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

No annotations are supplied, so the description carries the full burden, and it delivers: cost ($0.002 USDC/call), payment protocol and chain (x402, eip155:8453), provenance (ECB fixing, source attribution, rate date), derivation method (direct, cross, or peg-derived), and an explicit quality claim ('Deterministic, sourced, no estimate'). That is exactly the pre-call information an agent cannot get from the empty 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?

Front-loaded with the metering tag, then the core noun phrase, then qualifiers. Every clause carries information an agent needs (cost, source, method, determinism) and the trailing fragment 'Deterministic, sourced, no estimate' is a deliberate quality signal, not filler.

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 annotations and no output schema, the description still tells the agent what the response conceptually contains (rate value, rate date, derivation method, source attribution) and what it will be charged. Remaining gaps are minor but real: no statement of behaviour on non-publishing days or holidays, and no indication of number format/precision.

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 tool is 4. The description correctly implies the currency pair is fixed by the tool itself rather than passed as an argument.

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 at a specific vintage: 'Official EUR/MXN reference rate: latest European Central Bank fixing', plus the fallback ('official peg when MXN is pegged'). That is far more specific than the name alone. It does not explicitly distinguish itself from close siblings such as fx_convert, fx_latest or fx_timeseries, which is the only thing keeping it off a 5.

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 can infer 'call this when you need the current ECB EUR/MXN fixing'. There is no explicit when-to-use/when-not, no mention of when to prefer fx_convert (amount conversion) or fx_timeseries (history) instead. The commercial condition (paid call) is stated, which is a partial selection signal.

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