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

Annotations already declare read-only, idempotent, non-destructive, closed-world behavior. The description adds substantive output details (payment, interest, total, difference, break-even rate, sales-tax effect), going beyond the annotations, though it does not discuss assumptions or limitations.

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?

Two sentences, front-loaded with the comparison and its outputs, then a concise input instruction. The first sentence is long and enumerative, but every clause contributes to understanding what the tool returns.

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 17 parameters and no output schema, the description adequately explains the comparison result and the alternative input path. It does not cover every parameter edge case (e.g., trade-in handling), but the schema descriptions fill those gaps.

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 94%, so the schema provides most parameter semantics. The description still adds useful meaning by explaining that amount_financed_before_rebate and vehicle_price are alternative inputs, and that promo_cash is 'cash that combines with' the special APR.

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 comparison (cash rebate vs. special APR) and lists outputs, so the tool's purpose is unmistakable. It does not explicitly differentiate itself from siblings like loan_payment or out_the_door, which is why it falls 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?

The comparison context makes clear when to use this tool: deciding between a maker's rebate with an own loan and a promo APR. It gives input guidance ('Give the amount financed... or a vehicle price'), but it does not name alternatives or say when not to use it.

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