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

With no annotations supplied, the description carries the full burden and does well: it discloses the cost and payment mechanism ($0.002 USDC, x402, eip155:8453), the derivation method (direct, cross via EUR, or peg-derived), and asserts 'deterministic, sourced, no estimate' — behavior an agent cannot infer elsewhere. It stops short of stating freshness cadence or what happens when no fixing is available.

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, front-loaded with the payment/pricing tag an agent needs before calling, then the core purpose and output fields. Every clause (method, rate date, attribution, no-estimate guarantee) adds distinct information with no repetition.

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, and the description compensates by enumerating the returned fields: rate, rate date, method, and source attribution. For a zero-parameter, single-value lookup this is nearly sufficient; only failure/no-fixing behavior and update frequency are unstated.

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 disambiguate. The currency pair is fixed by the tool name itself, which is 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?

Names the exact resource and scope: 'Official RON/SAR reference rate' with 'latest European Central Bank fixing (or the official peg when SAR is pegged)'. The pair is unique in the sibling list (no fx_ron_sar or fx_sar_ron duplicate), so an agent can select it unambiguously without opening the schema.

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 phrase 'official reference rate' and the peg fallback note, which tells the agent this is the authoritative/regulatory source rather than a market mid. However, it never states when to prefer this over siblings like fx_convert, fx_latest, or fx_timeseries, nor any exclusion or prerequisite beyond the payment tag.

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