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

With no annotations, the description carries the full burden and does much of it: it discloses the paid x402 model and price ($0.002 USDC on eip155:8453), the authoritative data source, that output is deterministic and sourced rather than estimated, and that a peg may substitute for a fixing. It stops short of stating update cadence, cache/staleness behavior, or what happens on a non-publication day.

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 that front-loads the payment tag, then the resource, then the qualifiers. Every clause (rate date, method, source attribution) maps to a distinct return field, so little is wasted, though the bracketed pricing prefix competes with the purpose for first 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?

For a zero-parameter, read-only lookup with no output schema, the description returns enough: it enumerates the response contents (rate, rate date, method, source attribution) and warns that a peg may replace the fixing. Only the freshness/update schedule and error behavior are left uncovered.

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 semantics to explain and the baseline of 4 applies. The description correctly implies the currency pair is fixed by the tool identity rather than supplied as input.

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 resource (MXN/SEK reference rate) and a specific source (latest European Central Bank fixing), plus the peg fallback and the derivation method. The pair is named explicitly so an agent can pick it out of the hundreds of fx_* siblings without opening 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 implied — call this for the official MXN/SEK reference rate rather than an estimate. But it never routes the agent among the relevant alternatives (fx_convert for conversion, fx_latest, fx_timeseries, or the inverse fx_sek_mxn), and it gives no condition under which the derived-peg path applies.

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