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 if an email address is valid and reachable,' which implies a read-only operation, but does not disclose details such as rate limits, authentication needs, error handling, or what 'reachable' entails (e.g., network calls, response time). This leaves gaps in understanding the tool's behavior beyond the basic purpose.

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 front-loads the purpose ('Checks if an email address is valid and reachable') and includes the method ('using MailboxValidator') without unnecessary details. Every word earns its place, making it highly concise and well-structured for quick understanding.

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 low complexity (1 parameter, no output schema, no annotations), the description is minimally adequate. It states what the tool does but lacks information on behavioral aspects like rate limits or output format. Without an output schema, the description does not explain return values, leaving gaps in completeness for a validation tool that might return detailed results.

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 validate.' The description does not add any additional meaning beyond this, such as format examples or constraints. Since schema coverage is high, the baseline score of 3 is appropriate, as the description does not compensate but also does not detract from the schema's information.

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 specific action ('Checks if an email address is valid and reachable') and the resource ('email address'), using the verb 'Checks' which is precise. It also distinguishes from sibling tools by specifying the validation method ('using MailboxValidator'), whereas siblings like 'check_disposable_email' and 'check_free_email' focus on different aspects of email validation.

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 implies usage for general email validation, but does not explicitly state when to use this tool versus alternatives like the sibling tools. It mentions 'valid and reachable,' which hints at broader validation, but lacks clear guidance on exclusions or specific contexts where this tool is preferred over 'check_disposable_email' or 'check_free_email'.

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