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

Beyond annotations readOnlyHint=true, the description reveals the multi-provider waterfall, the credit-counting rule (one vs two credits), and which billing fields to trust for budgeting. No contradiction with annotations; it adds substantial behavioral context.

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 sentences, front-loaded with the core result, then usage timing, then the pricing rule. Every sentence adds necessary information; there is no filler.

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?

Even with no output schema, the description explains the return substance (perAddressCeilingUsd vs perAddressFloorUsd) and the billing-credit model, and there are no input arguments to document. An agent has everything it needs to invoke the call and interpret the result.

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, so the parameter-semantics burden is minimal and the baseline is 4. The description correctly focuses on output semantics rather than input details, which are absent.

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 opens with a specific verb and object ('Returns what one email verification costs right now') and grounds it in the actual billing plan the meter charges against. It clearly distinguishes this rate-lookup tool from the create_email_verification_job sibling by positioning itself as the pre-budget call.

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 directs the agent to call this before create_email_verification_job rather than assuming a price, and gives concrete budgeting guidance (treat perAddressCeilingUsd as the common case, not perAddressFloorUsd). That is clear when-to-use context with an alternative named.

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