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

Given no annotations, the description carries the full burden. It discloses that the operation is 'Pure CPU, no third-party lookup' and fully explains the paid nature and exact terms exchange. It does not cover error conditions or rate limits, but the core behavioral traits are transparent.

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 compact and well-structured: first sentence states the core purpose, second explains performance/trade-offs, and the final part details the payment flow. Every sentence earns its place with no redundancy.

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?

Despite no output schema, the description lists the key return values (E.164, line type, region, verdict). The complex payment flow is fully explained, and the pointer to the pricing tool covers alternative needs. Minor gaps like error handling are not critical for a validation tool.

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%, so baseline is 3. The description adds meaningful context for the x_payment parameter, explaining the exact usage flow (omit to get terms, sign, then provide). This goes beyond the schema's description and clarifies the parameter's role.

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 tool's function: 'Validate and normalize a phone number' and specifies what it returns (E.164 form, line type, region, verdict). This is specific and distinct from sibling tools like email_check or domain_intel.

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 provides clear context for the two-step payment process ('call without x_payment... then call again with x_payment') and points to the pricing tool as an alternative for price discovery. It does not explicitly mention when not to use this tool, but the context is strong for a paid validation flow.

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