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

Adds substantial context beyond the readOnly/idempotent/openWorld annotations: the 62-day threshold switches from a daily series with percentage change to monthly averages capped at 36 months, no company data is required, and 'quote rates from the result only' warns against fabrication. This is exactly the behavioral detail annotations cannot carry.

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?

Resource and mode are front-loaded, and the sentence flow (what it is → when to use → what each input shape returns) is logical. The usage examples are slightly list-heavy, but each pulls weight by mapping to a real user phrasing.

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?

With no output schema, the description still fully explains return shapes (closing rate, daily series plus percentage change, monthly averages) and prerequisites (no company data). Nothing an agent needs to invoke it correctly is missing.

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 baseline is 3, but the description adds the interaction semantics the schema only hints at – a single date returns that day's closing rate, a start/end range under 62 days returns a daily series, longer returns monthly averages. That materially clarifies how to combine date vs start_date/end_date.

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?

Opens with a specific resource and verb ('Exchange rates: the closing rate for a currency pair on a date, or a series over a range'), instantly distinguishing it from siblings like get_income_by_currency or get_balance. An agent knows exactly what the tool returns 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?

Gives rich when-to-use triggers ('user asks for an exchange rate, FX, a currency conversion, ... whether it is cheaper to buy in one currency or another'), covering several concrete phrasings. It stops short of naming the alternative sibling or stating when not to use it, so it lands just below a full 5.

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