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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, openWorld), so the bar is lower. The description adds meaningful behavior beyond them: it probes the domain's SMTP server directly, sends no email, tests for catch-all, and warns about up to ~30 s latency. It stops short of stating failure/rate-limit behavior, so a 4 rather than 5.

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 dense sentences, front-loaded with the core question, then mechanism and return values, then the routing/latency advice. No filler or restated metadata.

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 carries the return contract and does so fully: statuses (deliverable/undeliverable/risky/unknown), their sub-causes, plus reason, confidence, and category. Combined with the latency warning and no-email-sent guarantee, an agent has everything needed to call and interpret it.

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 coverage is 100% with one parameter, so the baseline is 3. The description adds semantic weight by stressing 'this exact email address', implying exact-match lookup rather than fuzzy or domain-level checks, which meaningfully complements the schema's format example.

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 opening question 'Does this exact email address accept mail?' states a specific verb (verify/deliverability check) and resource (email address) with scope. It is clearly distinguishable from siblings check_domain and check_email, which operate on different targets.

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?

Explicitly contrasts with the alternative: 'Slower than check_email (up to ~30 s); use it before sending, not for every sign-up.' This gives both a when-to-use (before sending) and a when-not-to-use (routine sign-up validation), naming the alternative tool.

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