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

The description goes far beyond the readOnlyHint/openWorldHint/idempotentHint annotations by disclosing the paid nature, the 402-challenge flow, charging edge cases (invalid address = paid verdict, DNS failure = 502 not charged), and the absence of an SMTP probe. This is exactly the behavioral context an agent needs for a metered tool. No contradiction with annotations; readOnlyHint is consistent with 'no SMTP probe'.

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 dense and front-loaded with the critical facts (PAID, purpose) before details. Every sentence earns its place: charging, checks, no-SMTP limitation, error behavior, and payment automation. It is a long single paragraph of compound sentences, which slightly hurts scannability, but there is zero redundancy or filler.

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 paid tool with a two-call x402 protocol, the description is exceptionally complete: it covers what happens without payment (402 challenge), on DNS failure (502, not charged), on invalid input (paid verdict), and what the tool refuses to guarantee (mailbox existence). With an output schema present, the return-value summary (risk verdict with reasons) is also mentioned.

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%, so both the email and payment parameters are fully documented in the schema itself, including the detailed two-call payment flow. The tool description adds the high-level framing ($0.001, second-call completion), but this largely restates the schema's payment parameter description, so it adds marginal value beyond the structured data.

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 ('check an email address') and enumerates the exact capabilities: syntax, DNS MX/A/AAAA, disposable/role lists, typo suggestion, normalization, and a risk verdict. It clearly distinguishes itself from the blockchain/x402 siblings (eth_balance, token_*, call_x402) by framing the purpose as 'Is this address worth sending to?'.

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?

The description frames the use case clearly ('Is this address worth sending to?') and includes an implicit when-not: 'No SMTP probe: it vouches for the DOMAIN, never the mailbox' instructs the agent that this tool is for deliverability-risk assessment, not mailbox confirmation. No explicit alternative is named, but none of the 21 siblings are email-related, so cross-routing guidance is unnecessary.

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