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

Annotations already state read-only/idempotent behavior, and the description adds useful operational disclosure: authoritative source (ECB SDW), no auth required, and daily series granularity. Nothing contradicts the 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?

Three compact sentences front-load the core purpose, then add filtering and source/auth context. Every sentence earns its place with 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 zero-required-parameter read tool with a full output schema, the description covers the essentials: what rates are available, filtering modes, data source, and access requirements. No missing information an agent needs to invoke it correctly.

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 schema already documents all four parameters, including enum meanings, patterns, defaults, and mutual exclusion. The description restates the rate types and filter relationship without adding new parameter-level meaning.

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?

Description uses a specific verb ('Get') and identifies the exact resource: official ECB key interest rates, listing all three rate types (deposit facility, main refinancing, marginal lending). This clearly distinguishes it from ECB macro-rate siblings like ecb-rates.macro.*.

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 tells the agent that data is daily and how to constrain the query: choose rate_type, and use either last_n_observations or start_period/end_period. It doesn't explicitly name alternative tools or exclusion conditions, so it lacks the strongest sibling-routing guidance.

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