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 paid x402 model, the exact price and chain (eip155:8453), the data source (ECB fixing / official peg), the derivation method (direct, cross via EUR, peg-derived), and that the value is deterministic rather than modelled. It does not cover failure or payment-error behavior, which is the main remaining gap for a paid endpoint.

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?

Two sentences plus a bracketed billing tag, with the resource front-loaded and every clause adding information (source, peg fallback, output fields, determinism). No filler or repetition of the tool name.

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 endpoint, the description tells the agent what comes back (rate, rate date, method, source attribution) and what it costs, which is what an agent needs to call it correctly. Only error/payment-failure semantics are 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; baseline 4 applies. The description correctly adds no parameter talk and instead spends its words on the fixed pair and sourcing.

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 and scope: the official NOK/MYR reference rate, sourced from the latest ECB fixing or the official peg. The currency pair in the name is confirmed and elaborated, so an agent can distinguish it from the hundreds of other fx_<pair> siblings 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 by the pair-specific name and the explicit note that this is a reference rate rather than an estimate, but there is no explicit when-to-use or when-not guidance. Nothing routes the agent to fx_convert, fx_latest, fx_timeseries, or an inverted fx_myr_nok fetch, which are the realistic alternatives for this data.

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