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

With no annotations, the description carries the full burden. It does disclose the meaningful commercial behavior — metered cost of 0.001 USDC per call via x402 — and names the data source (ECB), which implies daily reference-rate freshness. It omits rate limits, caching, and what happens on payment failure; the 402 handshake is only in the 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 short sentences, both front-loaded with the capability first and the cost second. Every clause carries information an agent needs; nothing is repeated from schema or title.

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 5-parameter, no-annotation, no-output-schema tool, the description is adequate on capability and cost but silent on return shape, single-date vs range history, supported currency list, and error behavior. An agent knows what it does and what it costs, but not what it gets back.

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 coverage is 80%, so baseline is 3, but the description adds real meaning: 'optional amount conversion' explains what the undocumented 'amount' parameter does, and 'latest or historical' clarifies that 'date' is optional and mutually exclusive with the default latest mode. Only 'symbols' and 'paymentProof' semantics come purely from 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 resource (European Central Bank reference rates), scope (latest or historical), coverage (30+ fiat currencies) and an extra capability (optional amount conversion). That is enough to distinguish it from crypto_price and funding_rates, though it never names those siblings explicitly.

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?

The 'latest or historical' phrasing implies the two usage modes and the 'fiat currencies' scope implies when to prefer it over crypto or conversion tools. However there is no explicit when-to-use, no exclusion of alternatives, and no stated prerequisite beyond the payment note.

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