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

With no annotations, the description carries the full burden and does substantial work: it discloses the paid model ($0.002 USDC/call, x402, eip155:8453), the authoritative source (ECB), the fallback behavior when IDR is pegged, and that results are deterministic and never estimated. What is not covered is payment/auth mechanics, freshness guarantees, or caching behavior.

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 commercial/payment tag followed by the substantive definition, with no filler. The bracketed payment prefix is dense and cryptic on first read but is placed where an agent will see it immediately, so it earns its position.

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 usefully enumerates what comes back — rate date, derivation method (direct, cross via EUR, or peg-derived), and source attribution. For a parameterless lookup gated behind x402 payment, this covers nearly everything an agent needs, with only value precision and freshness cadence left unspecified.

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-and-description burden for argument semantics is nil; the baseline for a no-param tool applies. The description's mention of rate date, method, and attribution describes the response, not inputs, which is consistent with a parameterless lookup.

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 and resource — the 'Official EUR/IDR reference rate' — and pins down the value source (latest European Central Bank fixing, or the official peg when IDR is pegged). The pair is stated unambiguously, so an agent can distinguish it from the hundreds of fx_* pair siblings without inspecting 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 only implied: an agent infers this is the tool for the EUR/IDR official rate. There is no explicit when-to-use framing, no guidance on when to prefer fx_convert or fx_latest, and no note on how this differs from the reverse pair fx_idr_eur. No exclusions or prerequisites are stated 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.