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

Annotations only declare readOnlyHint=true, so the description carries the extra burden and does: it discloses the capability boundary that a static file analysis cannot verify SPF, blacklists or reverse DNS. That is a concrete behavioral limitation an agent would otherwise assume works, and it materially affects tool selection.

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, zero filler: the capability is front-loaded, then the limitation and the alternative. Every clause earns its place.

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 read-only analysis tool with no output schema, the definition covers input, capability, limitations and the alternative route. The remaining gap is that it never hints at what the analysis returns (scores, checks performed), which would help an agent decide whether the result answers its question.

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?

Only one parameter with 100% schema description coverage, so the schema already documents format and expected source. The description's parenthetical ('full headers + body') reinforces the required input shape but adds no syntax beyond the schema; baseline is 3, with a slight bump for echoing the required input expectation in prose.

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 ('run a deliverability/content analysis') on a specific resource ('raw RFC 822 email (.eml source: full headers + body)'), and explicitly names the sibling (create_mailtest) that provides the authoritative alternative. An agent can distinguish this from check_blacklist or check_spoofability 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?

The 'IMPORTANT LIMITS' sentence gives an explicit when-not condition (no live SMTP session, so SPF/blacklist/reverse-DNS can't be verified from a file) and routes the agent to the correct alternative for the authoritative score. This is exactly the when/when-not/alternative guidance the dimension asks for.

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