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

Annotations already cover read-only, idempotent, open-world, non-destructive. The description adds substantial context beyond them: the exact cost (0.002 USDC per call via x402), supported payment chains, and the licensing/disclaimer that ECB rates are free-of-charge and informational only. This goes well past what the annotations declare.

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 usage trigger and content scope efficiently. Most sentences earn their place, though the ECB-website availability note is somewhat extraneous marketing/flavor that could be trimmed without losing agent-relevant 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?

With an output schema present, the description need not explain return values. It covers the essential agent-facing facts: what data is returned, the base/cross behavior, cost, payment methods, and the informational-only limitation. Nothing needed to call it correctly 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 coverage is 100% and the single 'base' parameter is fully documented in the schema. The description's mention of 'as published or as crosses for another base' largely restates the schema rather than adding new syntax or format detail, so the baseline 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?

States a specific verb and resource: returns ECB euro FX reference rates for ~30 currencies, in published form or as crosses, plus Hyperliquid HIP-3 FX/commodity marks. This clearly distinguishes it from the many crypto price siblings (get_btc_price, get_eth_price, get_sol_price) since it is fiat reference-rate focused.

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?

Opens with an explicit trigger: 'Use when an agent needs official daily exchange rates.' It also gives the important caveat that the rates are for information only, not for transactions, which guides appropriate use. It does not, however, name a sibling alternative (e.g. get_convert) or state when NOT to use it.

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