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

With no annotations, the description carries the full burden and does substantial work: it discloses data provenance (ECB statistics), a 'reference only' reliability caveat, hard limits (1999-01-04 floor, 366-day max, 1-10 symbols), defaults, output contents, and even the payment model ($0.002 USDC via x402). It does not mention rate limits, error behavior, or whether non-business days are absent from the series.

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?

Front-loaded with the when-to-use clause, then input, output, source, example, and price — a sensible order with no wasted framing sentences. It is dense to the point of being a run-on, and the example's 2026 dates are an odd artifact, but every block carries information.

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?

No output schema exists, yet the description enumerates the return payload (daily series plus per-currency first/last, % change, min/max with dates, average, daily volatility), and with no annotations it still covers limits, defaults, source, and cost. An agent has everything needed to construct a valid call.

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?

Schema description coverage is 100%, so the baseline is 3; the description adds a little by restating defaults and limits in prose and by giving a worked query example (?base=USD&symbols=KRW,JPY&start=...&end=...) that shows how the parameters combine. It does not add format or semantic detail beyond what the schema already documents.

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 verb and resource — 'historical exchange-rate series or a period summary' — and the title/description pin the source to ECB daily fixings, which separates it from generic rate tools. It never names its closest siblings (fx_rates, fx_convert), so the agent must infer that this is the historical/series variant rather than the spot-conversion one.

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?

'Use when you need a historical exchange-rate series or a period summary' plus a concrete scenario ('USD/KRW over the last quarter') gives a clear selection signal. There are no explicit exclusions or named alternatives (e.g. 'for a single current rate use fx_rates'), so the boundary with siblings is left implicit.

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