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 that the call is paid ($0.002 USDC via x402 on eip155:8453), names the authoritative source (ECB), explains the derivation method (direct, cross via EUR, or peg-derived) and asserts determinism with 'no estimate'. It stops short of covering freshness cadence, holiday/weekend behavior, or error modes for unavailable fixings.

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, dense but nearly every clause earns its place (price, source, method, determinism). The bracketed payment prefix is front-loaded but competes with the purpose statement for the opening position, pushing the actual what-it-does slightly late.

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, no-output-schema tool, the description supplies exactly what the schema cannot: what the number means, where it comes from, how it was derived, and that the response carries the rate date, method, and attribution. An agent can call this and interpret the result without further documentation.

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 no parameter meaning to add; the baseline for a parameterless tool applies. The description instead spends its words on the pair and the data semantics, which is the right allocation.

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 states a specific resource (the official CAD/CVE reference rate) and its provenance (latest ECB fixing or the official CVE peg), which is more than the pair-encoded name alone conveys. It does not explicitly differentiate itself from general alternatives such as fx_convert or fx_latest, which matters in a family of hundreds of fx_<base>_<quote> siblings.

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 name, so an agent can infer 'call this for CAD→CVE'. There is no explicit when-to-use statement, no guidance on when to prefer fx_convert, fx_latest, or fx_timeseries instead, and no note on how stale a fixing may be.

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