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 $0.002 USDC charge, the x402 scheme and network (eip155:8453), the ultimate source (ECB), the derivation method (direct, cross via EUR, or pegged), and promises the rate carries its own date and provenance. It does not describe the response payload shape or how stale a fixing can be, which matters for a rates call.

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 after a bracketed cost tag, front-loaded with price and then the resource. Every clause (source, peg fallback, date, method, attribution, no-estimate) conveys a distinct fact, and nothing is repeated from structured fields.

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 read tool, this covers the essentials an agent needs: what is returned, who publishes it, how it is derived, and what it costs. The remaining gaps are the quotation convention (base/quote orientation) and expected freshness cadence of ECB fixings, neither of which the schema or annotations supply.

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 the baseline is 4. The description correctly adds no parameter detail because there is nothing to parameterize, and it usefully clarifies that the currency pair is fixed by the tool identity rather than passed as an argument.

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 a specific resource (the ILS/NOK reference rate), its source (the latest European Central Bank fixing) and the fallback case (official peg when NOK is pegged). That is far more precise than the other pair tools' names alone, though it never states the quotation direction (NOK per ILS) or explicitly separates itself from the inverse sibling fx_nok_ils.

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 by the pair-specific naming; there is no statement of when to call this versus fx_convert, fx_latest, fx_timeseries, or a triangulated fx_ils_eur + fx_eur_nok path. The bracketed pricing tag does at least signal that this is a metered x402 call, which helps an agent weigh it against free alternatives, but no exclusions are given.

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