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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint, so safety is covered. The description adds genuinely new context: the data is official ECB data served via the Frankfurter API and is free of cost. It stops short of stating rate freshness/as-of date, caching, or failure behavior for unknown currency codes.

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?

Extremely short with zero redundant sentences and the key identifiers front-loaded. The trade-off is that it reads as fragments rather than a sentence, leaving the tool under-explained rather than padded.

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 simple two-optional-parameter read tool with rich annotations, the definition covers source and cost. But with no output schema, the description gives no hint of the response shape (rate date, base/rates structure) or behavior on invalid ISO codes, which an agent would benefit from knowing.

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 both parameters are documented with examples and the EUR default in the schema itself. The description's 'base + symbols' merely echoes those parameter names and adds no format, default, or constraint information beyond the 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 and source ('Official ECB cross-rates (Frankfurter)'), so an agent can tell it returns FX rate data. However, it is a telegraphic noun phrase with no verb, and it never distinguishes itself from the sibling 'kzt_rates', which is also a rates tool. Purpose is inferable but not stated as an action.

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, no prerequisites, and no mention of alternatives such as kzt_rates for Kazakhstani tenge rates. The word 'Free' hints that no API key or billing applies, but that is the only usage-adjacent signal.

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