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

Annotations cover the general safety profile (readOnlyHint=false, openWorldHint=true, destructiveHint=false, idempotentHint=false), and the description goes well beyond them: exact credit pricing per line type (1 vs 3, +2 for caller name), the up-to-5-credit pre-execution reservation, and an explicit boundary that it places no calls or sends no messages. That is unusually rich disclosure for a paid external lookup, and it is consistent with the non-read-only annotation since credits are consumed.

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?

Four dense sentences, zero filler, with the function front-loaded and the cost/caveat details placed after. Every sentence carries distinct information (capability, caveat, pricing, boundary).

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 two-parameter paid lookup with no output schema, the description covers capability, return attributes, cost, and behavioral boundaries, so an agent can invoke it correctly. It stops short of describing the response shape (e.g., formatting of carrier/line-type fields) or failure behavior on unparseable numbers, which is a minor gap rather than a blocker.

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 the baseline is 3, but the description adds meaning the schema cannot: caller_name costs an extra 2 credits and returns carrier-listing data rather than verified identity. That materially changes how an agent should decide to set the flag.

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 and resource — 'Check a user-provided phone number for carrier and line type' — and names the exact attributes returned. The resource (phone numbers) is distinct from the sibling tools email_verify, ip_lookup, and dns_check, so there is no routing ambiguity.

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 caveat that caller-name data is 'a carrier listing, not identity verification' is genuinely useful constraint-setting, and the cost model implicitly frames when the lookup is worth calling. However, there is no explicit when-to-use guidance and no comparison to the sibling lookup tools, so usage is only implied rather than stated.

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