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

With no annotations provided, the description carries behavioral disclosure. It states the exact data source (European Central Bank), the granularity (annual averages), and the period (from 1999), which meaningfully describes what an agent can expect. It does not mention output format or currentness, but for a parameterless read-only lookup this is a reasonable disclosure.

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 a single compact sentence that front-loads the core question, then adds the data range, source, and intended trigger. Every part earns its place and there is no redundant restatement of schema or title.

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 parameterless tool with no output schema, the description supplies the essential context: what question it answers, the data source and period, and when to use it. A bit more detail about the return shape would be nice, but it is not a serious gap given the tool's simplicity.

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 tool has zero parameters, so the baseline of 4 applies and there is no parameter documentation burden. The description usefully clarifies the fixed scope of the query: yearly averages since 1999, which is sufficient to invoke the tool correctly.

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 what the tool provides: historical USD/JPY annual average exchange rates from 1999 onward, sourced from the European Central Bank. The FX focus distinguishes it from siblings like nagai_crude and prices_now, though it does not explicitly name alternatives.

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 final clause, '物価が上がった背景を聞かれたときに' ('when asked about the background of price increases'), gives an explicit trigger context for using the tool. It does not state when not to use it or point to specific sibling tools, but the intended use case is clear.

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