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

With no annotations, the description carries the full burden and does well: it discloses the pricing model (x402, $0.002 USDC, base chain eip155:8453), the upstream authority (ECB, or the official peg when pegged), the derivation method, and that results are deterministic rather than estimated. It omits practical behavior such as publication/caching cadence for 'latest' and what happens on non-fixing days.

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 with the payment/transport metadata front-loaded, then the substantive payload information. Every clause adds signal (source, peg fallback, rate date, method, attribution), with no filler, though the bracketed payment prefix is boilerplate repeated across hundreds of siblings.

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 parameterless single-pair lookup with no output schema, the description covers source, fallback behavior, derivation method, returned fields, and cost — everything needed to call it correctly. The only residual ambiguity, quote direction, is minor for a deterministic reference-rate endpoint.

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 at the schema level and the baseline of 4 applies. The one semantic risk is quote direction (whether the value is NOK per MXN or MXN per NOK), which the description does not pin down.

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 a specific resource (official MXN/NOK reference rate) and a specific source and verb (latest European Central Bank fixing), so the agent knows exactly what is returned. It does not explicitly point at the inverse sibling (fx_nok_mxn) or the generic fx_latest/fx_convert alternatives, leaving differentiation to the pair-encoded name alone.

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 the 'reference rate' framing, and the per-call price is disclosed, which helps routing decisions. But there is no explicit when-to-use/when-not guidance, no mention of when to prefer fx_nok_mxn, fx_convert, or fx_timeseries for historical data, and no stated freshness guarantee for 'latest'.

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