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

With no annotations, the description must carry the behavioral transparency burden. It discloses the ECB/Frankfurter source, supports historical dates, and excludes crypto. However, it does not mention response format or whether rates are mid-market, which are relevant behavioral traits for a rate lookup tool.

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?

The description is a single, well-structured paragraph where every sentence adds value. It starts with a clear purpose statement, then expands with usage guidelines and exclusions, without redundancy or 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?

Given no output schema and no annotations, the description provides strong context: purpose, source, historical capability, usage conditions, and alternatives. It lacks an explicit mention of the return structure (e.g., converted amount and rate), which prevents a perfect score, but overall it is complete enough for effective tool selection.

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%, with each parameter (amount, from, to) already clearly described. The tool description adds no further parameter-level detail, so a baseline score of 3 is appropriate.

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 explicitly states the tool's function: 'Retrieves live and historical fiat currency exchange rates from the Frankfurter API (sourced from the European Central Bank).' It also clarifies its distinct capability of supporting historical date lookups, which clearly differentiates it from sibling conversion tools.

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 description gives explicit usage guidance with concrete conditions: 'Use currency_rates when a rate from a specific past date is required (e.g. accounting, tax, or audit), or when the ECB source must be documented.' It further distinguishes alternatives like currency_convert, currency_convert_lite, currency_fx_lite, and currency_convert_open, making the selection criteria very clear.

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