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

Annotations already declare readOnly/idempotent/non-destructive and the closed-world hint, so the safety profile is covered. The description adds genuinely new context beyond the annotations: it returns a boolean rather than the normalized number, and it explicitly excludes subscriber or reachability checks, which prevents an agent from over-trusting a true result.

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: what it does, when to use it, and what it returns/what it does not cover. The most decision-relevant information (the check itself) is front-loaded and nothing is padded.

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 must carry the return-value burden, and it does by stating the boolean result and that the normalized number is not returned. For a one-parameter, annotated validator, nothing an agent needs 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% for the single 'phone' parameter, so the baseline is 3. The description goes beyond the schema example by specifying that spaces and 00-prefixed forms are accepted after local normalization, which tells the agent it can pass loosely formatted input rather than only canonical '+66812345678' strings.

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 specific verb ('Check ... syntax') and resource ('phone number ... against E.164'), and the 'is-' prefix plus the explicit E.164 scope cleanly separates it from the generate-phone-number generator and the other is-*-safe validators. The added scope note ('after local normalization') makes the operation unambiguous.

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?

It states the use case directly: testing international number formats, including spaces or 00 prefixes, and clarifies it is not a subscriber/reachability check. It does not name a specific sibling alternative, but the boundary against the generator and the safety-check family is clear from how the use case is framed.

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