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

With no annotations, the description carries the full burden and does much of it: it discloses the price ($0.002 USDC/call), the payment protocol and network (x402, eip155:8453), the data provenance (ECB fixing), the deterministic non-estimated nature, and the peg fallback for KMF. It does not cover failure behavior (what happens if payment is rejected or no fixing exists for the date) or the update cadence beyond 'latest'.

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-loaded with the payment terms, then the rate identity, then the returned fields — the ordering matches what an agent needs to decide and then call. It is a single dense sentence that is slightly run-on but contains no filler.

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 must describe the return payload, and it does: the rate, the rate date, the derivation method (direct, cross via EUR, or peg-derived) and source attribution. Combined with the payment and determinism disclosures, an agent has everything needed to call this zero-parameter endpoint correctly.

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; there is nothing for the description to clarify. The pair and direction are baked into the tool identity, leaving no ambiguity about what is being requested.

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 USD/KMF reference rate' — and the tool name encodes the exact currency pair, so an agent can place it among the hundreds of fx_* siblings. It does not, however, explicitly contrast itself with fx_convert, fx_latest or fx_timeseries, which are the most plausible confusion points.

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 the pair-specific naming (fetch the USD→KMF reference rate), and the payer context signals this is a paid per-call endpoint. There is no explicit when-to-use/when-not guidance, no mention of when to prefer fx_convert or fx_timeseries, and no statement that the inverse direction is served by fx_kmf_usd.

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