email_check

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

No annotations are provided, so the description carries the full burden. It explicitly discloses a limitation: 'Does not probe the mailbox; mailbox_status is always not_checked' and also mentions cost: 'Costs 2 LookTwice API units.' These are valuable behavioral traits beyond basic function.

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 extremely concise, two sentences with no filler. It front-loads the core purpose and then adds critical behavioral information (mailbox status and cost) efficiently.

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 single-parameter tool with no output schema, the description covers purpose, validation scope, behavioral constraints, and cost. It is fully complete for the complexity level, requiring no additional context.

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 parameter 'email' described as 'The email address to check'. The tool description does not add additional parameter-level details beyond what the schema already provides, so baseline 3 is appropriate.

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 clearly states the tool's function: 'Validate an email address' and enumerates specific checks (syntax, MX records, disposable-domain, role-account, free-provider signals), distinguishing it from sibling tools that validate other entities like IBAN or VAT.

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 provides clear context that this tool is for email validation, implying it should be used when an email address needs to be checked. It does not explicitly state when not to use it or mention alternatives, hence not a 5, but the context is sufficiently clear for selection.

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