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 good job: it discloses the cost and payment rail ([PAID $0.002 USDC/call, x402, eip155:8453]), the freshness source (latest ECB fixing), the fallback semantics (official peg when XAF is pegged), and that the result is deterministic and non-estimated. It does not address publication-gap behavior (weekends/TARGET holidays) or error cases, which prevents a 5.

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 payment requirement, then the resource, then the qualifiers in a single tight sentence plus a short trust clause. No filler, and the ordering matches what an agent needs first (cost/access, then what it returns).

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?

With no output schema, the description must convey return content, and it does: the rate, rate date, derivation method, and source attribution. For a 0-param deterministic lookup this is nearly complete; only edge-case behavior (holiday gaps, what happens when no official rate exists, response shape) is left unstated.

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 0-param schema applies. The description correctly spends its words on output meaning (rate date, method, attribution) instead of inventing parameter guidance.

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 the exact resource (the MXN/XAF reference rate), the authority behind it (European Central Bank fixing, or the official peg when XAF is pegged), and the alternates to a plain fixing (cross via EUR, peg-derived). Because the sibling set is hundreds of near-identical fx_<BASE>_<QUOTE> tools, the pair identification plus the sourcing/method detail is exactly what an agent needs to tell this apart from fx_mxn_xof or fx_usd_xaf.

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 by the currency pair and by 'Deterministic, sourced, no estimate', which implicitly steers away from cases needing forecasting. However, no sibling is named, and there is no explicit statement of when to prefer this over fx_convert, fx_latest, or fx_timeseries, nor when-not-to-use (e.g., non-ECB or historical needs).

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