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

Annotations already declare readOnly, idempotent, non-destructive behavior, and the description adds meaningful context beyond them: the default interest rate falls back to Banco de España's three-year average (latest month), and the tool also returns the 12-month Euribor. It does not describe output formatting, but it does disclose what is computed, which is the key behavioral fact for a calculator.

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?

A single dense sentence with the market scope front-loaded and no filler. It is efficient, though the semicolon-joined clause about the default rate and the Euribor return is slightly crammed for easy parsing.

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 read-only calculator with no output schema, the description covers what is computed, the default-rate behavior, and the secondary return value. The one gap is that it never states the required loan_amount input or the term default of 25 years in prose, but the schema carries those details.

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 three parameters, setting a baseline of 3. The description adds value by pinning down the default for the optional interest_rate — the Banco de España three-year average rather than the bare 'optional' the schema gives — which is exactly the ambiguity an agent would otherwise face.

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 computation (monthly payment and total interest of a repayment mortgage) for a specific market (SPAIN), plus a secondary return (12-month Euribor). The 'SPAIN' prefix cleanly distinguishes it from the sibling mortgage_payment_br, mortgage_payment_mx, and home_loan_emi_in without needing to open any 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?

Usage is implied by the name and market scope — an agent can infer this is the Spanish mortgage calculator. However, there is no explicit when-to-use/when-not guidance and no reference to the sibling alternatives, so it stays at the implied level.

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