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

With no annotations, the description carries the full burden and does a solid job: it discloses the paid access model ($0.002 USDC/call, x402, eip155:8453), the data provenance, the derivation method, and the guarantee that values are sourced rather than estimated. It does not cover failure modes (e.g. behavior on non-fixing days) or response shape in detail.

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?

One dense sentence with the payment/access notice front-loaded in brackets, followed by what the data is and its provenance. Every clause carries information; only the trailing 'Deterministic, sourced, no estimate' is mildly repetitive of the earlier provenance claim.

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 parameterless read tool with no annotations and no output schema, the description supplies the essential missing pieces: cost and payment protocol, data source, the three return attributes (rate date, method, source attribution), and the peg edge case. An agent has enough to invoke it correctly and interpret the result.

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 the baseline is 4. The description correctly implies a parameterless single-pair call with no currency arguments needed.

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 names a specific resource (official EUR/HUF reference rate), its source (European Central Bank fixing) and the fallback case (official peg when HUF is pegged). It is clearly distinguishable from the hundreds of sibling pair tools by naming the pair and source, though it never distinguishes itself from the non-pair siblings fx_latest, fx_timeseries, or fx_convert.

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?

Usage is implied rather than stated: an agent can infer this is for an official, non-estimated ECB reference rate for one specific pair. There is no explicit guidance on when to prefer this over fx_latest, fx_timeseries (history) or fx_convert (conversion), which is the main routing ambiguity given the sibling set.

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