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

Goes well beyond the readOnly/idempotent/openWorld annotations: discloses the 366-day cap, working-day-only cadence, per-currency return statistics (first, last, min, max, percent change), and full payment behavior ($0.002 via x402 USDC on Base, PaymentRequired on unpaid calls). This is exactly the kind of operational context 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?

Two dense sentences, front-loaded with the use case and return format before the payment details. It is information-rich but each clause earns its place; the pricing sentence is wordy yet operationally necessary.

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 carries the return burden and does so by naming the per-currency statistics, plus it documents the date range, currency cap, and the full payment flow. Nothing needed 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 description coverage is 100%, so the baseline is 3, but the description adds constraints not in the schema: the 366-day maximum span and the re-basing behavior of the base parameter. These give the agent calling guidance beyond field labels.

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 daily ECB reference rates) and scopes it to trend/history queries, which clearly separates it from the current-rate siblings like fx_latest and fx_convert. An agent can tell the historical nature immediately 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 explicit triggering contexts ('trend questions', 'back-dated conversions'), which implies the siblings fx_latest/fx_convert handle current rates. It does not name those alternatives outright or state exclusions, so it stops short of the 5-level routing guidance.

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