Calculate mortgage payment (local)

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

Annotations already declare readOnly, idempotent, and openWorld=false, so the safety profile is covered. The description adds genuinely new behavior: the returned PITI components, total interest over loan life, and the conditional PMI rule (LTV > 80%). It does not discuss failure/validation behavior for bad inputs, keeping it at 4.

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-loads the purpose and return value before usage notes and parameter rules; each sentence carries distinct information with no filler. Slightly dense in the final sentence but not wasteful.

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 steps in to describe the return (PITI breakdown plus lifetime interest), and it resolves the key input ambiguities. It is essentially complete for invocation, missing only explicit sibling routing.

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 only 40% across 10 parameters, so the description must compensate, and it does: it clarifies that down_payment and down_payment_percent are alternatives with a 20% default, and that property tax can be supplied as an annual figure or a rate. It leaves several parameters (hoa_monthly, insurance_annual, loan_term_years default) to the schema, so not fully compensating.

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 verb and resource ("mortgage payment calculator") plus the distinctive attribute "Local-only", so the agent immediately knows it is a deterministic computation rather than a network fetch. It does not explicitly differentiate itself from the related sibling zillow_calculate_affordability, keeping it just short of a 5.

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?

Gives a clear use context — "safe to use for scenario comparison without burning a fetch" — which tells the agent when this local tool is preferable to a network call. It never names the alternative calculator siblings (e.g. affordability) or states exclusions, so it stops at clear context rather than explicit when/when-not routing.

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