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

Annotations already declare read-only, idempotent, non-destructive, non-open-world. The description adds genuinely new facts: the per-call cost (EUR 0.002) and the update cadence (business-day granularity, no intraday). It still doesn't say where the rates come from or what currencies/dates are covered, but the cost and freshness disclosures are meaningful beyond annotations.

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 tight sentence, front-loaded with what the tool returns and immediately followed by the two facts an agent most needs: granularity and cost. No filler.

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 simple read-only lookup with no output schema, the description should hint at the return shape (base currency, per-currency entries, date field), which it omits. Cost and cadence are covered, but an agent still can't predict what the response contains or how limit affects it.

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?

Only one parameter (limit) exists; the schema supplies type, default 50, and bounds 1-500, though with no prose description. The tool description says nothing about what limit controls (number of currencies? rate entries?), so it adds no meaning beyond the schema. Baseline 3 is appropriate given the schema's constraints do most of the work.

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 the specific resource (FX daily reference rates) with a scope qualifier that rules out intraday data. The resource is distinct enough from siblings like get_commodities and get_crypto_tickers that an agent can select it without ambiguity, though it never explicitly contrasts itself with them.

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?

'Each business day, no intraday' implies a usage boundary by excluding intraday/real-time use cases, which is useful guidance. However, there is no explicit when-to-use statement, no prerequisites, and no named alternative for intraday needs.

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