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

Annotations already declare readOnlyHint=true and destructiveHint=false, covering safety. The description adds behavioral context by specifying the return value (mail servers and priorities), which is useful for the agent. It does not contradict annotations and adds meaningful details beyond them.

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 extremely concise, only two sentences. It front-loads the action ('Look up') and efficiently states the resource and return value. Every word earns its place, with no fluff or repetition.

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 tool is simple with one parameter and no output schema. The description fully covers what the tool does and what it returns, which is sufficient for an agent to invoke it correctly. Given the simplicity and availability of annotations, no additional detail 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?

Schema description coverage is 100%, so the schema already explains the domain_name parameter with an example. The description does not add any additional parameter semantics beyond what the schema provides, so the baseline score of 3 is appropriate.

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 uses a specific verb 'Look up' with a clear resource 'MX (Mail Exchanger) records' and a target 'for a domain'. It distinguishes itself from sibling tools like cname_lookup and dns_lookup by explicitly naming the record type, and adds the return value (mail servers and priorities).

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 context is clear: this tool is for looking up MX records, so an agent can infer when to use it. However, it does not explicitly mention alternatives or exclusions, such as 'for general DNS lookups use dns_lookup'. The clarity of purpose makes it obvious, but explicit guidance is absent.

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