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

With no annotations, the description carries the full burden of behavioral disclosure. It reveals the accepted format loosely, but does not state the output type, whether the match must be exact, how edge cases are handled, or whether leading plus is required. This is only a partial behavioral profile.

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?

The description is a single compact sentence with the core rule front-loaded. However, 'Is Phone:' duplicates the tool name, and 'Provide value' is cryptic rather than informative, leaving some room for improvement.

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 tool with no annotations, no output schema, and zero documented parameters, the description is incomplete. It defines the heuristic check but leaves invocation mechanics, return value, and relationship to sibling validators unexplained, so an agent would struggle to call it correctly.

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?

The input schema has zero properties, yet the description says 'Provide value.' This is confusing: an agent cannot determine how to pass the value or what parameter name to use. The description adds the concept of a value but no parameter semantics that align with the schema.

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 clearly states the verb and resource: check whether a string looks like a phone number, with specific criteria (7-20 digits, optional +,-,space,parens). This distinguishes it from related validators like x402-validate-phone, though it doesn't explicitly name them.

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?

There is no guidance on when to use this tool versus alternatives such as x402-validate-phone, x402-phone, or other is-* validators. The only instruction is 'Provide value,' which is an invocation hint rather than usage context.

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