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

Annotations already declare readOnlyHint, idempotentHint and destructiveHint=false, so safety is covered structurally. The description contributes beyond that by disclosing the shape of the payload (current + previous rate with effective dates, recent decision history) and by flagging that this surfaces data from specific FM flows, which no annotation communicates. It says nothing about data lag or refresh cadence for a 'recent decision history' claim.

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 sentences, front-loaded with the identity of the resource and then the use cases. The parenthetical explaining why the DFR is now the primary rate is domain context rather than filler, but the first sentence is dense and could be split for scanability.

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 no parameters and no output schema, the description carries the entire burden of describing return values — and it does so precisely: current rate, effective date, prior rate and its date, and decision history. An agent can decide to call this and interpret the result without further information.

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?

The tool takes zero parameters, so there is nothing to disambiguate and the baseline of 4 applies. The description does not introduce any parameter-like options, correctly matching the empty schema.

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 (the ECB's three official key rates) and enumerates exactly what is returned for each — current rate, effective date, previous rate and its effective date — plus the underlying series keys (DFR/MRR_FR/MLFR). This clearly separates it from the generic ecb_get_data and from ecb_exchange_rate, which covers a different domain.

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?

It gives explicit trigger phrasings ('ECB interest rate', 'ECB deposit rate', 'has the ECB cut rates') and states a negative case ('instead of guessing an FM series key'), which is strong routing guidance. It does not, however, name a concrete sibling alternative such as ecb_get_data or ecb_list_dataflows, so the when-not is only implied rather than resolved to a tool.

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