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

Annotations already declare readOnlyHint=true and openWorldHint=true, so the safety profile is covered. The description adds only that the output is raw data rather than advice, which is useful but thin; it says nothing about update cadence, caching, or the shape of the returned 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?

Two short sentences, zero filler, with the subject of the data front-loaded before the non-advice caveat.

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 read-only data tool with no output schema, the description covers what the response contains at a high level. It does not characterize the structure or units of the series, which an agent assembling a response would eventually need, but it is adequate for correct invocation.

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% and the single 'history' parameter is fully documented in the schema (default 4, min 1, max 24). The description's phrase 'recent history' loosely echoes it but adds no syntax or semantics beyond the schema, so the baseline of 3 applies.

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 (the three ECB key interest rates) and what the payload contains (latest change dates and recent history), so the agent knows exactly what data comes back. It does not, however, distinguish itself from the close sibling fx_rate_ecb, which also deals with ECB-published rates.

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?

No when-to-use guidance, no exclusions, and no pointer to fx_rate_ecb for currency conversion versus policy rates. 'Data only, not advice' is a liability disclaimer, not usage guidance.

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