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/Base payment requirement and price, the authoritative source, the pegged-currency fallback, the derivation method, and explicitly promises a deterministic, non-estimated value. It omits failure and staleness behavior, such as what happens on TARGET holidays or when a fixing is unavailable, and it does not describe the response shape beyond the fields it lists.

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 dense sentence preceded by a bracketed payment header, with the essential facts (pair, source, included fields, determinism) front-loaded and no filler. It is a long run-on clause chain, but every phrase carries information an agent needs before paying for the call.

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-argument, no-output-schema tool the description compensates well by enumerating what the response contains (rate date, method, source attribution) and by flagging the pay-per-call cost, which is decisive for an agent deciding to invoke it. The remaining gap is the missing quotation direction and holiday/staleness behavior, which matter for a reference-rate tool.

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, which is the baseline-4 case; the description correctly adds no parameter claims and instead spends its words on the returned rate's provenance. Nothing is misrepresented about inputs.

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 concrete resource and scope: the official SEK/IDR reference rate from the ECB fixing, with rate date, derivation method and source attribution. It clearly distinguishes this from a market/estimate rate, but it never states the quotation direction (IDR per SEK vs SEK per IDR) and never names siblings such as fx_convert or fx_timeseries that an agent might reach for instead.

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 by the pair-specific name and the 'official ECB' framing, which signals fitness for audit/reference purposes rather than trading. There is no explicit when-to-use statement, no exclusion of alternatives, and no note about when an agent should prefer fx_convert for arbitrary conversion or fx_latest for multi-pair lookups.

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