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

With no annotations, the description carries the full burden. It discloses the cost, authentication/payment behavior, and the notion of effective date ranges. It does not describe output format or error handling, but the most critical behavioral traits (cost, auth, historical scope) are covered.

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, front-loaded with the paid warning, no filler. All information is relevant and efficiently presented.

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?

Without an output schema, the description should more concretely explain the return shape (e.g., array of date-value pairs). 'Historical values with effective date ranges' gives the gist but not enough detail for a paid API. It also leaves the relationship to get_series implicit.

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. The description adds marginal value by explaining api_key's optionality and the YYYY-MM-DD format for from/to, but it does not elaborate on series or country beyond what the schema already 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 clearly states the tool returns historical values of a reference series with effective date ranges, distinguishing it from sibling tools like get_series or get_snapshot by the 'historical' focus. Though it lacks an explicit imperative verb, the intent is unambiguous.

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?

It provides clear context: the tool is paid ($0.005), supports optional from/to filtering, and explains the api_key vs x402 payment flow. However, it does not explicitly mention when to prefer this over get_series or list_series, leaving some use-case ambiguity.

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