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

With no annotations present, the description carries the full burden and does so well: it discloses the once-daily cadence, reproducibility, the 90-day lookback limit, what the response contains (rate, publication date, euro-cross flag), and the cost/payment mechanism ($0.005 per call over x402 USDC). This is exactly the behavioral context an agent needs to pick and budget for the call.

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-loads the core conversion scope, then stacks scope, return shape, positioning and price in three tightly packed sentences with little waste. Slightly dense, and the pricing sentence is arguably a separate concern, but every sentence carries actionable 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 4-param, no-output-schema tool, the description covers inputs, the historical limit, the return shape, and the payment model, so an agent knows both how to call it and what it will get back. Nothing material to correct invocation 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 all four parameters (from, to, amount, date) are already documented, including the ISO 4217 format and the YYYY-MM-DD/90-day constraint. The description reinforces the currency family size and default-window behavior but adds no meaning beyond the schema, so 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 ('Converts an amount between any two of the ~30 currencies in the ECB's official reference rates') and pins the scope to a fixed daily publication rather than a live quote, which is what separates it from any market-rate tool. An agent can identify the tool's job 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 a clear when-to-use framing ('the auditable rate accounting and customs ask for') and an explicit when-not ('not a live market quote'), plus the 90-day historical window. It doesn't name an alternative sibling tool for live rates, so it stops short of full routing guidance.

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