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 earns credit by disclosing the return taxonomy (valid/invalid/catch-all/unknown) and MX provider details, which no structured field provides since there is no output schema; it stops short of cost, quota, or rate-limit context.

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 tight sentences: what it does and returns, then when to use it. No filler, and the purpose is front-loaded ahead of the usage context.

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 one-parameter tool with no output schema, the description responsibly enumerates the possible return statuses and the supplementary MX data, so an agent knows what to expect. Missing only operational details such as rate limits, cost, or latency expectations.

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% for the single parameter, which already carries its own example and description. The tool description adds no syntax, format, or normalization guidance beyond what the schema states, so the baseline 3 applies.

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+resource ('Validate an email address for deliverability') and names the exact outcome space (valid/invalid/catch-all/unknown plus MX details). It is clearly distinguishable from sibling 'find_email', which locates rather than evaluates addresses.

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 says when to use it: verifying addresses from a CRM, lists, or other sources before sending outreach. It gives a clear triggering context but never names an alternative tool or a when-not-to-use condition relative to siblings like find_email or send_message.

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