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

Annotations already cover the safety profile (readOnlyHint=true, openWorldHint=false), and the description adds genuinely new behavioral context: the upstream source (ECB), the invariant unit convention, the rule that date filters are ignored without pair, and the cost of '1 request unit'. It does not say what happens on an invalid or non-EUR pair, or whether a bad date range errors or returns empty.

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?

Five short sentences, front-loaded with the resource and the mode-selection rule, then unit semantics, then the date-filter constraint, then cost. Every sentence carries distinct information; nothing repeats the name or the schema.

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 read-only, three-optional-param tool with no output schema, the description covers both modes, the date-filter condition, the rate's unit convention, and the request cost. What's left uncovered is the shape of the returned list or series and the currency universe available, which the agent must infer.

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 the schema already documents all three params, setting the baseline at 3. The description does add real meaning beyond the schema — the conditional applicability of from/to and the 'EUR plus quote currency' construction of pair — but it adds no format edge cases (e.g. lowercase, non-EUR bases) that the schema line 'EUR plus the quote currency, such as EURUSD' doesn't already imply.

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 the exact resource (ECB euro foreign-exchange reference rates) and the two operating modes — omit pair to list all series, pass pair such as EURUSD to read one. It also states the unit semantics ('units of the quote currency for one euro'), so an agent knows both what the tool returns and how to choose between modes without opening the schema.

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?

Conditional usage is explicit: 'Omit pair to list them', 'Pass pair ... to read one series', and 'from and to ... apply only with pair'. That removes the main ambiguity for a tool with three optional params. It stops short of naming any exclusions or alternatives (none of the siblings are FX tools), which keeps it off a 5.

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