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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so the safety profile is fully covered structurally. The description adds the genuinely useful trait that it will not fabricate prices or credit coverage, but says nothing about how contingency is applied or what the calculation produces.

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, zero filler, with the core mechanism stated first and the negative guardrail second. Nothing repeats the tool name or title.

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?

An output schema exists, so return values need not be described, and annotations cover the safety profile. The remaining gap is the unexplained contingencyPercent and item limits, which an agent would need before constructing a valid call.

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 0%, so the description carries the full burden and only partially delivers: 'unit rates' maps to pricePerUnit and 'source URLs' to source. The required contingencyPercent, currency enum (USD/BRL), quantity, unit, priceDate, and the 50-item cap are all left unexplained, and contingencyPercent materially drives the result.

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 ('Calculate') and object ('auditable scenario') and pins down the mechanism: caller-supplied unit rates and source URLs. The negative clause ('Does not guess AWS prices') usefully separates it from pricing-lookup siblings like finops_unit_economics, though 'auditable scenario' remains slightly abstract as a resource name.

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 'caller-supplied' and 'does not guess AWS prices' phrasing implies the condition for use — you already have rates and sources — but it never states when to call this versus finops_unit_economics or aws_knowledge_search, nor any prerequisites beyond supplying inputs.

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