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

With no annotations, the description carries the full burden. It does disclose two useful traits: 'Pure CPU, instant', signaling no side effects or external calls. But it omits what happens when the optional rate/years are left out, whether results are approximations, and what the response looks like beyond naming three figures.

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 short sentences with the computational scope front-loaded and nothing redundant. The efficiency is good, though the last sentence about comparing offers is thin enough to border on filler.

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 simple three-parameter calculator with no output schema, naming the returned figures (monthly repayment, total interest, total cost) covers the essentials. The remaining gap is the undefined behavior of the optional parameters when omitted.

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 the three parameters are documented in the schema itself; the description adds no parameter meaning beyond it. It also doesn't clarify the significance of rate and years being optional while principal is required, leaving default behavior unknown.

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 concrete verb and resource: 'Compute monthly repayment, total interest and total cost for a loan', and even names the three outputs. It does not, however, differentiate itself from close siblings like loan_calculator, mortgage, and mortgage_calculator, which an agent would see in the same list.

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?

'Use to compare offers' implies a comparison scenario but gives no exclusions, prerequisites, or guidance on when to pick this over loan_calculator or mortgage. The usage context is only hinted at rather than specified.

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