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

Annotations already establish readOnly, idempotent, non-destructive, and closed-world, so the bar is lowered; the description still adds genuinely useful behavioral context by clarifying that no SMS/call is placed and that nothing is persisted. It also discloses the response payload fields, which annotations cannot convey. It stops short of saying what happens on invalid input (error vs. validity=false), which is the main remaining unknown.

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?

Four short sentences, front-loaded with the purpose, followed by return values and two disambiguating negatives. Nearly every sentence earns its place; 'Uses Google's libphonenumber plans' is the one phrase that is vague and arguably wasteful.

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 correctly compensates by listing the returned fields, and the tool is simple (two scalar params, read-only). The only material omission is any explanation of defaultCountry, which leaves an agent guessing on a non-trivial input.

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 0%, so the description must carry parameter meaning and it largely does not: 'defaultCountry' is never mentioned, and the agent gets no hint that it is a 2-character ISO code used to interpret numbers lacking a country code. The reference to E.164 output weakly implies expected input format but does not explain the 'phone' parameter's constraints.

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 opens with a specific verb+resource ('Validate and format a phone number') and then enumerates exactly what comes back: validity, country, E.164, national/international formats, and line type. That level of specificity makes it unmistakable against siblings like validate_email or validate_iban 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?

Usage is implied by the name and purpose but never stated: there is no guidance on when to reach for this versus validate_email or other validators, and no mention of what input form is expected (raw digits vs. international prefix) or when defaultCountry must be supplied. An agent can infer the general use case but not the boundaries.

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