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

Annotations already cover the safety profile (readOnly, non-destructive, idempotent, closed-world), so the bar is lower; the description still adds real context beyond them by declaring the operation is free, incurs no payment, and makes no network request. It stops short of describing the shape of the returned price values.

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 'Free.' and then a single dense sentence covering return contents and constraints; nothing is wasted. The stacked parentheticals ('and of the HTTP-only email test report') add minor parsing friction.

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 input parameters and no output schema, the description is responsible for signalling what comes back, and it does list the three returned items. It leaves open the exact format/units of the price, but for a zero-argument pricing endpoint this is largely complete.

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?

The schema has zero parameters at 100% coverage, so the baseline is 4. The description reinforces this with 'No input', which is consistent and adds mild confirmation value but nothing beyond the empty 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 (Return) and precisely enumerates the resource: the price of check_email_address, the payment network (Base), and the receiving address. It is immediately distinguishable from the sibling validation tools, which do work rather than quote cost.

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 usage by naming check_email_address as the priced item, so an agent can infer this is a pre-flight cost check, but it never states when to call it versus proceeding straight to the paid tool. No alternatives or exclusions are given, leaving usage to inference.

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