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

Annotations already cover the safety profile (read-only, idempotent, non-destructive, open-world), so the bar is lower. The description adds genuine context beyond them by scoping the operation to 'DNS only: it does not connect to the mail servers', clarifying that no SMTP probing occurs, and by naming the returned data (IPs and reverse DNS) since there is no output schema.

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?

Two tight sentences with the primary purpose front-loaded, immediately followed by the scope constraint and return contents. Every clause carries information; nothing is wasted.

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?

For a single-parameter read tool with rich annotations and no output schema, the description covers purpose, scope boundary, and the shape of the result (IPs, reverse DNS). Nothing an agent needs to invoke it 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?

Only one parameter and schema description coverage is 100%, so the schema already documents 'domain' with an example. The description adds no formatting or validation nuance (e.g. punycode, subdomain handling), leaving this at the baseline 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?

States a specific verb and resource ('Look up a domain's MX records') and glosses the resource ('its incoming mail servers'), so the agent knows exactly what is returned. It does not explicitly contrast itself with the sibling dns_lookup, which is the likely generic alternative, so it stops short of 5.

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?

No explicit when-to-use or when-not-to-use statement, and no sibling alternative (e.g. dns_lookup) is named. The narrow MX framing makes appropriate usage inferable, but the agent must do that inference itself.

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