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

Annotations already cover safety (readOnly, idempotent, closed-world), and the description layers on material context they cannot express: the $0.001 x402/USDC-on-Base payment requirement, the returned shape {type, valid, normalized, reason}, and the privacy contract that the input is never echoed or stored and PII is masked. An agent knows cost, output, and data-handling behavior before invoking.

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 tight sentences, purpose and scope front-loaded, then cost, then return shape and privacy. No sentence is filler — each adds a distinct fact (scope, price, output, data handling).

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?

With no output schema, the description supplies the return fields {type, valid, normalized, reason} itself, plus cost and privacy guarantees. For a one-parameter, closed-world, idempotent validator, nothing needed to call it correctly is 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 coverage is 100% and the single parameter is fully documented in-schema, so the baseline would be 3. The description adds genuine semantics beyond the schema by stating the input is never echoed or stored and that identifiers are masked, which tells the agent how the 'value' parameter is handled. It stops short of noting the 512-char bound, hence 4 rather than 5.

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?

Names the exact resource and the exact scope boundary: 'Email address syntax (dot-atom local part, LDH domain; no DNS)'. This distinguishes it instantly from the sibling validators (validate_iban, validate_eu_vat, validate_pl_pesel), which validate entirely different identifier types, and the 'no DNS' clause prevents an agent from assuming deliverability checking.

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 syntax-only, no-DNS qualifier is an implicit when-to-use/when-not statement: call this when syntactic well-formedness is enough, not when MX/deliverability matters. It does not explicitly name an alternative tool or state prerequisites/rate limits, so it falls short of the explicit routing seen in a 5.

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