check_mx

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

Annotations already declare readOnlyHint and openWorldHint, so safety is covered; the description adds the valuable non-annotation detail that the call consumes no verification credits. It does not describe what the response contains (raw MX list vs. a boolean deliverability verdict), leaving some ambiguity.

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 short sentences, zero filler; the core purpose comes first and the cost note second. Every clause earns its place.

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 DNS lookup with annotations covering safety and the schema covering the input, this is nearly complete. The one gap is what an agent gets back — whether it's a record list, a boolean, or both — since no output schema exists to compensate.

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 the schema documents it fully (100% coverage) with a format example. The description adds no further meaning about domain handling (IDN, subdomains, casing), 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 (look up) and resource (a domain's MX records) plus the derived outcome (can receive email). It implicitly distinguishes itself from check_spf/check_dkim/check_dmarc, but never names those siblings or check_domain_health, so the routing signal is weaker than it could be.

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 'Free — no verification credits' note gives a cost-based reason to prefer this over paid checks like verify_email, which is useful implied guidance. It stops short of saying when to call this versus check_domain_health or the other DNS record checks, or what preconditions exist.

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