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

Annotations cover the safety profile (readOnly, openWorld, non-destructive), and the description adds substantial context beyond them: publication schedule (~16:00 CET on working days), source authority (ECB), a paid x402 flow with per-call price and accepted chains, and a free sample invocation. This is exactly the behavioral detail an agent needs to decide and pay.

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 what the tool is and its scope, then payment mechanics. Slightly dense and the payment/sample sentences lengthen it, but each sentence carries actionable information for the caller.

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 explains the return shape (rates object mapping codes to rates relative to base), the data source and freshness, and the payment/auth requirement. Nothing an agent needs to call it correctly is missing.

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 the description goes further by clarifying cross-rate semantics for 'base' and giving a worked example ({'base':'EUR','symbols':['USD','CHF','GBP']}) plus the empty-symbols behavior. It adds modest value over the schema rather than just restating it.

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 (official ECB currency exchange rates), names the source, the ~30-currency scope with examples, and the cross-rate capability. An agent can distinguish it from siblings like crypto_prices or macro_data without opening a 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?

Explicitly names use cases ('currency conversion in invoices, pricing, or financial calculations') and gives a concrete example input, but does not name a sibling alternative or state when NOT to use it. Clear context without exclusions.

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