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

Annotations declare readOnlyHint and openWorldHint, so the safety profile is covered; the description adds genuinely new behavior beyond that: the two data sources with approximate currency counts, cross-rate computation, and the cost model ($0.002/call via x402, 5 free calls/day). This pricing detail is exactly the kind of context annotations cannot carry.

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 core purpose, then sources and pricing; every clause carries information. Slightly dense in the provider/currency enumeration but no wasted sentences.

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 no output schema, the description covers sources, currency breadth, base flexibility, and cost, which is sufficient to call correctly. It does not state the return shape or rate-date behavior, a minor gap for a data-fetch tool.

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 baseline is 3, but 'Any base, cross rates computed' adds real semantic value about how the base parameter behaves and what can be derived, going beyond the schema's terse 'default USD'.

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 resource (official daily reference exchange rates) with named providers (ECB, Bank of Russia) and explicit currency coverage, which cleanly separates it from siblings like treasury_yields and macro_series. An agent can identify this as the FX-rate tool without opening the 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?

No when-to-use, when-not-to-use, or alternative-tool guidance is provided. The pricing and free-call note imply a paid context but do not tell the agent when this tool is preferable to a sibling.

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