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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is largely covered. The description adds the authentication requirement and the 'preview' semantics, which reinforce that no mutation occurs. It does not describe additional behavior such as response shape or whether fees are calculated live, but the output schema covers return values and the annotations cover the core side-effect profile.

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?

The description is short and front-loaded, with the core purpose stated in the first clause and the authentication note appended. It wastes few words, though the redundant 'previewPaymentFees:' prefix repeats the tool name. Overall it is concise and scannable.

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 relatively simple read-only preview tool, the description combined with the full parameter schema and output schema is complete. The agent knows what the tool does, what parameters are expected, that it is safe and idempotent, that it requires authentication, and what output structure to expect. No critical information for invoking 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 description coverage is 100%, so each parameter is already well documented in the input schema. The description adds no parameter-level details beyond the schema, such as formatting or relationships between amount and currency. Baseline 3 is appropriate because the schema carries the semantic load and the description does not contradict or expand 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?

The description states a specific verb, 'Preview', and a specific resource, 'the fees that would be charged for a given payment amount'. This clearly distinguishes it from payment execution tools like createPaymentSession and payInvoice, and from pricing/quote tools focused on domains or transfers. The purpose is unambiguous even without sibling differentiation.

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 implies the tool is used before a payment to estimate associated fees, and it adds the prerequisite that authentication is required. However, it does not explicitly state when to use this tool versus alternatives such as createPaymentSession, getTransferQuote, or getProductCatalog. There is no direct exclusion or alternative routing, so the guidance is only implied.

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