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 x402 payment cost ($0.002 USDC/call on eip155:8453), the data source (ECB), the derivation method (direct, cross via EUR, or peg-derived), and that results are deterministic and non-estimated. It does not cover failure modes (e.g., what happens on a non-fixing day) or staleness handling beyond returning a rate date.

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?

It is a single dense sentence with no filler, and the cost/transport tag is front-loaded for immediate budget awareness. Slightly better structure would lead with the purpose before the pricing prefix, but nothing is wasted.

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 supplies the essentials: what rate, from which source, how derived, and which fields accompany it (rate date, method, source attribution). Only edge-case behavior on missing fixings is left unaddressed.

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 of 4 applies. The description correctly implies a parameterless single-pair lookup, consistent with the empty schema.

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 exact resource (IDR/XOF reference rate) and the source (latest ECB fixing, or the official peg when XOF is pegged), so the agent knows precisely what value it gets. It does not explicitly distinguish itself from siblings like fx_xof_idr (inverse pair), fx_latest, or fx_convert, which keeps it 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?

Usage is implied by 'latest ... fixing' and 'no estimate', which signals a spot-reference lookup rather than a conversion or time series. However, there is no explicit statement of when to prefer fx_convert, fx_latest, or fx_timeseries instead, so the agent must infer routing from the tool name.

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