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

Annotations already establish readOnlyHint=true, destructiveHint=false, and openWorldHint=true, so the safety profile is covered. The description adds non-obvious context the annotations cannot: a stated cost of 2 credits per call and a plan-tier gate that silently withholds extraPayment, startDate, paymentFrequency, and fees output on free plans. That paid-plan caveat is exactly the kind of behavioral detail that prevents a confused call, though the description does not discuss failure modes for invalid inputs.

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?

The description is dense but front-loaded: the core computation comes first, then optional down payment, then paid-plan extras, then cost. It loses a little to the redundant 'Loan Calculator: Loan Calculator is...' opener that merely restates the title, and the paid-plan sentence packs five distinct features into one long clause that is harder to scan than a short list.

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 an output schema present, the description need not explain return values, and annotations cover the safety profile; what remains is purpose, cost, and plan gating, all of which are stated. The one material gap is the absence of any disambiguation from MortgageCalculator and the interest-rate tools, which matters given how crowded this catalog segment is.

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 every parameter is already documented in structured form and the baseline is 3. The description does add a small amount beyond the schema by grouping which optional inputs produce which extra blocks and noting the paid-plan restriction, but it does not clarify units (years vs. months, decimal vs. percent) or interaction between downpayment and fees beyond what the schema already states.

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 names a specific computation (monthly payment, total interest, overall loan cost) and the exact inputs that drive it (principal, annual interest rate, term length), so the tool's function is unmistakable. It does not, however, distinguish itself from close siblings in the catalog such as MortgageCalculator, InterestRate, or MortgageRate, leaving an agent to guess which calculator to pick.

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 useful conditional guidance embedded in the text – optional down payment for adjusted financing balances, and extraPayment/startDate/paymentFrequency/fees only producing extra output on paid plans – which tells an agent what each optional input unlocks. What is missing is any explicit when-to-use/when-not guidance relative to the many other loan- and rate-related siblings in the catalog.

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