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

With no annotations provided, the description carries the full burden and does substantial work: it discloses the payment protocol (x402), the network (eip155:8453), the exact per-call cost, the upstream source, the possibility of a peg fallback, and an explicit determinism/no-estimate guarantee. It omits freshness/latency characteristics (how stale the latest fixing can be) and any failure behavior, which keeps it from a 5.

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?

One dense sentence plus a bracketed pricing tag — no filler, and the cost/payment context is front-loaded so an agent sees the price before committing. The parenthetical about methods is slightly cluttered but conveys real 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?

No output schema and no annotations means the description must describe the return shape, and it partially does — rate date, derivation method, and source attribution are named. However, it never states the rate value field itself, the quotation direction (OMR per XAF or the inverse), or numeric precision, all of which an agent needs to interpret the response.

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 schema has nothing to document and the baseline of 4 applies. The fixed pair and fixed source are correctly encoded in the name and description rather than in inputs.

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 the exact currency pair (XAF/OMR) and states it returns the official reference rate — a specific verb+resource, not a restatement of the name. It adds the source (ECB fixing) and the fallback case (official peg). It stops short of distinguishing itself from siblings like fx_convert or fx_timeseries by name, which keeps it at 4.

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 description implies when to use it — when you need the authoritative spot reference for this pair rather than a computed conversion — but never states that explicitly or names an alternative. The paid-cost note ($0.002 USDC/call) gives an implicit incentive to consider cheaper options but offers no guidance on when that trade-off is worth it.

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