MailboxValidator Email Validation MCP Server

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

No annotations are provided, so the description carries the full burden of behavioral disclosure. It states the tool checks for disposable emails but doesn't explain what constitutes a 'disposable email', how the check is performed (e.g., API call, local database), potential rate limits, error handling, or what the output might look like. This leaves significant gaps in understanding the tool's behavior.

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 a single, efficient sentence that clearly states the tool's purpose without unnecessary words. It is front-loaded with the core functionality and includes the method ('using MailboxValidator'), which adds value without verbosity. Every part of the sentence earns its place.

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?

Given the tool's simplicity (1 parameter, no output schema, no annotations), the description is adequate but incomplete. It covers the basic purpose but lacks details on behavioral aspects like output format, error cases, or usage context relative to siblings. For a straightforward tool, this is minimally viable but leaves room for improvement in guiding the agent.

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 input schema has 100% description coverage, with the parameter 'emailAddress' documented as 'Email address to check.' The description doesn't add any additional semantic context beyond what the schema provides, such as examples or constraints. Since schema coverage is high, the baseline score of 3 is appropriate, as the description doesn't compensate but also doesn't detract.

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 purpose: 'Checks if an email address is a disposable email using MailboxValidator.' It specifies the verb ('checks'), resource ('email address'), and method ('using MailboxValidator'). However, it doesn't explicitly differentiate from sibling tools like 'check_free_email' and 'validate_email', which likely have overlapping functionality.

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 no guidance on when to use this tool versus its siblings ('check_free_email' and 'validate_email'). It doesn't mention prerequisites, alternatives, or specific contexts for usage, leaving the agent to infer when this tool is appropriate based solely on the name and description.

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