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

Annotations already establish readOnly, idempotent, non-destructive, and closed-world. The description adds meaningful context beyond that: 'offline', 'No DNS lookup, no network request', clarifying that it will not verify deliverability or reachability, which is exactly the boundary an agent needs to know. Only the exact return shape (field names) is left unstated.

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 compact sentences, front-loaded with the key differentiator ('Free.') and the core action, with the no-network/no-payment caveats following. Zero waste.

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?

There is no output schema, so the description correctly compensates by naming what is returned ('normalized address and domain'). Combined with the offline/no-payment guarantees and the sibling routing hint, an agent has everything needed to call and interpret it.

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% with a single, self-explanatory 'email' parameter including a format example, so the schema does the heavy lifting. The description adds no syntax constraints or formatting guidance beyond the schema, so the baseline 3 applies.

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 ('check the syntax of an email address') plus the return value ('normalized address and domain'), all in one clause. It distinguishes itself from the sibling check_email_address by positioning itself as the pre-step.

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 the sibling tool ('before paying for check_email_address') and the condition that selects this one instead (syntax-only, free). An agent has no ambiguity about when to call this versus the paid alternative.

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