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

With no annotations supplied, the description carries the full burden and does well: it discloses the cost and payment protocol ("$0.002 USDC/call, x402, eip155:8453"), the data source (ECB), and the peg fallback behavior when NOK is pegged. It stops short of explaining payment flow or refresh cadence, which keeps it from a 5.

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, front-loaded with the pricing bracket followed by the purpose and guarantees. Every clause adds information (source, fallback, method, attribution), with only the bracketed payment prefix acting as mild clutter.

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 no-parameter, no-output-schema tool, the description covers what is returned (rate, rate date, method, source attribution) and reliability (deterministic, no estimate), which is enough to call it correctly. What is missing is freshness/latency expectations and the practical payment mechanics for x402.

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 has zero parameters, so there is nothing to disambiguate and the baseline is 4. The description usefully clarifies that the pair direction and source are fixed by the tool itself rather than by 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?

The description states a specific verb and resource: "Official ISK/NOK reference rate: latest European Central Bank fixing," naming the currency pair, source, and fallback ("official peg when NOK is pegged"). This separates it from the many fx_* siblings by pair and by source, though it never names which sibling to use for conversion or historical series.

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 context is implied by "Deterministic, sourced, no estimate" and by naming the ECB as the authority, which suggests it is the canonical reference-rate lookup rather than an arbitrary rate. However, there is no explicit when-to-use versus fx_convert, fx_latest, or fx_timeseries, and no guidance on whether a paid call is warranted over those alternatives.

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