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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint=false. The description adds valuable behavioral context beyond that: it specifies the data source (IMF IFS, keyless), the shape of the response (latest value, month, and recent monthly history), and a concrete current rate snapshot, which helps calibrate 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 compact and front-loads the core definition and return behavior, then adds usage guidance, source, and a helpful current-rate note. The final sentence about negative rates and the January 2025 hike is contextually useful but slightly beyond what is strictly required to invoke the tool, so it is not maximally lean.

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 single-parameter, read-only tool with no output schema, the description is complete: it states what is returned, the data source, the units, and even a current example. An agent has everything needed to select and invoke it correctly for the intended query.

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 description mentions 'recent monthly history,' which aligns with the months parameter, but the schema already fully documents that parameter with a clear meaning and default. With 100% schema coverage, the description adds no additional parameter-level value beyond reinforcing the idea of history length, so the baseline of 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 clearly identifies the resource (Bank of Japan policy interest rate) with a specific verb ('Returns the latest value, its month, and recent monthly history'). It also uses precise qualifiers ('short-term policy rate / Monetary Policy-Related Interest Rate, % per annum') that distinguish it from the sibling boj_inflation tool.

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 gives explicit guidance: 'Use this for any Bank of Japan interest rate / BOJ rate / Japan policy rate question.' This clearly states when to use the tool, though it does not explicitly mention exclusions or the alternative boj_inflation, so it stops short of a 5.

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