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

With no annotations, the description carries the full load and does meaningfully disclose cost and payment rail ('[PAID $0.002 USDC/call, xsvc... x402, eip155:8453]'), data provenance (ECB fixing), the derivation method (direct, cross via EUR, or peg-derived), and the peg fallback. It stops short of stating publication cadence/freshness or failure behavior (e.g., weekend/holiday fixing), which would matter for a paid 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?

A single dense sentence preceded by a bracketed payment tag; the resource is front-loaded and every clause (rate, date, method, attribution) adds information an agent would otherwise have to guess. No filler or restatement of the name.

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-annotation, no-output-schema read tool, the description covers the important unknowns: what is returned (rate plus date, method, source), where the number comes from, and what it costs. Missing only timing/freshness expectations and error conditions, which are moderate gaps for a paid endpoint.

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; baseline 4 applies. The description correctly implies a parameterless call by fixing both currencies in the tool name.

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 verb+resource with direction: 'Official TRY/NOK reference rate: latest European Central Bank fixing' plus what accompanies it (rate date, method, source). It is clearly distinguishable from unrelated siblings, though it never states that the inverse direction lives in fx_nok_try or that fx_try_eur etc. are different pairs, leaving the agent to infer directionality from the name.

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?

'Deterministic, sourced, no estimate' implies the use case: authoritative reference rates rather than approximations. However, it gives no explicit when-to-use vs alternatives (fx_convert for conversion math, fx_latest, fx_timeseries) and no exclusions, so routing must be inferred from the pair name alone.

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