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

With no annotations, the description carries full burden. It discloses data freshness (published ~16:00 CET on working days), pricing ($0.01 per call via x402/USDC on Base), and that rates are reference not live. It could further mention rate limits or idempotency, but current info is sufficient for safe use.

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 contains redundant sentences (e.g., 'Reference rates (published ~16:00 CET on working days), not live market rates.' appears twice). Could be more concise by removing repetition and front-loading essential info.

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?

Given no output schema, description explains it returns the EUR reference rate with real observation date, which is adequate for a simple lookup tool. Could specify response fields (e.g., rate value, currency, date) for completeness, but current info is sufficient for basic usage.

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 baseline is 3. Description reiterates schema descriptions (3-letter ISO code, ISO date format, frequency options) without adding new information beyond what the schema already provides.

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 clearly identifies the tool as retrieving official ECB euro reference exchange rates, specifying 29 currencies, latest or by date, and distinguishes it from live market rates. It uses specific verb 'lookup' and resource 'ECB euro reference exchange rates', making purpose unambiguous.

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 provides context for use cases: pricing, accounting, FX, treasury agents. It states 'not live market rates', implying when not to use. However, it does not explicitly exclude alternative tools among siblings (e.g., crypto exchange rate tools) or provide a direct when-not-to-use statement.

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