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

Annotations already indicate a read-only, idempotent, non-destructive operation, and the description adds valuable behavioral context: it is free, deterministic, requires no login, and the is_pec_domain result can be null, explicitly distinguishing null from false. This goes well beyond the structured annotations.

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 three focused sentences: what the tool does, the critical three-valued output semantics, and the operational characteristics. Every sentence adds essential information and there is no redundant 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?

With no output schema, the description does well to explain the key is_pec_domain semantics, which is the most important nuance. However, it does not fully specify the overall response shape or behavior for a malformed email, leaving a small gap in return-value understanding.

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 only parameter 'email' is already described in the schema with format and example. The description reinforces that the address is a PEC address and mentions domain checking, but it does not add substantive new parameter semantics beyond what the schema already provides.

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 names a concrete verb and resource: validate an Italian PEC address, checking format and domain against the AGID list of certified providers. This clearly separates it from sibling validators like validate_iban and validate_partita_iva by focusing on PEC-specific semantics.

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 establishes clear context: use this tool when an Italian PEC address needs validation against the AGID provider list. It does not explicitly mention when not to use it or name alternative validators, but the domain-specific framing makes the intended use unambiguous.

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