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

Annotations already declare readOnlyHint=true, idempotentHint=true, openWorldHint=true, and destructiveHint=false, which cover safety. The description adds valuable context: coverage start date, weekend/holiday fallback to preceding business day, no auth required, and public domain data. These are useful behavioral details beyond the structured 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 concise, uses a single paragraph with clear sentences, and front-loads the primary purpose. It includes necessary caveats (weekend/holiday, start date) without unnecessary filler. Slightly could be tighter but thoroughly informative within a compact structure.

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?

The description provides all relevant context for an agent to call this tool correctly: the API source, historical date scope, currency coverage, optional parameters, and the lack of auth requirements. Since an output schema exists, the description does not need to explain return values. Given the sibling tools and annotations, this is complete.

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% – every parameter has a description in the schema. The tool description does not add significant semantic meaning beyond what the schema already provides for base, date, amount, and symbols. It does mention the currency list and amount scaling, but this is redundant with the schema descriptions. Therefore, baseline 3 is appropriate.

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 uses a specific verb ('Retrieve') with a clear resource ('ECB reference exchange rates') and scope ('for a specific historical date'). It distinguishes from sibling tools by emphasizing the historical date aspect裠which makes it clear this is not the 'latest' or 'series' variant.

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 implicitly signals when to use this tool (single historical date) versus the 'latest' and 'series' siblings, and it explains the weekend/holiday fallback behavior. However, it does not explicitly name alternative tools or state 'use this for historical point-in-time queries, use latest for current rates, use series for date ranges' – a slight gap.

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