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

No annotations are provided, so the description carries the full burden. It discloses the $0.005 cost, the return metadata (official source citation, effective date, last-confirmed date, staleness flag), and the api_key/x402 payment flow, which is valuable behavioral context beyond just saying 'get current value'.

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 compact sentences convey cost, purpose, return value composition, and authentication flow with no unnecessary words. Every sentence 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 the absence of an output schema, the description sufficiently explains what the return values contain and how payment works. It does not mention error handling or invalid inputs, but for a simple lookup tool with three params, the description is largely complete.

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 input schema already covers all three parameters with descriptions (100% coverage), but the description adds value by giving examples for series, explaining api_key as a payment bypass, and implying the country enum with ISO codes. This goes beyond what the schema alone provides.

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 states 'Current value of a reference series' with examples, clearly identifying the tool's action (fetch current value) and resource (reference series). This distinguishes it from siblings like get_series_history and list_series, which are for historical or listing purposes.

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 indicates this tool is for current values and mentions the payment process, but it does not explicitly contrast with alternative tools like get_series_history or list_series. The usage context is implied rather than fully spelled out.

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