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 on cost, payment protocol, determinism, and sourcing ('no estimate'). It does not address failure modes (stale fixing, weekend/holiday gaps, what happens if the ECB has no print) or response shape beyond the fields listed.

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?

Front-loads pricing and payment scheme, then the purpose, in a tight two-clause structure with no filler. The middle clause (rate date, method, source attribution) is dense but each element is informative.

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 0-param tool with no output schema, describing the returned rate, date, derivation method (direct/cross via EUR/peg) and attribution covers what an agent needs. The main omitted piece is how to satisfy the x402 payment flow before calling.

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?

Zero parameters, so the baseline of 4 applies; there is no argument semantics to explain. The description spends its space on return-value meaning instead, which is the right trade-off.

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 precise verb+resource: the official XAF/SAR reference rate, and pins down the source (European Central Bank fixing) and the peg fallback. Despite hundreds of near-identical fx_* siblings, the pair and the sourcing rule make the tool's identity unmistakable.

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?

The bracketed prefix tells the agent this is a paid call ($0.002 USDC via x402 on eip155:8453), which is real selection signal, but nothing says when to pick this over fx_convert, fx_latest, fx_timeseries, or the inverse fx_sar_xaf. Usage is only implied by the 'reference rate' framing.

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