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

Annotations already cover read-only, idempotent, non-destructive, and open-world behavior, so the lower bar applies; the description nonetheless adds real context the annotations don't: the European Central Bank as the upstream source and a $0.01 USDC per-call payment via x402 (Base or Solana). That cost/payment requirement is operationally important and not inferable from structured fields.

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?

Three short sentences, front-loaded with purpose, then source, then format, then price. The date-format sentence duplicates the schema's own pattern, which is a minor redundancy, but nothing is padded.

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 two-parameter read-only lookup with no output schema, the description states what is returned (a single rate) and where it came from, which is sufficient. It omits edge cases such as non-publishing days or missing-date behavior, and return value shape is only 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% and the enum of nine G10 pairs plus the date pattern are fully documented in the schema. The description restates the YYYY-MM-DD format but adds no syntax or semantic detail beyond it, so the baseline 3 applies.

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?

States a specific verb and resource ('Returns the exchange rate for a G10 currency pair on a specific date') and names the data source (ECB). The 'on a specific date' phrasing implicitly separates it from get_fx_rate_current and get_fx_rate_series, but no sibling is named explicitly.

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 only guidance is 'Use YYYY-MM-DD format,' which is a formatting rule, not a usage rule. Nothing tells the agent when to pick this over get_fx_rate_current, get_fx_rate_series, or the get_bundle_fx_dashboard bundle.

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