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 paid x402 access model, the exact price ($0.002 USDC per call, eip155:8453), the determinism guarantee ('no estimate'), and the peg edge case where the fixing is replaced by an official peg. It stops short of describing the 402 challenge flow, error behavior, or ECB publication cadence.

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 plus a bracketed payment prefix — front-loaded with the resource, then the source, resolution method and return fields. No filler, though the bracket cost prefix is somewhat cluttered and the sentence is long enough that key routing information (latest-only) is buried mid-clause.

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 trivial no-param, no-output-schema tool, the description covers what matters: what is returned (rate date, derivation method, source attribution), where it comes from, and what it costs. Minor gaps are timeliness (how fresh the fixing is) and whether the response includes the numeric rate alongside the metadata.

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 nothing for the description to disambiguate; the pair and source are baked into the tool identity. Baseline 4 applies for a no-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?

Names a specific resource (INR/SEK official reference rate) and the authoritative source (European Central Bank fixing), including the peg fallback — far from a tautology. It does not explicitly distinguish itself from the generic siblings fx_latest, fx_convert or fx_timeseries, so an agent must infer the division of labor from the pair-specific name.

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: 'latest ... fixing' signals this is a single current-rate lookup rather than a time series or a conversion, which weakly routes away from fx_timeseries and fx_convert. There is no explicit statement of when to prefer this over fx_convert or fx_latest, and no stated prerequisites such as payment setup beyond the price tag.

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