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

Annotations already cover the safety profile (readOnlyHint, openWorldHint), and the description adds genuinely non-redundant behavior: the data provenance (ECB reference rates, coverage since 1999) and the paid-call cost ($0.003 USDC via x402). The payment requirement in particular is critical context an agent cannot get from annotations or schema.

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 core capability, then output shape, then cost. No filler; every clause adds information.

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 helpfully enumerates the returned series and statistics, and discloses the payment model, which is sufficient for an agent to invoke it correctly. The remaining gap is the undocumented parameter contract at 0% schema coverage.

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 description coverage is 0% across all four parameters (start, end, base, symbols), so the description carries the full burden. It only vaguely gestures at 'date range' and 'per currency' without naming the parameters or specifying accepted date formats and symbols syntax (e.g. comma-separated ISOs), leaving most of the parameter contract undocumented.

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?

Specific verb+resource: historical ECB reference rates since 1999 over a date range, with an explicit enumeration of computed outputs (change, min, max, mean, volatility). The 'Historical' qualifier implicitly separates it from the sibling 'fx' tool, but it never names the alternative, so differentiation is left to inference.

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?

Usage is implied by 'for any date range' and 'per currency', so an agent can infer the call shape. However, there is no explicit when-to-use statement, no exclusion, and no reference to the sibling 'fx' for spot/current rates, leaving the routing decision to the agent.

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