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

Annotations already declare readOnly, idempotent, and non-destructive behavior, and the description adds meaningful context beyond that: the Worker cron refresh from Frankfurter, daily rather than live data, the snapshot.captured_at and snapshot.age_hours response fields, the payment requirement and failure mode, and the no-data-no-charge policy. No contradiction with 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?

The description is front-loaded with the core purpose and then layers caveats, response metadata, cost, and payment behavior. It is dense but each sentence adds relevant operational information, though the equivalent HTTP route line is slightly peripheral for an MCP agent.

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 tool with no output schema, the description explains the core behavior, data freshness, key response metadata, and the important payment-challenge failure mode. It does not spell out the full rate response structure, but an agent has enough to call and interpret the tool 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 description coverage is 100%, and each parameter already has a clear description and example. The tool description adds useful context about base currency and ECB observation date, but it does not need to compensate for schema gaps because the schema is already thorough.

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 resource as European Central Bank reference foreign-exchange rates for a chosen base currency, with the ECB observation date. It is immediately distinguishable from the sibling crypto, defi, and prediction-market tools, and the 'not live tradable quotes' caveat sharpens the scope.

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 states these are daily reference rates and explicitly warns they are not live tradable quotes, which tells an agent when not to use them. It also explains the as_of historical lookup behavior via the schema. It does not name a specific alternative tool, but the exclusion is clear enough in context.

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