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

With no annotations, the description carries the full burden and does well: it discloses the commercial model and auth path up front ("PAID $0.002 USDC/call, x402, eip155:8453"), and explains the derivation method (direct, cross via EUR, or peg-derived) plus that attribution and rate date are returned. It stops short of describing what happens on an unpaid or failed call (e.g. a 402 challenge flow), which matters for a paid endpoint.

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?

Front-loaded with the billing/payment terms an agent needs before invoking, then the resource, then the data-quality guarantee. One long sentence, but every clause (source, date, method, attribution, determinism) carries information. Minor cost: the bracket prefix is dense and competes with the purpose for first-read attention.

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?

No output schema exists, and the description compensates by enumerating the returned fields: rate, rate date, derivation method, and source attribution. The notable gap is quotation direction (PLN per HUF versus HUF per PLN) and rounding/precision, which an FX consumer needs and which neither schema nor description supplies.

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?

The tool takes zero parameters, so there is nothing for the description to disambiguate and the baseline applies. The description correctly frames the pair as fixed rather than parameterized.

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 pair: the PLN/HUF reference rate, and names the exact source (latest ECB fixing, or the official peg when HUF is pegged). The pair is unambiguous from the name plus description, so an agent can distinguish it from the other fx_* pair tools. It does not, however, name or contrast with non-pair siblings such as fx_convert, fx_latest, or fx_timeseries.

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?

"Deterministic, sourced, no estimate" implies when this is the right choice (official reference data rather than a market estimate), which is useful implied guidance. But there is no explicit statement of when to prefer fx_convert or fx_timeseries, no mention of staleness/holiday behavior, and no exclusions. Usage must be inferred.

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