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

With no annotations, the description carries the load and does so well: it discloses the negative capability ("Does not check whether the line is in service"), the quota/credit model (20 free checks per day per network without a key, 1 check with a key). It stops short of saying whether an invalid number raises an error or returns a response, and doesn't quantify latency or quota-exceeded behavior.

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 sentences, each carrying distinct payload: capabilities front-loaded, then the limitation, then the cost model. No redundant restatement of the tool name or title.

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?

No output schema exists, so the description has to describe the return shape, and it does by naming all seven result fields and the three suggested_action values. The remaining gap is input-format guidance for the undocumented phone parameter, which matters for a validation tool that will fail differently depending on input convention.

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 50%: the country parameter is documented in-schema as ISO 3166-1 alpha-2, but phone has no description in either place, and the description never clarifies whether phone should be supplied in national or E.164 form or how country interacts with it. Baseline 3 given the partial schema coverage and no compensating detail.

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 ("Parse and validate") plus the exact resource (a phone number) and enumerates the concrete outputs (valid, possible, type, region, E.164, national, suggested_action). The resource name makes the boundary against the email validators unambiguous without opening either 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 resource (use this for phones, the siblings for emails) and the suggested_action field hints at decision-making use, but there is no explicit statement of when to choose this over validate_email/validate_emails or of preconditions such as input format requirements.

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