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

Annotations cover the safety profile (readOnlyHint=false, idempotentHint=true, openWorldHint=true), and the description adds genuinely useful context beyond them: exact credit costs per tier and the fact that a null result is free. It also discloses the premium data source (domain registration records). It stops short of explaining what the tier or cache flags do to 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?

Compact and front-loaded: the core purpose leads, then tier definitions and pricing in tight parentheticals. Every clause earns its place, though the pricing parenthetical is dense enough to require a second read.

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?

There is no output schema, so the description must carry more weight, and it partially does by naming the returned fields per tier (caller ID name, emails, phones). However, it says nothing about response shape, empty-result handling beyond 'free', or the two undocumented input parameters, leaving real gaps for a 4-parameter 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 description coverage is 0%, so the description carries the burden, and it only documents the tier parameter (standard vs premium). The phone, bypass_cache, and default_country parameters are unexplained in both schema and text; bypass_cache in particular is opaque given the credit-based pricing.

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 states a specific resource and outcome: identifying who is behind a US/Canada phone number, with a scope limit (US/Canada). It is clear enough to distinguish from reverse_email, but it never names siblings like carrier_lookup or validate_phone, so the agent must infer the boundary between 'who owns this number' and 'which carrier serves it'.

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 explains what each tier returns and costs, which effectively guides the standard-vs-premium choice, but there is no explicit when-to-use-this-tool versus alternatives (carrier_lookup, validate_phone, verify_contact) and no stated preconditions or exclusions. Usage context is implied rather than declared.

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