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

With no annotations, the description carries the full behavioral burden, and it does disclose the most important operational facts: a paid call ($0.002 USDC, x402, eip155:8453) and the fallback logic (direct, cross via EUR, or peg-derived). What it omits is equally behavioral: no statement of staleness handling on weekends/holidays, no error behavior when the ECB has not published, and no response shape despite the absence of an output schema.

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?

Cost and payment rail are front-loaded in brackets, then purpose, then data provenance, in roughly two sentences with no filler. Every clause conveys something actionable.

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 tool the description is nearly sufficient because it enumerates the meaningful returned components (rate, rate date, method, attribution) and rules out estimation. The remaining gap is the concrete field names/units of the response, which the absent output schema leaves unaddressed.

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 applies. The description correctly signals that the pair is fixed by the tool rather than supplied by the caller, which is the only parameter-adjacent point of interest.

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 concrete resource and pair (AED/PLN), the exact data source (ECB fixing) and the payload contents (rate date, derivation method, attribution). Among the hundreds of fx_* pair siblings this is enough to identify the right tool, but it never states the quote direction explicitly (1 AED expressed in PLN) nor points at fx_convert for arbitrary pairs, which is the one differentiation an agent might still need.

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: the agent can infer 'call this when you need the AED→PLN reference rate'. There is no explicit when-not guidance, no mention of fx_convert for computed conversions or fx_timeseries for historical series, and no statement about how this differs from fx_latest.

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