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

Annotations already cover the safety profile (readOnlyHint, idempotentHint, destructiveHint=false, openWorldHint), so the bar is lower. The description does add real context by disclosing that the check goes beyond DNS to fetch an HTTPS policy file and validates mode and MX patterns, implying external network access and latency. It does not say what happens when no policy exists, whether the result is a pass/fail verdict, or anything about rate limits.

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?

One dense sentence, front-loaded with the primary resource and parenthetically enumerating sub-checks. There is no filler and nothing redundant with the title or schema.

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?

There is no output schema, so the description carries some burden for explaining results; it partially discharges this by naming the artifacts inspected (TXT record, policy file, mode, MX patterns, TLS-RPT record). What is still missing is whether the return is structured per-record or a verdict, and behavior for a domain with no MTA-STS deployment, but the enumeration makes it substantially complete for a one-parameter diagnostic.

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?

Single required parameter with 100% schema description coverage, so the schema already documents the domain argument. The description's "for a domain" adds no format detail such as whether a bare domain vs. URL vs. wildcard should be supplied, so it neither adds nor detracts meaningfully. Baseline 3 applies when the schema carries the parameter semantics.

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?

Uses a specific verb ("Check") with a named resource pair (MTA-STS and TLS-RPT) and enumerates the exact artifacts inspected: the TXT record, the HTTPS policy file, mode, MX patterns, and the TLS-RPT reporting record. This is specific enough that no sibling in the list (dmarc_check, spf_validate, dns_lookup) could be confused for it.

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?

Scope is implied ("for a domain") but there is no explicit when-to-use or when-not guidance, and no mention of alternatives such as domain_health_report or how this complements dmarc_check/spf_lookup even though MTA-STS is allied to those. An agent must infer usage from the tool name and record type alone.

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