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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the tool's safety profile is known. The description adds valuable behavioral context: the ECB publication update frequency, the fail-closed behavior for unsupported currencies, the fact that it's free, and the exact return fields including cache_age_minutes. This goes beyond the annotations and sets clear expectations.

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 four sentences that efficiently pack purpose, use case, update schedule, failure behavior, cost, and return format. It is front-loaded with the main action and every sentence serves a purpose. The 'Returns' line is a helpful structured addition. It could be slightly tighter, but it's well-organized and not wasteful.

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?

Despite having no output schema, the description explicitly lists the return fields: rate, from, to, source, fetched_at, cache_age_minutes. It also covers update frequency, fail-closed behavior, and cost. For a simple two-parameter, read-only tool with rich annotations, this is fully complete — the agent has everything needed to invoke it and interpret the result.

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 input schema covers 100% of the parameters with descriptions and examples, so the baseline is 3. The description adds a list of supported currency codes (USD, EUR, INR, SGD, CNY, JPY) and mentions the 'official ECB daily series,' which gives a bit more context. However, this is marginal since the schema already indicates 3-letter codes with examples.

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 opens with a specific verb+resource: 'Get the current certified ECB reference rate between two supported currencies.' This clearly differentiates the tool from every sibling tool, since none of the other shippingrates_* tools handle currency conversion. The scope is well-defined and 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 explicitly states when to use this tool: 'useful for converting shipping costs quoted in different currencies' and 'Use this to normalize costs from different carriers/countries to a common currency for comparison.' It does not explicitly mention alternatives or exclusions, but since it is the only FX tool among the siblings, the use case guidance is clear and sufficient.

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