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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint=false. The description adds valuable non-obvious behavior: the data source (Frankfurter API over ECB reference rates), publication time (~16:00 CET working days), currency coverage, and free pricing. No contradiction with annotations.

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?

Three compact sentences with no redundant wording. The core functionality is front-loaded, followed by source/schedule details and the crypto alternative. Every sentence earns its place.

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 read-only lookup tool with no output schema, the description covers the essential context: data source, scope, date range, update schedule, pricing, and sibling routing. It lacks only an explicit return-shape explanation, but that is not critical given the straightforward API and the schema's parameter documentation.

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 60%, and the description adds moderate operational context by describing the three modes (latest/historical, conversion, time series) that map to date, amount, and end_date. However, it does not explain how from/to/amount combine or clarify parameter format beyond what the schema already states, so it does not fully compensate for the undocumented parameters.

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?

The description identifies a specific resource (ECB reference exchange rates for 30 major currencies) and three distinct operations: latest/historical rates, amount conversion, and time series. It also clearly differentiates from crypto-focused siblings by naming crypto_price as the alternative.

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?

The description gives clear usage context: official fiat exchange rates from ECB, available since 1999, updated on working days, and free. It explicitly routes crypto requests to crypto_price, but it does not contrast against other potentially overlapping siblings like price or stablecoins, so it stops short of a full 5.

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