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

No annotations are provided, so the description carries the burden. It discloses meaningful traits: regex-based (hence approximate, prone to false negatives/positives), no deliverability lookup, and no sending. It omits any statement about the return shape or what an invalid input yields.

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?

A single compact sentence plus a scope clause, front-loaded with the purpose. Every clause earns its place; the only waste is the redundant restatement of the tool name as a prefix.

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 simple single-input validator with no output schema and no annotations, the description covers purpose and exclusions adequately, but never hints at the return value (boolean vs. structured result) — a gap the agent must guess at since nothing structured fills 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 compensate. It implies the input is an email string but adds no format, length, or example guidance beyond the tool name. Marginally helpful, not a full substitute for the missing schema documentation.

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: email syntax validation with RFC 5322 approximation plus TLD/length checks. An agent understands exactly what the tool does. However, it never distinguishes itself from the many near-siblings (x402-validate-email, x402-is-email, x402-email-verify), so it does not earn 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?

"纯正则，不查送达、不发信" establishes clear scope boundaries — this is a local format check, not a deliverability or send tool — which implicitly routes the agent elsewhere for those needs. It stops short of naming the actual alternative siblings, so it is contextual rather than fully explicit.

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