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

Goes well beyond the readOnlyHint/openWorldHint annotations: it discloses the per-call price ($0.01 USDC on Base), the required x402-capable wallet client, the no-interpolation/no-carry-forward data semantics, and the hard 90-day/50-item caps. Cost and auth requirements are exactly the kind of behavioral 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?

Front-loaded with the core operation and its limits, then data semantics, pricing, and auth. Nearly every sentence carries load, though the pricing/auth sentences could be tightened slightly without losing 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?

For a parameterized read-only range query with no output schema, the definition supplies the constraints, data semantics, cost, and access requirement an agent needs to decide and invoke. The only gap is the shape of the returned rate records, which the description leaves 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?

With 100% schema coverage the baseline is 3, but the description adds real meaning: "Quote units per base unit" clarifies the relationship between the `currency` (quote) and `baseCurrency` (base) parameters, and it reinforces the 50-item/90-day bounds on maxItems and the date range.

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: reading ECB reference or same-day cross rates for one currency pair, with explicit scoping (daily rates, ≤90 days). This is unambiguous. Sibling differentiation is a non-issue since no other sibling tool covers FX data, so it sits just below the top of the scale.

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?

"Reference data, not transaction quotes" gives a useful usage caveat about the nature of the data, and the ≤90-day/≤50-item bounds imply scope. However, it never says when to reach for this tool versus an alternative source or what to do when a range exceeds the limit — usage is implied rather than stated.

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