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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint and destructiveHint=false, so the safety profile is covered externally. The description still adds value beyond that by noting the records are "live" (fresh, real-time resolution rather than cached) and that priorities are included in the result, though it says nothing about failure behavior for nonexistent or unresolvable domains.

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 sentences, both earning their place: the first front-loads the usage condition, the second states the return content. No filler, no restating of the title.

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?

No output schema exists, and the description compensates by naming what comes back (MX records with priorities), while the annotations carry the safety and open-world semantics. For a single-parameter read-only lookup, nothing needed for correct invocation 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?

There is only one parameter and schema description coverage is 100%, so the schema already fully documents domain_name including an example. The description adds no syntax or format detail beyond the schema, which is the expected baseline at full coverage.

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 verb (look up / receive email for) and resource (MX records for a domain), framed from the user's intent ("which mail servers receive email for a domain"). It is clear on its own, but it does not distinguish itself from overlapping siblings such as dns_lookup or smtp_finder, which an agent might reasonably consider interchangeable.

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?

"Use this when the user wants to know which mail servers receive email for a domain" gives a concrete triggering condition, so the agent knows when this tool applies. It stops short of naming alternatives or exclusions (e.g. "use dns_lookup for general record types"), so there is no explicit routing guidance against its many sibling lookup tools.

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