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

With no annotations, the description carries the full disclosure burden and does well: it states the per-request price (€0.0002), the billing mechanism, and the required Authorization Bearer header, plus a hard 90-day lookback limit. What it omits is failure behavior, such as what happens for an unsupported currency code or a date outside the window.

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?

Capabilities are front-loaded, then prohibitive facts (cost, auth, history limit), then parameters — a sensible ordering with essentially no filler. The parameter block is a single run-on clause, which slightly reduces scannability.

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 shape need not be described, and the description covers the remaining essentials: available currencies, defaults, date bounds, pricing, and authentication. An agent has enough to invoke this correctly on the first attempt.

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 must compensate, and it does: all four parameters are documented with defaults (base EUR, symbols all) and formats (comma-separated codes, YYYY-MM-DD, 90-day bound). One inconsistency is left unresolved — the schema types amount as a string while the description calls it a number — and the interaction between amount and symbols is not spelled out.

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 opening clause names a specific resource and scope: daily reference rates for 31 currencies plus UAH, any base, amount conversion, and 90-day history. That is far more informative than the bare name 'fx'. It stops short of 5 because the closest sibling, unit_convert, also handles currency conversion and the description never distinguishes itself from it.

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 listed capabilities (spot rates, conversion, history) implicitly signal when to call it, and the 90-day window constrains the historical use case. However, there is no explicit when-not guidance and no named alternative, so an agent choosing between fx and unit_convert gets no help.

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