregated response contains or how multi-dimension grouping nests results, which matters given there is no output schema.

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 short statements with no filler; the core capability is front-loaded, followed by the filter delegation and the safety note in descending order of importance.

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?

Given 13 parameters, no output schema, and a sibling that shares the filter set, delegating filters to billomat_list_invoices is an efficient and complete strategy. The one remaining gap is that the return shape of a grouped-totals response is never hinted at, which is meaningful without an output schema.

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%, so the schema already documents all 13 parameters including group_by's pattern and ordering rule. The description's example (group_by=client,year) reinforces but does not add meaning beyond the schema, making the baseline 3 appropriate.

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 operation (aggregate invoice totals) with the exact grouping dimensions available, plus a worked example. It is clearly distinguishable from billomat_list_invoices, which returns individual invoices rather than grouped totals.

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?

Points the agent at billomat_list_invoices for filter semantics ('takes the same filters'), which is useful routing context, and signals read-only usage. It does not state an explicit when-to-use/when-not rule (e.g. use this instead of listing invoices when you only need aggregates), 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.