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

Annotations only say readOnly/openWorld; the description adds the pricing model ($0.002 USDC per call via x402), that failed calls are free, the three-free-calls-per-day limit, the two-step quote/sign flow, the data provenance (ECB, published free), and the important caveat that rates are informational and not valid for transactions.

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 capability, then cleanly sectioned as 'When to use', 'Parameters', and 'Returns'. The payment block is longer than strictly necessary and the 'for information only, not for transactions' caveat is repeated verbatim at the start and end.

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?

Although there is no output schema, the description names the return values (rate, converted amount, ECB date used) and fully covers the auth/payment path. An agent has everything needed to invoke this correctly, including what happens on failure and without payment.

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 from/to/date/amount/payment are all already documented. The description restates the same facts, adding only the '30 supported' currency count and the default amount of 1; it does not enumerate the supported codes or add format detail beyond the schema.

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 plus scope: 'Convert between currencies at European Central Bank reference rates (30 currencies)'. The ECB-rate and fiat-currency framing clearly separates it from crypto siblings like token_price and crypto_market_pulse.

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?

Provides an explicit 'When to use' line and an unusually clear payment workflow (call without `payment` for a quote, sign and call again; first 3 calls/day free). It does not name an alternative tool for adjacent needs (e.g., crypto conversion), so it stops short of full when-not guidance.

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