convert_currency

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

With no annotations, the description carries the burden and does so well: it discloses the non-trading-day fallback-to-prior-day behavior and the fact that the response flags the substitution, and it states that unsupported currencies are refused with the supported list returned. That is real behavioral context beyond the schema. It omits rounding/precision behavior and rate-source caveats, which keeps it from a 5.

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?

A single dense sentence with the core action front-loaded and the parenthetical caveats tucked at the end where they belong. It is slightly long, but every clause (date format, fallback, refusal behavior) earns its place.

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 4-param, no-annotation, no-output-schema tool, the description covers the important edge cases (fallback days, unsupported currencies, response noting the substitution). It leaves the returned value shape and error payload structure unspecified, which is tolerable given the tool's simplicity but is still a gap.

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 0% across 4 parameters, so the description must compensate. It does document the date format ('YYYY-MM-DD'), the 'latest' semantics, and that currencies must be Frankfurter-supported, but amount and the two currency parameters get no format, casing, or ISO-code guidance, and the supported-currency list is only referenced, not enumerated.

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+resource ('Convert an amount between two ... currencies') plus the scope ('Frankfurter-supported') and the rate mode. None of the siblings (list_countries, get_country_metric, etc.) do currency conversion, so there is no ambiguity to resolve, and the description pins down exactly what is being computed.

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?

It clearly frames the two supported modes of use (latest rate vs. a pinned historical date) and explains the fallback rule, which tells an agent what to expect before calling. It does not name when another tool should be used instead, but no sibling overlaps this function, so the omission is low-cost.

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