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

Annotations already declare readOnly/idempotent/non-destructive, so the safety bar is low. The description adds real value beyond them: the ECB data source, the default fixation period (fixed 5 up to 10 years, latest month), and the fact that other fixation periods are also returned. It stops short of describing rounding, currency handling, or amortization detail.

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?

Front-loaded with the country scope and the core computation; two clauses of dense, relevant content. The trailing 'Also returns the other fixation periods' is slightly cryptic and would read better tied to the output, but nothing is wasted.

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 return-value burden and mostly does so (monthly payment, total interest, other fixation periods). The default-rate explanation closes the main parameter ambiguity. Minor gaps remain around output units/format and precision, but the essentials are present.

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, but the description adds genuine semantics for interest_rate by explaining the default ECB rate and its fixation period when the parameter is omitted. The term_years and loan_amount semantics are left to the schema, which already documents them.

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 constant-instalment loan) and scopes it to Italy via the leading 'ITALY.' marker, which cleanly distinguishes it from the country-specific siblings mortgage_payment_de/es/br/mx. An agent can tell what it does and which country it targets 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?

Usage is only implied (compute an Italian mortgage payment) with no explicit when-to-use, when-not-to-use, or named alternatives among the mortgage_payment_* siblings. It does explain the default-rate context, but that is behavioral detail rather than routing guidance.

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