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

With no annotations, the description carries the full burden, and it does well: it discloses the paid x402 price ($0.002 USDC per call on eip155:8453), the determinism guarantee, the source attribution, and that the value is a fixing or peg rather than an estimate. What is missing is fallback behavior when no fixing exists (weekends/holidays) or typical error conditions.

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 compact sentence plus a short qualifier, with the key identifying information (pair and source) up front; the bracketed payment prefix is upfront and useful for an agent that must budget a paid call. Every clause carries information; slightly dense but not padded.

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 tool with no output schema, the description covers the essential return contents (rate, rate date, derivation method, source attribution) and the pricing model. The only real gap is the absence of edge-case behavior for non-fixing days, which matters for a daily-fixing-based rate.

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 and baseline 4 applies. The description correctly avoids inventing parameter talk and instead spends its words on output semantics.

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 resource (INR/XOF reference rate) and the specific source (latest ECB fixing, or the official peg). That makes it distinguishable from the hundreds of `fx_*` pair siblings by name alone, though it never explicitly contrasts itself with `fx_convert` or `fx_latest`, which also return rates.

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: the pair is baked into the tool name, so an agent can infer this is for INR/XOF lookups. There is no explicit statement of when to prefer this over `fx_convert`, `fx_latest`, or `fx_timeseries`, and no mention of when it is inappropriate (e.g. historical series requests).

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