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

Annotations already declare the full safety profile (readOnly, idempotent, non-destructive, closed-world), so the bar is lower. The description adds value beyond that by disclosing the USD-pivot mechanism and the exact return keys, which the annotations cannot convey.

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 tight clauses: purpose first, return shape second, cross-reference last. Nothing is padded and the action is front-loaded, so an agent can parse it in one pass.

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 deterministic two-input math tool with annotations covering safety and an output schema present, the description is essentially complete. Restating the return keys is mildly redundant given the output schema, which is the only reason it is not a 5.

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% and both parameters are already explained as 'Units of currency X per 1 USD'. The description's 'via USD' reinforces that convention but adds no syntax or edge-case detail beyond the schema, so the baseline 3 applies.

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 ('calculate cross exchange rate between two currencies') and adds the mechanism 'via USD', which is meaningful for currency math. It does not explicitly differentiate itself from the many currency siblings (calculate_currency_exchange, calculate_exchange_rate_margin), so it stops 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 only routing signal is 'See list_bundles for related conversions calculators', which points at a bundle rather than stating when to pick this over the sibling exchange-rate tools. Usage is implied rather than stated, so it sits at minimum viable.

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