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

With no annotations, the description carries the disclosure burden and does it well: it states the cost ($0.002 USDC/call), the payment protocol and chain (x402, eip155:8453), the data source (ECB), the derivation method (direct, cross via EUR, peg-derived), and explicitly that the value is deterministic and not an estimate. It does not cover staleness windows or error/failure behavior, which keeps it from 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?

Two sentences, front-loaded with the payment terms and then the substantive purpose, with no filler. The bracketed pricing prefix is dense but earns its place by surfacing a cost the agent must know before calling. Minor density cost keeps it just below top marks.

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, the description tells the agent what comes back: the rate, the rate date, the method, and source attribution. It does not specify the response structure, field names, or quoting convention/precision, which is a real gap given no output schema exists to cover it.

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 baseline for a no-param tool applies. The description correctly assumes no input is needed and spends its space on output and provenance instead.

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 (MXN/USD official reference rate) and pins down its provenance: latest ECB fixing, or the official peg when USD is pegged. That is far more precise than a bare pair name. It does not, however, differentiate itself from the hundreds of other fx_* pair siblings or from fx_convert/fx_latest/fx_timeseries, so it stops short of 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?

There is no when-to-use guidance: nothing says to prefer this over fx_convert for arbitrary conversions, fx_latest for multi-pair snapshots, or fx_timeseries for history. The 'or the official peg when USD is pegged' clause describes internal fallback behavior, not selection guidance. In a sibling set of this size, routing help is the main missing piece.

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