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

With no annotations provided, the description carries the full burden and does disclose the outcome taxonomy ('valid / risky / invalid with reasons') and the breadth of checks, including network-dependent ones (domain/MX, SPF/DMARC). It does not mention auth needs, rate limits, or that lookups hit DNS, so a small amount of behavioral context is still missing.

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?

It is a single dense sentence that front-loads the action and the timing cue, followed by the return contract. Every listed check earns its place by telling the agent what signal it will get back.

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 must describe returns, and it does so concisely with the three-way verdict plus 'reasons'. For a one-parameter, read-only lookup tool, nothing material is left out.

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 at 100% schema coverage, so the schema already defines it; the baseline would be 4. The description adds meaning by implying the many dimensions analyzed from that one input (domain reputation, typos, role type), which helps the agent understand the input is more than a regex check.

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 gives a specific verb ('check') and resource ('an email address'), then enumerates exactly what is verified (format, MX, disposable domains, typo suggestions, role addresses, SPF/DMARC). It is trivially distinguishable from siblings like check_website, check_token, and check_wallet.

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?

It states the usage context clearly: 'before sending or signing someone up', which tells the agent when this check belongs in a workflow. It does not explicitly name an alternative or a when-not-to-use case, but the sibling tools operate on entirely different resources, so no exclusion is needed.

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