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

Annotations already declare readOnly, idempotent, non-destructive, and open-world, so the safety profile is covered. The description adds real behavioral value beyond that: it returns null MX per RFC 7505 rather than flagging it broken, and it deliberately refuses to guess the provider behind a gateway because that is unobservable — useful disclosure about output fidelity.

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?

Three sentences, front-loaded with what is returned, then the bulk-mode difference, then the two edge-case behaviors. No filler and every sentence carries information an agent needs.

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?

With only two optional parameters and no output schema, the description carries the return-value burden well: priorities, resolved IPs, provider, gateway, and the bulk breakdown are all named. Minor gaps remain on failure behavior (nonexistent domain, no MX) and the null-MX format itself.

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?

Schema coverage is 100%, so baseline would be 3, but the description adds meaning the schema does not: the `domains` array mode additionally returns a provider breakdown, and the 100-domain cap is stated in prose as well as schema. It does not clarify precedence when both parameters are supplied.

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) plus the derived output (provider identification, security gateway detection). This is clearly distinguishable from siblings like check_dns, lookup_txt, or check_google_domain, which cover different record types or single-provider checks.

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?

Explains the choice between the two parameters: use `domains` for up to 100 at once, which also returns a provider breakdown. It gives clear context for the tool's purpose (mail provider discovery) but never names an alternative sibling or states when not to use it.

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