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

Annotations already establish the safety profile (readOnlyHint=true, idempotentHint=true, openWorldHint=true, destructiveHint=false) and the description does not contradict them. The description adds useful scope context ('available for purchase' and the allowed search filters), but it does not disclose operational behavior such as whether already-owned numbers are excluded or whether availability changes between calls. Given annotations cover safety fully, a 3 is appropriate.

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?

A single 12-word sentence front-loads the verb and resource, then adds the two relevant filter dimensions. There is no filler, no repetition of schema details, and no parenthetical clutter — every phrase earns its place.

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 read-only search tool with a complete four-parameter schema and annotations covering safety, the description is nearly sufficient. What is missing is workflow context: it does not say that a found number can be purchased via buy_phone_number or that list_phone_numbers returns numbers you already own, so the agent must infer the tool's place in the sales flow.

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 100% and every parameter already has inline documentation, including defaults (limit default 10, country default US) and validation patterns (area_code matches ^\d{3}$). The description repeats 'area code or phone number' without adding semantics beyond what the schema provides, so the baseline 3 applies.

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 uses a specific verb ('search') and a specific resource ('phone numbers available for purchase'), and states the two filtering dimensions ('by area code or digit pattern'). It implicitly distinguishes itself from siblings like list_phone_numbers (owned numbers) and buy_phone_number (the purchase action), but it never names a sibling explicitly, so the differentiation is left to inference rather than stated.

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 'available for purchase' implies the use case — searching look for numbers you don't yet own, before executing buy_phone_number — but this is implied rather than stated. There is no explicit guidance on when NOT to use it, e.g. 'use list_phone_numbers to view numbers you already own' or 'use buy_phone_number afterwards to purchase a result with digit pattern.'

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