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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety is covered by structured data. The description adds genuinely new operational context: the per-call price ($0.0035 USDC) and the x402 payment requirement, which an agent must know before invoking. It does not disclose rate limits or failure behavior, so not a 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 short sentences, front-loaded with what is checked, then when to use it, then cost. Every sentence earns its place with zero padding.

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 single-input tool with no output schema, the description covers the checks performed, the use case, and the payment model. It would be slightly more complete if it hinted at the result shape (booleans vs. score) or deliverability semantics of the flags returned.

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 exists and its schema description coverage is 0%, so the description must carry the burden. "Verify an email address" implies the input is an email string, but there is no guidance on accepted formats or edge cases. Baseline 3 for a single-parameter tool with the implicit naming.

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 and enumerates exactly what the verification covers (syntax, MX lookup, disposable/role flags), so the agent knows the deliverable. It does not differentiate from the sibling email_analyze, which is the only thing keeping this below a 5.

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?

"Use before sending or storing an address" gives a clear triggering context for the tool. It stops short of naming exclusions or pointing at alternatives such as email_analyze, so it is clear context without routing.

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