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

With no annotations, the description carries the full burden and does add real behavioral context: it is a paid call ($0.002 USDC via x402 on eip155:8453), the value is deterministic and sourced rather than estimated, and it explains that a peg-derived value may be returned when NOK is pegged. It stops short of refresh cadence, failure behaviour, or what the payment flow requires on the caller side.

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 front-loaded sentence: the payment/metadata bracket first, then what the value is, then provenance and guarantees. No filler, and every clause (method, peg fallback, attribution) conveys distinct information.

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 tool, the description covers the main things an agent needs: cost model, data authority, method reporting and peg handling. It could go slightly further by sketching the returned fields or noting the fixing's update cadence, but nothing essential to invoking it is missing.

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 input schema defines zero parameters, so there is nothing the description must disambiguate; baseline for a no-param tool applies. The description correctly implies the pair and source are fixed at the tool level rather than supplied as arguments.

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 verb-equivalent: the official IDR/NOK reference rate, the ECB fixing (or official peg), plus the rate date, derivation method and source attribution. The currency pair is unambiguous in the name and description, so it is distinguishable from other fx_* pairs, but the description never contrasts it with the generic siblings fx_convert, fx_latest or fx_timeseries.

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: 'official reference rate' and 'no estimate' suggest citing a canonical fixing rather than performing a conversion, which weakly separates it from fx_convert. There is no explicit when-to-use statement, no mention of when a calculated cross or timeseries would be preferable, and no guidance on what to do if the ECB has not published a fixing yet.

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