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 paid cost and rail ($0.002 USDC/call, x402, eip155:8453), the authoritative source (ECB), the derivation method (direct, cross via EUR, or peg-derived), and that values are deterministic and not estimated. It stops short of describing the response shape, staleness/caching, or failure behavior.

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 payment/access constraint front-loaded in brackets, followed by what the value is and how it is derived. Every clause adds information; only the 'Deterministic, sourced, no estimate' tail is mildly redundant with the earlier attribution language.

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 zero-parameter, no-output-schema lookup, the description covers the essentials: what value is returned, its date, its derivation method, and its source. Only the concrete response fields and freshness guarantees are left unstated, which is a minor gap at this complexity.

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; the baseline for a parameterless tool applies. The description correctly implies no inputs are needed for this fixed pair.

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 (the XAF/PLN official reference rate) and its source (European Central Bank fixing, or the official peg when PLN is pegged), so an agent knows exactly which rate this returns. It is distinguishable from fx_convert/fx_latest/fx_timeseries by being a single fixed pair, though it never names those siblings explicitly.

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 only implied: an agent infers this is the tool for the XAF/PLN reference rate. There is no statement of when to prefer it over fx_convert (conversion), fx_latest (multi-pair snapshot) or fx_timeseries (history), and no exclusions or prerequisites beyond the payment note.

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