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

Annotations already declare readOnly, openWorld and non-destructive, and the description usefully adds the data source (ECB), a freshness signal ('the date the rates apply to'), and a detailed cost model ($0.002/call, first 25 free, failed calls free). That billing note also explains the non-obvious idempotentHint=false. The line 'reads or computes only and changes nothing' largely restates the annotations, and nothing is said about rate limits or staleness beyond the returned date.

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 source, scope and return shape, then usage, then pricing. Nothing is truly wasted, though 'reads or computes only and changes nothing' duplicates the annotations and the pricing sentence is dense.

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 still tells the agent what comes back (rates, converted amounts, and the date the rates apply to), where data originates (ECB) and how it is billed. For a three-parameter read tool that is sufficient to call it correctly.

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 description coverage is 100%, so from/to/amount, ISO 4217 format and comma-separated targets are already documented. The description restates the same facts (base currency, optional targets, amount, default all currencies) with only the from=USD example as added value, so the baseline of 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 (fetch ECB foreign exchange rates, return converted amounts) plus scope (~30 major currencies, examples USD/EUR/GBP/JPY). The terse tool name 'fx' is fully disambiguated. It does not, however, contrast itself with the visible siblings fx_all and fx_symbols, leaving the agent to guess which of the three to pick.

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 concrete usage contexts ('currency conversion, invoice and price calculations, and reporting in another currency') and a worked example (from=USD). It stops short of explicit exclusions or routing to fx_all/fx_symbols, so the agent gets context but no negative guidance.

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