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

With no annotations supplied, the description carries the full behavioral burden and does a good job: it discloses the paid x402/eip155:8453 access model and $0.002 cost, the data provenance (ECB fixing), the peg fallback branch, and the derivation method options. It does not cover stale-fixing behavior on non-publication days or error responses, which keeps it below 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?

Two tight sentences with no filler; every clause (cost, source, peg fallback, method, attribution) carries information. The only structural weakness is that the bracketed payment metadata precedes the statement of what the tool actually does, delaying the agent's primary recognition cue.

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 rate lookup with no output schema and no annotations, the description supplies source, cost, fallback semantics and the shape of the returned fields (rate date, method, attribution). The remaining gap is failure/staleness behavior, which is minor for this tool complexity.

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 schema is empty and there is nothing for the description to disambiguate. The core selection input (the currency pair) is baked into the tool name rather than a parameter, and the description makes that explicit.

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 (CAD/XOF reference rate), the authoritative source (European Central Bank fixing) and the fallback (official peg). An agent can distinguish it from the ~600 sibling pair tools by the pair encoded in the name plus the source statement, though it never names generic siblings like fx_convert or fx_latest to explain the difference.

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: the 'official reference rate ... no estimate' framing tells the agent this is for authoritative single-pair lookups, not conversions or time series. There is no explicit when-to-use, no exclusion against fx_convert/fx_latest/fx_timeseries, and no note about prerequisites such as the x402 payment flow beyond disclosing the price.

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