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

With annotations already declaring readOnlyHint=false and idempotentHint=false, the description adds substantial behavioral context: credit spend, the confirm gate, the 10-second timeout, the fact that the charged GET is never retried, and that unknown/catch-all results do not establish deliverability. This is exactly the kind of hidden operational detail an agent needs for a charged external API call.

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?

The description is front-loaded with the action, followed by the cost/confirm requirement and then operational caveats. Every sentence provides distinct and useful information, with no redundant restatement of the title 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 charged, non-idempotent external verification call with no output schema, the description covers the critical unknowns: cost, confirmation requirement, timeout, retry behavior, interpretation of uncertain results, and recovery if the response is lost. Nothing essential for correct invocation appears to be missing.

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 0%, so the description must carry parameter meaning. It clearly explains the operational role of confirm:true, which is the non-obvious parameter. The email parameter is only described as 'one email', but its format and length constraints are fully specified in the schema.

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 states a specific verb and resource: 'Verify one email using MillionVerifier's real-time API.' The phrase 'one email' implicitly distinguishes it from the bulk file-oriented siblings like millionverifier_upload_file and millionverifier_file_info. An agent can identify both the action and its single-email scope without opening the 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?

It gives clear usage context: the tool can spend credits, requires confirm:true, and has a 10-second vendor timeout. It also warns what to do if the response is lost. It does not explicitly name the bulk alternative or state when not to use this tool, so it stops short of a full when/when-not/alternatives statement.

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