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 per-call price and payment rail ($0.002 USDC, x402, eip155:8453), the upstream source (ECB), the peg fallback, and that results are deterministic and not estimates. What it omits is failure behavior when a fixing is unavailable and any freshness/latency expectation.

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 with no filler, and the returned fields are enumerated efficiently. The one weakness is ordering: the payment preamble occupies the front slot ahead of the purpose statement, so the tool's job is not the very first thing read.

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 tool with no output schema, the description covers the essentials an agent needs: the data source, the rate value, the rate date, the derivation method and the attribution. Missing only edge-case behavior when no fixing exists.

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 nothing needs documenting beyond confirming no input is required, which the description implicitly does by naming the fixed pair. Baseline of 4 applies for a 0-parameter tool.

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 (ISK/CHF) and a specific data product (latest ECB fixing, or the official peg when CHF is pegged), which an agent can distinguish from generic conversion tools. It does not explicitly say how it differs from fx_convert, fx_latest or fx_timeseries, leaving that inference to the naming convention.

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?

There is no when-to-use or when-not-to-use guidance and no named alternative, despite a large sibling set containing fx_convert, fx_latest, fx_timeseries, fx_isk_eur and fx_chf_isk. The description implies the tool is for the official ISK/CHF reference rate, but an agent must guess whether to use it versus a conversion or timeseries tool.

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