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

Adds value beyond annotations by describing the output structure (fields and examples). Annotations already indicate read-only, idempotent behavior. Could detail error handling for invalid dates, but still good.

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?

Two sentences, zero filler. Front-loaded with purpose and output. Second sentence provides critical usage hint. Every word earns its place.

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?

Given no output schema, the description fully specifies the return structure. Annotations cover safety, idempotency. Input behavior is clearly explained. Complete for a simple read-only tool.

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 covers 100% with format description. The description adds behavioral semantics: omitting date gives today's rates. This complements the schema without redundancy.

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?

Clearly states it fetches official NBU exchange rates for ALL currencies on a single day, distinguishing it from siblings like nbu_currency_rate (single currency) or nbu_currency_history (historical data). The verb 'fetch' is implied by context.

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?

Provides explicit guidance: 'Omit date for today's rates.' This tells the agent when to use the default vs. specify a date. Does not explicitly exclude other tools, but the sibling list makes alternatives available.

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