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

Annotations already declare readOnlyHint, destructiveHint=false and a closed-world scope, and the description adds genuinely new context: an ownership/permission restriction, an authentication requirement, an ordering requirement, and an explicit no-side-effect guarantee ("nothing is purchased"). Missing only return-shape/pagination details, which is tolerable for a read-only lookup.

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 short sentences, no filler, and the access gate is front-loaded ahead of the action and the safety clarification. Every sentence conveys a distinct operational fact (permission, action, prerequisite, no purchase).

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 one-parameter read-only lookup with no output schema, the description covers the essentials an agent needs: who can call it, what it does, what to read first, and that it has no purchasing side effect. The only real hole is the undocumented required parameter and an unstated hint that results come back as a set of candidate numbers to choose from.

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?

The schema has 0% description coverage on a single required parameter, so the description needs to carry the meaning of "area_code" and it does not. It never mentions the parameter, its required status, or the regex-constrained 3-digit format, leaving the agent to infer that the search is area-code-driven at all.

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 ("Search") and a precisely scoped resource ("available US local numbers included in the owner's plan"). The scope qualifiers (US local, plan-included) plus "nothing is purchased by searching" separate it from the activation/purchase flow without the agent having to open another schema.

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?

Gives real gating conditions: owners-only, sign-in required, and a prerequisite step ("Read receptionist_service first"). It also clarifies the negative case implicitly (searching is not buying). It does not name the concrete next tool (e.g. activate_phone_number) that the owner would use after choosing, so the routing is slightly incomplete.

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