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

Annotations cover the read-only/idempotent/open-world profile, and the description adds critical context beyond them: the $0.004 USDC-on-Base x402 pricing and the payment-gating behavior ('Returns the x402 payment requirements unless payment_signature is given'). This is exactly the kind of behavioral trait an agent needs to call the tool correctly and is not derivable from 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?

Front-loads the category tag and purpose before routing guidance, pricing, and payment behavior. It is dense and largely waste-free, though the list-like stacking of pricing, payment, and sample notes is slightly choppy.

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 5-parameter tool with no output schema and no required params, the description covers purpose, usage context, cost, and the payment gating flow, which is enough to invoke it. It stops short of describing the response payload shape, but the payment-requirements return behavior is at least indicated.

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 all five parameters (base, date, amount, symbols, payment_signature) are already documented, establishing a baseline of 3. The description's note about returning payment requirements unless payment_signature is given loosely reinforces that parameter but adds no new syntax or format detail beyond the schema.

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 ('Get an official, citable euro exchange rate') and explicitly frames scope around invoices, ledger entries, and tax returns. It also distinguishes itself from the market-feed alternative, so an agent can differentiate it from sibling data tools 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?

Gives a clear when-to-use condition ('when the rate must come from an official source rather than a market feed'). It lacks an explicit when-not or a named sibling alternative, but the official-vs-market distinction is actionable guidance.

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