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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description still adds meaningful context beyond that: determinism (same input, same output), no network and no stored state, both reduction options computed, and concrete limits (64 KB input, 1 s/call, 1 MB result).

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?

Content is front-loaded with the core computation first, then when/not-for, side effects, determinism and limits — every sentence carries information. It is somewhat dense and the EN block largely duplicates the Spanish text and the schema description field, which adds mild redundancy.

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 deterministic financial calculator with an output schema and full annotation coverage, the description supplies everything else an agent needs: scope, the excluded alternative tool, side-effect profile, determinism and resource limits. Nothing material is missing.

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. The description adds genuine semantic value on top by clarifying that results are produced for both prepayment strategies (shorten term vs. lower payment) and by tying payments_made to the prepayment moment, which the schema alone does not convey.

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 first sentence gives a precise verb and resource: it computes the effect of an extraordinary principal prepayment on a fixed-payment loan after N payments, and names the specific outputs (balance, new payment, periods, interest, savings for both options). This is clearly distinguishable from the sibling fin-loan-amortization, which is explicitly called out as out of scope.

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?

There is an explicit 'No para' clause naming the alternative tool (fin-loan-amortization) for full amortization schedules, plus a 'Cuándo' clause about needing exact, reproducible financial math. The when-to-use framing is slightly generic (money/tax/financial-math precision) rather than prepayment-specific, and it does not mention fin-loan-payment as a related alternative, so it falls short of a perfect 5.

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