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

Annotations already declare readOnlyHint and idempotentHint, and the description reinforces this with 'Read-only — connects and quits without sending mail.' It goes beyond annotations by disclosing the exact server measurement context, the null behavior for non-responding hosts, and the 'No auth' requirement, giving the agent critical operational detail.

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 thorough but every sentence earns its place: it front-loads the core operation, lists what is validated, states read-only behavior, details return values, gives usage context, and clarifies edge cases. The length is justified by the tool's complexity, and there is no redundant or filler content.

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 tool with no output schema, the description fully explains what the agent will receive (per-MX TLS protocol, certificate details, expiry days, FCrDNS results). It also covers the important caveat about null startTlsSupported and the server location, leaving no missing operational detail that an agent would need to call the tool correctly.

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% and the schema already documents the domain parameter (including format constraints). The description adds contextual value by explaining that the domain is used to resolve MX records and that only the first 4 hosts are tested, but this is more about the tool's scope than the parameter's meaning. Since the schema fully covers the parameter, baseline 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 clearly states the tool's function: a live check of the first 4 MX hosts covering TLS handshake, certificate validation, EHLO capabilities, and FCrDNS. It explicitly distinguishes itself from the sibling tool check_mta_sts by noting they are complementary, so an agent can immediately tell them apart.

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?

It provides explicit guidance: 'Use to verify inbound mail TLS posture' and directly recommends pairing with check_mta_sts for the policy layer. It also warns about the 10-30s latency and explains the null vs false distinction for startTlsSupported, which are essential for correct interpretation of results.

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