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

Annotations already cover the safety profile (readOnlyHint=true, openWorldHint=true, destructiveHint=false), so the description's main added value is the 10-credit cost disclosure and 'in real time', which the agent cannot get from structured fields. The rest (return shape: hostnames plus priority values) largely duplicates what the output schema already conveys.

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 short sentences, front-loaded with the core action, and the cost note is appended efficiently. Minor waste in the redundant 'MX Lookup: MX Lookup queries' opening that restates 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?

For a single-parameter read-only lookup with a full schema and an output schema in place, the description covers purpose, output nature, latency expectation, and cost. Only the sibling-routing guidance is absent, which is a small gap at this complexity level.

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%, with the schema itself supplying the example 'yahoo.com'. The description adds no syntax rules, TLD constraints, or IDN/punycode handling beyond what the schema states, so baseline 3 applies.

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: queries DNS for MX records of any domain, and adds what is returned (mail exchange hostnames plus priority values). It is clearly distinguishable from generic resolution tools, though it never names the closest siblings (DNSLookup, SRVRecordParser, CAARecordParser) to draw the boundary explicitly.

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 guidance on when to use this versus DNSLookup or the other DNS-record tools in the sibling list, and no prerequisites or exclusions. The use case (mail routing inspection) is only implied by the word 'MX', leaving the agent to infer the selection criteria.

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