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 substantial work: it discloses the price and payment rail ($0.002 USDC/call, x402, eip155:8453), the upstream authority (ECB fixing), the derivation method (direct, cross via EUR, or peg-derived), and that the result is deterministic and not an estimate. It stops short of stating freshness cadence, rate-limit/retry behavior, or what happens on non-fixing days (weekends/holidays).

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 following a bracketed payment prefix; every clause carries information (source, fallback, method, attribution, determinism) and the purpose is front-loaded after the cost notice. Slightly overwrought with the trailing 'Deterministic, sourced, no estimate' fragment, which partly restates the preceding clauses.

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 quote tool, the description covers what the value is, where it comes from, how it was derived, and what metadata accompanies it (rate date, method, attribution). Missing only edge-case behavior such as stale/non-publication days and response error modes, which is a minor gap for a paid per-call 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 no parameter meaning to add and the baseline of 4 applies. The description correctly implies the pair is fixed at the tool level rather than passed as an argument, which prevents an agent from looking for a currency-pair parameter.

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 names the exact resource (official NOK/SAR reference rate) and its source (latest European Central Bank fixing, or the peg where applicable), so an agent knows precisely what value it gets. It does not, however, explicitly distinguish itself from the general siblings fx_latest, fx_convert, or fx_timeseries, which would require the agent to infer that the pair-encoded name implies a single-pair quote.

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' plus 'no estimate' signals this is for authoritative/audit-grade rate lookups rather than conversions. There is no explicit when-to-use or when-not-to-use guidance, and no pointer telling the agent to prefer fx_convert for arithmetic or fx_timeseries for historical series, which matters given hundreds of near-identical siblings.

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