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

Beyond the read-only annotation, it discloses that data is passive and possibly old, that no active SMTP checks are performed, and that pricing is $0.002 per successful call with failed calls uncharged. These are material behavioral traits for selection and invocation.

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 purpose and scope are front-loaded in the first sentence, while limitations and pricing follow compactly. Every sentence contributes useful selection or invocation context.

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?

The description covers data source, scope, freshness caveats, active-test exclusions, and payment behavior. With an output schema present and full parameter coverage, no further explanation of return values is needed.

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 one required parameter with 100% schema description coverage, so the schema already documents the domain input. The description adds no additional parameter syntax or format guidance beyond what the schema provides, which fits the baseline of 3.

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 states a specific resource and output: up to 3 MX hosts, their IPv4 addresses, and open SMTP ports from Shodan passive scan data. It clearly distinguishes this passive lookup from active SMTP/TLS testing, so an agent can identify it without opening the schema.

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 gives clear context for use on a mail domain and explicitly rules out active SMTP conversation, banner, STARTTLS, and certificate checks. However, it does not name specific alternative tools for those active checks, leaving routing to be inferred.

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