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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), but the description adds real behavioral scope beyond them: it discloses the return shape ({ok, normalized?, type, issues}) and, critically, that it does not verify IRS issuance or identity. That negative claim prevents a common misinterpretation of the tool's guarantees.

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, zero filler. Purpose comes first, the safety constraint second, and the return contract plus scope limitation last — an appropriate front-loaded ordering for a validation tool.

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 response shape itself, states the input safety constraint, and bounds what the tool does and does not prove. Nothing an agent needs to invoke or interpret this tool 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 description coverage is 100%, so the single 'tin' parameter is already documented and baseline would be 3. The description adds marginal value by enumerating the accepted identifier families (EIN/SSN/ITIN) and reinforcing the synthetic-only constraint, telling the agent what kinds of strings are valid.

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 precise verb (check) plus the exact resource (synthetic US EIN, SSN, or ITIN formats), and explicitly frames itself as a format validator rather than an identity/issuance verifier. An agent can distinguish it from generate-ssn and the is-*-safe siblings without opening any schema.

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?

Gives clear context ('use with tax-ID input fixtures') and an explicit when-not ('never real personal identifiers'), which is the key safety boundary for this tool. It stops short of naming alternatives (e.g., generate-ssn for producing fixtures), so it is strong context but not full routing guidance.

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