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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety is covered; the description adds value beyond that with the auth requirement ("No API key needed") and the returned fields (code, name, latest publication date, behind-schedule flag). It does not mention response size handling for ~129 entries or any rate limiting, which is the only remaining gap.

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 imperative "Call this FIRST", followed by the return contract and then the auth note — a good priority order with no filler sentences. The parenthetical list of all six tax authorities is slightly long but earns its place by letting the agent answer coverage questions without a follow-up call.

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 zero-parameter, read-only discovery tool with no output schema, the description supplies everything needed: when to call it, what comes back, the field names returned, and the absence of an auth requirement. There is no output schema, yet the return shape is described well enough to use results immediately.

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?

Zero parameters, so the schema baseline is 4. The description still clarifies the semantics of the returned `code` value as the `bank` argument for other tools, which is genuinely useful contract information even though there are no inputs to explain.

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?

Specific verb and resource: "Returns every covered source — 123 central banks plus 6 tax authorities" with named examples. The enumeration of the six tax authorities makes the scope unambiguous and tells the agent exactly what a result set contains, distinguishing this discovery tool from the rate-fetching siblings.

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?

"Call this FIRST when you need a bank code" gives an explicit ordering rule, and the two quoted user phrasings ("which central banks do you cover?", "do you have rates from X?") map intents directly to this tool. It also explains the downstream consequence — the returned `code` is what other official-rates tools take as `bank` — so the agent knows why to call it before the siblings.

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