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 well: it discloses the paid x402 requirement ($0.002 USDC/call on eip155:8453), asserts determinism ('sourced, no estimate'), and names what the payload reports (rate, rate date, derivation method, source attribution). It does not state freshness latency or failure behavior for unsupported dates, which keeps it short of 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?

Essentially one sentence after the bracketed pricing tag, with the substantive claim front-loaded after the (necessary) cost disclosure. Every clause carries information; the bracketed pricing block is dense but justified since it is the only place the payment requirement is stated.

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 tool, the description covers everything an agent needs: what is returned, where it comes from, the peg fallback, determinism, and the payment prerequisite. Nothing material is missing or left to inference.

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 and the baseline of 4 applies. The description correctly adds no parameter noise for a fixed-pair lookup.

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 the exact currency pair, the specific data source (European Central Bank fixing), the fallback case (official peg when BRL is pegged), and the derivation method. An agent can distinguish this from the hundreds of sibling pair tools (fx_eur_usd, fx_usd_brl, fx_convert) purely from the description without opening a schema.

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 by the pair name and the 'latest' qualifier, but the description never states when to pick this over fx_convert, fx_latest, or fx_timeseries, nor any exclusions (e.g., historical rates require a different tool). The agent must infer the boundary from tool names alone.

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