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

With no annotations supplied, the description carries the full disclosure burden and does well: it declares the paid x402 model ($0.002 USDC/call on eip155:8453), the source (ECB), the deterministic non-estimated nature, and the peg fallback behavior. It stops short of stating caching/freshness timing or what happens if no fixing exists for a date, which prevents 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 with the pricing tag front-loaded in brackets, then the core purpose, then the fallback/method detail. Every clause earns its place, though the packed phrasing ('direct, cross via EUR or peg-derived') is slightly list-like.

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?

No output schema exists, yet the description enumerates the returned fields (rate, rate date, method, source attribution), covers the peg edge case, states the cost, and asserts determinism. For a zero-parameter, annotation-free rate lookup, nothing an agent needs 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 tool takes zero parameters, so the baseline is 4; there is nothing for the description to disambiguate. It nonetheless clarifies that the returned rate carries a date, a derivation method, and source attribution, which sets expectations for the response shape.

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 specific resource (Official NOK/MXN reference rate) and the specific verb/operation (latest ECB fixing retrieval), so an agent can tell it apart from the hundreds of other fx_* pair tools by the currency pair alone. It does not explicitly route away from fx_convert, fx_latest, or fx_timeseries, which is the only thing keeping it from a 5.

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 implied: fetch the latest ECB fixing for this pair, or the official peg if MXN is pegged. However, no explicit guidance is given about when to prefer this over fx_timeseries (historical) or fx_convert (conversion math), even though both siblings exist in the toolset.

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