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

With no annotations, the description carries the full burden and does unusually well: it discloses the paid x402 flow ($0.002 USDC/call, eip155:8453), the authoritative source (ECB fixing), the derivation method (direct, cross via EUR, or peg), and that results are deterministic with no estimation. It falls short of stating failure behavior, caching/freshness windows, or what a pegged result looks like.

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 plus a bracketed price tag; every clause (source, method, date, attribution) earns its place. The pricing tag leads the description, which is useful for cost gating but displaces the purpose statement slightly from the very front.

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 and no parameters, the description must convey what comes back, and it does: rate, rate date, derivation method, and source attribution. Missing only operational details such as error behavior when a fixing is unavailable or cache staleness of the 'latest' fixing.

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 no parameter meaning to add and the baseline applies. The description correctly refrains from inventing inputs and instead spends its words on the returned fields.

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 states a specific resource and verb-equivalent ('Official USD/CVE reference rate: latest European Central Bank fixing'), so an agent knows exactly which currency pair and which source is returned. It does not explicitly name the sibling it is not (e.g. fx_convert for arbitrary pairs, or fx_usd_eur), so pair differentiation rests on the tool name rather than the prose.

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?

There is no when-to-use or when-not-to-use guidance: nothing says to prefer this over fx_convert, fx_latest, or fx_timeseries, nor under what conditions the peg-derived path matters. The only decision-relevant context given is the cost, which implies 'use sparingly' but never states it.

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