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

Annotations are all false/absent, so the description carries the full burden. It discloses the key behavioral traits: returns both directions, never reports a reciprocal as the published figure, shortens out-of-window dates instead of refusing, and names the actual rate date. It also explains the free-window cutoff and Pro coverage. This is rich, non-obvious behavior that an agent needs to know before calling.

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?

The description is compact and front-loaded: the first sentence states the tool's purpose, the second clarifies the return behavior, and the third covers edge cases. Every sentence earns its place, and the parameter descriptions in the schema carry the detailed semantics without bloating the main description.

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 3-parameter tool with no output schema, the description plus schema covers everything an agent needs: what the tool returns, how dates are handled, the free-window limitation, and the currency convention. The sibling list provides enough contrast to avoid confusion with rate_history or rates_latest. No critical gap remains.

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 coverage is 100%, so the baseline is 3. The description adds meaningful context beyond the schema: it explains the ECB convention (quotes every currency per 1 euro), warns that 'the ECB rate for USD' means from EUR to USD, and clarifies that the date parameter's fallback behavior. The only reason it's not a 5 is that the schema already documents the parameters thoroughly, so the description's added value is complementary rather than essential.

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 states a specific verb ('Call this tool for'), a precise resource ('the ECB rate of one pair on one date'), and immediately distinguishes it from siblings like rate_history and rates_latest by scoping it to a single pair and a single date. It also clarifies the return shape ('both directions and the rate date'), which removes ambiguity about what the tool does.

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?

The description explicitly says when to call this tool ('for the ECB rate of one pair on one date') and implicitly contrasts with siblings that handle multiple dates or multiple pairs. It also gives concrete behavioral guidance: a reciprocal is never reported as the published figure, and older dates are shortened rather than refused. This is enough for an agent to select it over rate_history or rates_latest.

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