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

Annotations already declare readOnlyHint=true, destructiveHint=false, openWorldHint=true, and idempotentHint=false. The description reinforces a few of these ('reads or computes only and changes nothing', 'each call is charged, so repeats are not free') and adds genuinely new cost context: $0.01/call via x402 USDC or a card key, first 25 free, failed calls free. However, the explicit restatement of annotation-covered safety traits dilutes the added value, and no return shape or pagination detail is given, so a 3 is appropriate rather than a 5.

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 resource definition before use guidance and pricing, which is the right order. It is a dense single paragraph with no wasted sentences, though the pricing sentence is long and could be tightened.

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?

There is no output schema, so the description carries the burden of describing the return: ~30 currencies, ISO code, English name, and pair counts. Combined with cost, safety, and sibling routing, nothing an agent needs 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 sole parameter 'only' is fully documented in the schema, including its example and edge-case behavior. The description says nothing about 'only' and 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+resource: the list of ~30 currencies with ECB daily reference rates, ISO 4217 code, English name, and pair counts. This clearly distinguishes it from the sibling conversion tools /fx and /fx/all, which it names directly.

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 explicit when-to-use guidance: check which currencies /fx and /fx/all can convert before calling them, or populate a currency picker. It names the alternative tools and the condition that selects this one over them.

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