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

With no annotations, the description carries the burden and does well: it discloses the paid access model ($0.002 USDC/call over x402 on eip155:8453), the upstream source (ECB), the derivation method (direct, cross via EUR, or peg-derived), and that the value is deterministic rather than estimated. It stops short of describing failure behavior (e.g., what happens when no fixing exists) or whether the result carries a timestamp-based staleness guarantee.

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?

Effectively one dense sentence after a bracketed payment tag; every clause (source, peg fallback, date, method, attribution, determinism) earns its place. The payment/billing metadata is front-loaded ahead of the actual purpose, which slightly delays the agent's primary decision, but the whole thing is short enough that it costs little.

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 covers what matters: the returned fields (rate date, method, source attribution) are named, and the data provenance is given. The remaining gap is response shape detail — units/quoting convention (MXN per SAR or SAR per MXN) and error behavior — which nothing else in the definition 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 to disambiguate and the baseline of 4 applies. The description correctly adds no parameter guidance, and instead spends its words on what the fixed pair returns.

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 the specific resource ('Official MXN/SAR reference rate'), the data source ('latest European Central Bank fixing'), and the fallback ('or the official peg when SAR is pegged'). The pair itself is the discriminator against the hundreds of sibling fx_<base>_<quote> tools, so an agent can pick it without opening anything else.

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 but never stated: there is no explicit 'use this when you need X' framing and no routing to alternatives such as fx_convert (for conversion math), fx_sar_mxn (the inverse pair), or fx_latest / fx_timeseries (other rate shapes). An agent can infer the case from the purpose line, but nothing excludes misuse.

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