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

With no annotations, the description supplies substantial operational context: a $0.002 USDC per-call charge, x402 payment on eip155:8453, deterministic sourced results rather than estimates, and possible direct, EUR-cross, or peg-derived methods. It also discloses that the response includes the rate date, method, and source attribution.

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 compact sentences front-load the cost and then efficiently cover purpose, source, returned metadata, calculation methods, and result quality. The payment and methodology jargon is consequential rather than decorative, and there is no substantial 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?

For a zero-argument latest-rate lookup, the description covers source, recency, derivation methods, provenance metadata, determinism, and payment requirements. Despite the absence of an output schema, it provides enough information to understand and invoke the tool correctly; stronger sibling routing would be an enhancement rather than a necessary invocation detail.

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 has zero parameters, so the no-parameter baseline applies and no parameter documentation is necessary. The description introduces no hidden inputs or invocation options that would require further explanation.

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 unambiguously identifies a lookup of the latest official NOK/BRL reference rate, based on the European Central Bank fixing or an applicable official peg. Its emphasis on an authoritative reference rate with methodology and attribution distinguishes it from generic conversion or historical-rate siblings.

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?

'Latest,' 'official ... reference rate,' and 'no estimate' establish a clear current, authoritative-reference use case. It does not explicitly route amount conversions to fx_convert or historical requests to fx_timeseries, so the guidance lacks explicit alternatives and exclusions.

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