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

With no annotations, the description carries the full burden and does disclose the important commercial terms: a $0.002 USDC charge per call via x402 on eip155:8453, meaning a payment/auth step is required. It also states provenance and that results are deterministic rather than estimated. It says nothing about failure behavior (e.g., what is returned when no fixing exists or OMR's peg changes), which matters for a paid, deterministic data call.

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, dense but largely waste-free, with the returned fields (rate date, method, source) enumerated compactly. The payment metadata is placed first rather than after the purpose, which slightly delays the most important content, but it is genuinely load-bearing information.

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?

There is no output schema, so the description usefully enumerates the return fields (rate, rate date, derivation method, source attribution) and explains the method variants. Given the trivial zero-parameter schema, the only real gap is error/edge-case behavior and the exact rate convention.

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 beyond what the schema already shows. The baseline applies.

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 a specific verb+resource pair ("Official XOF/OMR reference rate") and pins the source to the ECB fixing or official peg, so an agent knows exactly which rate is returned. It does not differentiate from nearby siblings such as fx_convert, fx_latest, or fx_timeseries, which overlap in intent.

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 by the fixed currency pair and the "latest fixing" wording; there is no explicit statement of when to call this versus fx_convert (for arbitrary amounts) or fx_latest (for multiple pairs at once). No exclusions, prerequisites, or fallback guidance are given.

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