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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is known; the description reinforces this by framing the tool as a pure summation ('It only sums what it is given') that performs no interpretation. It does not describe the shape of the returned object beyond naming the computed boxes, which is a minor gap for a calculation tool with 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 front-loaded sentences: what it sums, what the input is with an example, what it does not do, then the sibling alternative. No filler, and the scope limitation is placed before the routing hint where it matters most.

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?

For a single-parameter, read-only calculation tool with no output schema, the description covers purpose, input format, limitations, and the alternative path. The only missing piece is what the response envelope looks like, though the enumerated formula boxes substantially telegraph the output.

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 baseline is 3, but the description adds real meaning: the values are tax amounts ('cuotas') per box rather than rates, and it supplies a concrete example object ({"09": 17430, "29": 9870}) clarifying the key format. That goes beyond the schema's accepted-box enumeration.

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 and resource — 'Add up the boxes (casillas) of the Spanish quarterly VAT return Modelo 303' — and enumerates the exact formula boxes (27, 45, 46, 64, 66, 87, 69, 71). It also explicitly distinguishes itself from the sibling get_modelo, so an agent can route between the two without opening a 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 gives both sides of the routing decision: use this tool when you have tax amounts per box already, and use get_modelo with model 303 when computing from an account's own invoices and gastos. It also sets explicit exclusions — 'it does not decide which rate or deduction applies and it is not a tax return.'

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