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: it discloses the paid access model ($0.002 USDC per call via x402 on eip155:8453), the upstream source (ECB fixing), the peg fallback, and the derivation method that will be reported. It omits freshness/TTL, error behavior, and whether results are cached, which are the remaining gaps for a paid data endpoint.

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 pricing tag followed immediately by the resource definition. Nearly every clause earns its place (source, fallback, method, attribution, determinism). The bracketed payment prefix is terse to the point of being cryptic for an agent unfamiliar with x402.

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 exists, and the description compensates by naming what the response contains: the rate, its date, the derivation method, and source attribution. For a zero-parameter, deterministic lookup, that plus the payment and sourcing details is everything an agent needs to call it 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 there is nothing for the description to disambiguate beyond the fixed currency pair, which the name and first clause already cover. Baseline for a parameterless tool is 4, but the description adds no extra semantic constraint on the request itself (e.g., whether a date can be requested at all).

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: the official MYR/XOF reference rate, sourced from the latest ECB fixing with an explicit peg fallback. The currency pair is unambiguous and matches the tool name. It does not differentiate itself from siblings such as fx_latest, fx_timeseries or fx_convert, which sit in the same large family.

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 rather than stated: the phrase 'no estimate' signals this is for authoritative reference rates, not approximations. However, there is no explicit guidance on when to use fx_convert (for amount conversion), fx_timeseries (for history), or fx_latest (for multi-pair spot), all of which are plausible alternatives in this sibling set.

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