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

Annotations already cover the safety profile (readOnlyHint=true, idempotentHint=true, openWorldHint=true), so the bar is lower. The description adds real value by disclosing the cost model (0.003 USDC via x402 or 2 Factline credits) and the dual payment path — critical operational context an agent needs before calling. It does not mention rate limits or latency.

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?

Content is front-loaded (what it validates, then who it's for, then the national-number rule, then cost). It is dense but every clause carries information; the enumeration of line types and check types is long but purposeful.

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 compensates by enumerating the returned validation facets (valid/possible, formatting, country/calling code, line type, SMS hint), so an agent knows what it gets back. Cost is disclosed. Only minor gaps remain (no error behavior, no rate limits).

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 both parameters are documented, making 3 the baseline. The description goes beyond the schema by explaining the relationship between the two params — national numbers are accepted when paired with a country code, with a concrete example ('0612345678 with country=NL') — which is genuinely useful semantics not obvious from the schema alone.

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+resource ('Phone number validation for any country') and enumerates the exact checks performed (allocated number ranges, E.164, formatting, country/calling code, line type, SMS-capable hint). This cleanly distinguishes it from the closest sibling, email_verify.

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?

Names concrete use cases (signup, KYC, CRM cleanup, SMS and calling agents), giving clear context for when to reach for it. However, it never names an alternative or states when NOT to use it (e.g., vs email_verify or kyc_check), so it stops short of explicit routing guidance.

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