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

Annotations already cover safety (readOnly, idempotent, non-destructive), so the bar is lower. The description adds real behavioral context: exact rational arithmetic, nearest-cent half-up rounding, a 360-row cap, and explicit exclusion of taxes/insurance/PMI/fees. It does not describe error behavior for invalid inputs, but the disclosed computation semantics are substantial.

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?

Three dense sentences, each carrying distinct content (computation, optional inputs, arithmetic/rounding, exclusions), front-loaded with the core purpose. Slightly list-like, but no filler sentences.

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?

Output schema exists so return values need not be re-specified, yet the description still summarizes the amortization table shape. Combined with the 360-row cap, rounding rules, and exclusions, an agent has everything needed to call it correctly.

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 baseline is 3, but the description adds meaning beyond the schema: down payment 'reduces principal' and extra monthly principal 'shortens the modeled term'. That clarifies the interaction between optional parameters that the schema documents only individually.

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+resource ('Computes fixed-rate monthly principal and interest payments') plus the full output scope (total interest, payoff month, amortization table). It is clearly distinguishable from sibling calculators like compound_interest_calculator because it names mortgage-style P&I plus amortization.

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 clear context for when the tool applies (fixed-rate amortized loans) and explains optional behaviors like down payment and extra principal. It never explicitly routes the agent away from a sibling (e.g. compound_interest_calculator or debt_to_income), so it stops short of full when-not guidance.

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