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

Annotations already declare this a safe read (readOnlyHint=true, destructiveHint=false, openWorldHint=false), so the bar is lower. The description nonetheless adds real behavioral value beyond them: it defines the status vocabulary (OK, WARNING, MISSING, ERROR), explains that blacklistsChecked accompanies blacklistsListed so the count isn't misread as a total, and states that a -1 score means 'not measured' rather than zero — three interpretation rules an agent could easily get wrong.

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?

Three sentences, front-loaded with the purpose, then two sentences of output-interpretation caveats. Every clause carries information (sentinel value, counter pairing) and nothing is repeated from the name or 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?

There is no output schema, so the description must carry return-value meaning, and it does for the salient fields and sentinels. It stops short of describing the overall response shape (e.g. whether results are a per-mailbox array) or freshness/cadence of 'latest health check', which leaves a small gap, but an agent can call this zero-parameter tool correctly from what is written.

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?

With zero parameters there is nothing to disambiguate, so the baseline is 4. The description adds no parameter information, correctly, and instead spends its words on output 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?

The description opens with a specific verb and resource: 'Returns the latest health check for every connected mailbox', and enumerates the exact signals returned (SPF, DKIM, DMARC, MX, blacklists, placement, score). The scope phrase 'every connected mailbox' implicitly separates it from per-sender tools like get_sender_email, but no sibling is named explicitly, which keeps it off a 5.

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?

There is no when-to-use guidance, no when-not-to-use, and no named alternative. An agent cannot tell from this text whether to call it before or instead of get_sender_email_dns, get_deliverability_insights or get_inbox_placement_run; it must infer the aggregate-vs-per-sender distinction from the wording alone.

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