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

Adds real behavioral context beyond the annotations: 'Does not contact the mailbox' clarifies this is a passive check rather than a deliverability probe, and the pricing/credit/trial details let the agent reason about cost. Annotations already cover read-only and open-world traits, so this is additive.

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-loads the purpose, then delivers the check list and the two constraints (no mailbox contact, pricing) in compact clauses. No filler sentences and nothing repeated from the title.

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?

An output schema exists so return values need not be explained, and the description still covers scope, safety caveat, and commercial terms. Nothing an agent needs in order to call this one-parameter tool 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?

There is a single parameter with 100% schema description coverage, so the schema already carries the semantics. The description adds no format, length, or normalization guidance for the email input beyond what the schema states.

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 an email address') and then enumerates exactly what facets are checked (syntax, MX with fallbacks, disposables, role accounts, free providers, typo suggestions). This is far more discriminating than any sibling tool name suggests.

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 opening clause 'before you use it' gives clear intended timing/context for invoking the tool, which is genuinely actionable. It stops short of naming when NOT to use it or pointing at alternatives, but no sibling competes directly for this job.

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