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

Annotations only provide readOnlyHint and openWorldHint; the description adds the important paid nature of the call—$0.01 USDC on Base—and the payTo address, which is behavior an agent would not know from structured data. The 'Not investment advice' disclaimer is extra context. The payment is an external side effect rather than a data mutation, so it does not contradict readOnlyHint.

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?

The description is concise and front-loads the key fact that the tool is paid, then gives parameters, scope, and payment address. Only the 'Not investment advice' phrase is arguably non-operational, but it does not significantly bloat the description.

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 tool with three optional parameters and no output schema, the description covers payment mechanics, defaults, fiat-only scope, and the alternative for crypto. It does not describe the response format, but that is not essential for invoking the tool correctly.

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?

The input schema already documents all three parameters with coverage at 100%, including defaults and comma-separated format. The description restates this shorthand without adding substantial new parameter-level meaning, so the baseline score of 3 is appropriate.

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?

The description clearly identifies the tool as a fiat FX converter using Frankfurter/ECB, and it explicitly separates this from crypto pricing by directing to '/price'. The verb and resource are specific, and the fiat-only note makes the tool's scope unambiguous.

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 description states 'Fiat only — use /price for crypto', giving an explicit when-not-to-use condition and naming the alternative tool. It also provides defaults for from, to, and amount, so an agent knows how the tool behaves when parameters are omitted.

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