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

Annotations already declare readOnly, idempotent, non-destructive and closed-world behavior, so the description's real contribution is the caveat that EINs have no check digit and validation therefore catches typos and fake prefixes but not every invalid number. That is exactly the limitation an agent needs before treating a pass as proof of a real EIN. It stops short of describing the return value or any rate/permission considerations.

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?

Two tight sentences; the positive statement of what is validated comes first and the scope caveat second. No filler, boilerplate, or restatement of the tool name.

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 one-parameter read-only validator this covers what is checked and how far the result can be trusted. With no output schema present, the description is slightly short of fully self-sufficient because it never says what a result looks like (boolean, normalized EIN, or failure reason).

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?

There is a single parameter with 100% schema description coverage ('EIN, with or without dash') plus a format example, so the schema carries the burden. The description's '9-digit format' phrasing is about validation logic rather than the input's accepted syntax, adding little beyond the schema. Baseline 3 applies.

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 (Validate) and resource (US Employer Identification Number) and even specifies the exact checks performed: 9-digit format and a valid IRS campus prefix. Against siblings named validate_bic, validate_iban, validate_eu_vat and validate_gb_vat, the resource noun alone makes it unambiguous which validator to pick.

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?

Usage is only implied: an agent should call this when it has an EIN to check. No explicit when-to-use, when-not-to-use, or named alternative is given, though the single-purpose nature and distinctive resource name make misrouting unlikely.

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