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

With no annotations at all, the description carries the full burden and does well: it discloses the paid x402 mechanism ($0.002 USDC per call on eip155:8453), the data provenance (ECB fixing), the fallback logic (official peg when SAR is pegged), the disclosed derivation method, and that values are deterministic and never estimated. It stops short of describing freshness/update cadence guarantees or failure 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?

A single dense sentence plus a bracketed pricing tag — no filler. The payment tag is placed ahead of the purpose, which slightly delays the front-loaded statement of what the tool returns, but the cost information is genuinely useful to see early.

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, annotation-free tool with no output schema, the description covers cost, source, derivation method, peg handling, and the fields an agent can expect (rate date, method, attribution). Only freshness cadence and error/empty-result behavior are left unstated.

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; baseline is 4. It correctly implies the pair is fixed by the endpoint rather than passed as an argument.

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 (ISK/SAR reference rate) and the specific verb-like action of returning the latest ECB fixing or official peg. The currency pair in the name plus the stated source make it unambiguous against the hundreds of fx_* siblings, which each cover a different pair.

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 by the resource: an agent needing an ISK/SAR rate would pick this. However, the description never mentions the obvious alternatives in the sibling set — fx_convert, fx_latest, fx_timeseries — nor when to prefer a cross-rate pair like fx_isk_eur or fx_eur_sar instead of this direct pair.

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