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

Annotations already declare readOnlyHint=true, destructiveHint=false, so the safety profile is covered. The description adds value beyond them by disclosing the return shape ({valid, status, confidence_score}) despite no output schema, and by flagging the redundant-call hazard with discover_email. It does not mention rate limits, auth, or what each status value means.

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 and return shape, then usage and the routing exclusion. Every sentence carries distinct information; 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?

For a one-parameter read-only tool with no output schema, the description covers purpose, the return fields, when to use it, and the sibling routing rule. An agent has everything needed to invoke it correctly and to avoid misusing 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?

One parameter with 0% schema description coverage, so the description must carry the burden. It conveys that the input is a single email address, but gives no format, length, or normalization details beyond that. Marginal compensation for the coverage gap.

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 and resource ('Verify if a single email address is real and deliverable') with the scope narrowed to a single address. It is clearly distinguishable from sibling discover_email, which serves a different job.

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 ('when you already know the address... confirm it before sending') and names the alternative for the other direction ('name → verified email... use discover_email'). It even warns not to call validate_email on discover_email output because validation already runs internally — a real exclusion rule.

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