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

Goes well past the readOnlyHint/openWorldHint annotations by disclosing the data staleness model (allocation-time carrier, no porting awareness, no live status) and the full commercial model ($0.002 USDC per successful call via x402 on Base/Solana/Arc, failed calls uncharged). Those are exactly the facts an agent needs before spending money on a call.

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?

Three sentences, front-loaded with the operation and its outputs, followed by the caveat and then pricing. No filler; every clause (including 'failed calls are not charged') carries decision-relevant information.

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?

An output schema exists, yet the description still summarizes returned fields and, more importantly, supplies the semantic caveats the schema cannot express (allocation-time carrier, no live status) plus cost. Nothing an agent needs to invoke or interpret this tool 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?

Schema coverage is 100% and the single 'number' parameter is documented with format and examples, so the baseline is 3. The description only echoes the E.164 framing and adds no parsing rules, normalization behavior, or error cases beyond what the schema already states.

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 specific verbs and resource: parses an E.164 number, checks it against libphonenumber metadata, and enumerates exactly what is returned (country, line type, formats, allocated carrier). It also disambiguates against the name's implication by declaring 'Not a live HLR query', so an agent knows precisely what class of answer to expect.

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?

Explicitly names the negative case: it is not a live HLR query, ported numbers retain the original carrier, and active/inactive status is not checked. That is real when-not-to-use guidance, though no sibling alternative is offered for callers who actually need live HLR or reachability data.

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