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

Annotations already declare readOnlyHint, idempotentHint, openWorldHint and destructiveHint=false, so the safety profile is covered. The description adds genuine behavioral context beyond them: the outbound-SMTP block on port 25 and the fact that the result includes chain trust and hostname coverage. It does not cover extraction failure modes or rate limits, which keeps it at a 4.

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 front-loaded with the useful part: the return contract first, then usage plus the one hard limitation. Nothing repeats the schema or the annotations.

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 two-parameter read-only probe with no output schema, the description enumerates the returned fields and states the one environmental constraint that would otherwise cause a mis-call. An agent has everything needed to decide and invoke.

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%, with both `domain` and `port` documented in the schema, including the port default and the port-25 rejection. The description adds no syntax or format detail beyond that, so baseline 3 is correct.

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 ('Read a host's TLS certificate') and then enumerates exactly what is returned: issuer, validity, SANs, key type/size, signature algorithm, chain, protocol, trust and hostname coverage. This is far more specific than the sibling checks and lets an agent tell it apart from check_mta_sts, check_dns or check_tracking_domain without opening a schema.

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?

Gives concrete when-to-use guidance ('on an MTA-STS policy host or a click-tracking domain') with a real consequence ('expired certificate stops mail or turns every click into a browser warning'), and an explicit when-not ('Port 25 is not reachable... the certificate an MX host presents cannot be read this way'). It does not name an alternative sibling tool the way a 5 would, but the usage context is unambiguous.

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