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

Annotations already cover read-only/idempotent/open-world safety, and the description goes well beyond them: 422 vs 5xx error semantics, that failed calls are not charged, latency expectations (under 0.1 s, up to 10 s on refresh), pricing (USD 0.001) and a free tier of 10 calls per IP per UTC day. It also discloses provenance (ECB euro reference rates, Tanod-computed cross rates) and a licensing disclaimer.

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 purpose and alternative in the first two sentences, then attaches operational facts. The licensing/attribution and 'not for transactions' clauses are somewhat boilerplate for a paid reference-rate API but are justified by the data source, so the length is defensible rather than padded.

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?

An output schema exists, so return values need no explanation. Everything an agent needs to call it correctly — required param semantics, date window, error-and-billing behavior, latency, pricing, and the sibling alternative — is present.

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%, so the schema documents base/date/symbols and the baseline is 3. The description adds real semantic value about the base parameter — non-EUR bases are cross rates computed by Tanod rather than raw ECB figures — plus a corroborating note that older dates are rejected, which is slightly more than the schema alone conveys.

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 ('foreign-exchange reference rates for a base currency') and explicitly distinguishes itself from the sibling get_live_fx_rates by naming that alternative for live on-chain rates. An agent can route between the two without opening either schema.

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 an explicit when-to-use-elsewhere rule: 'For live on-chain rates use get_live_fx_rates.' It also names the conditions that produce failures (unknown currency, older dates → 422; ECB outage → 5xx), so the agent knows the boundaries of correct invocation.

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