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

Annotations already establish read-only, idempotent, non-destructive, non-open-world behavior, so the bar is lower, and the description still adds substantive context: the data is indicative rather than real-time or executable, and callers should inspect observation_date for freshness. That staleness/provenance caveat is exactly the kind of behavioral nuance annotations cannot convey.

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?

Three short sentences, no filler, with the core identification front-loaded before the freshness caveat and the attribution note. The final sentence about the original ECB source is slightly tangential but does signal provenance/licensing, so it largely earns its place.

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?

With an output schema present, return-value explanation is not required, and the annotations cover safety. The description supplies the crucial freshness caveat for a low-complexity single-parameter tool; the only meaningful gap is not routing the agent away from the history siblings.

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% and the schema itself documents both the format (uppercase ISO code, e.g. USD) and the unit semantics ('currency units per one EUR'), plus the omit-for-all behavior. The description adds nothing beyond that, so the baseline 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?

The description names a specific resource and scope: 'ECB indicative daily currency reference rates against EUR,' which tells an agent exactly what data flows back. It does not, however, distinguish this tool from its siblings get_exchange_rate_history or list_exchange_rate_history, nor does it say whether the response is a latest snapshot or a range, leaving real ambiguity about which of the three rate tools to pick.

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?

There is no when-to-use guidance and no mention of the obvious alternatives (get_exchange_rate_history, list_exchange_rate_history). The caveat 'Not real-time or executable prices' tells an agent how to interpret the values, but nothing tells it when this tool is the correct choice versus its siblings.

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