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

Beyond the annotations (which already signal a safe, read-only, idempotent, no-network operation), the description discloses failure semantics (N11 and unassigned area codes fail), the fictional_range edge case that still resolves, data provenance (NANPA, libphonenumber), and an explicit 'informational, not legal advice' disclaimer. This is substantive behavioral context not available from structured fields.

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 long and dense, but it is front-loaded with the core purpose and every clause carries content — return fields, risk grades, and edge cases — rather than filler. The enumeration is justified for a tool with no output schema, though it could be trimmed slightly.

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 carries the full burden of describing return values, and it does so comprehensively: validity, E.164 form, area-code geography, service status, overlays, time zones with local time, and a graded toll-fraud risk with named reasons. Nothing an agent needs to interpret the result 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?

With one parameter at 100% schema description coverage, the schema already documents the accepted input formats (including extensions and alphanumeric forms). The description adds only the +1 geographic scope, which is context rather than parameter syntax, so the baseline of 3 is appropriate.

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 (check) and resource (a +1 phone number) and scopes it to US/Canadian/North American numbers before calling or texting. It enumerates exactly what the result covers, so an agent can distinguish it from siblings like can_i_call_now or list_state_calling_rules.

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?

The phrase 'before calling or texting it' clearly establishes the pre-contact validation context in which the tool should be used. It does not, however, name or explicitly exclude the sibling tools (can_i_call_now, list_state_calling_rules) that might be confused with it, leaving that routing to inference.

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