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

Annotations already mark the tool as read-only, non-destructive, idempotent, and open-world, but the description adds valuable context: it connects and quits without sending mail, may take 10-30 seconds due to live handshakes, and requires no authentication. These behavioral details go beyond what annotations provide.

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 tightly structured: it opens with the core action, lists the specific checks and return values, then gives usage guidance and caveats. Every sentence contributes meaningful information with no redundancy, achieving depth without verbosity.

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?

With no output schema, the description explicitly enumerates the returned data (per-MX cipher/version, cert SANs, expiry days, FCrDNS verdict, STARTTLS-required flag), covering the return contract. It also mentions latency, authentication, and the scope (every MX host), leaving little ambiguity for invocation.

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?

The input schema fully documents the single 'domain' parameter with an example and constraints. The description does not add any parameter-specific semantics beyond what the schema already states (domain name only, no URL/path/port), 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 clearly states the tool performs a live SMTP TLS check on every MX host, enumerating the exact protocol steps (TCP 25, EHLO, STARTTLS) and validation checks. It also distinguishes itself from siblings by naming check_mta_sts as the policy-layer counterpart, making the tool's scope unmistakable.

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 description explicitly says 'Use to verify inbound mail TLS posture' and recommends pairing with check_mta_sts for the policy layer, providing clear when-to-use and an alternative tool. It also warns about potential slowness (10-30s) due to live SMTP handshakes, which helps the agent decide when to invoke it.

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