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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the 'DOES NOT BUY' line mostly reinforces structured data. However, the description adds genuinely new behavioral guidance: the country code is mandatory and must not be guessed, with a named fallback tool, which is operational context the annotations do not provide. It stops short of describing result shape or pagination.

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 short sentences with zero filler, and the critical constraint (no purchase) is front-loaded before the prerequisite guidance. Every sentence 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 listing tool with no output schema, the description supplies the action, the negative scope, and the prerequisite chain needed to call it correctly. It does not describe what the results contain or how limits/pagination behave, which is a minor gap given the annotations already cover the safety profile.

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 80%, so the schema already documents country, locality, area_code, and phone_number_type. The description's only parameter content is that country is required and must not be guessed, which the schema already marks as required. Baseline 3 is appropriate since the schema does the heavy lifting.

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 ('Lists purchasable phone numbers in a country') and immediately scopes it with 'DOES NOT BUY — it only shows them', which cleanly separates it from siblings like call2me_purchase_number, call2me_checkout_number, and call2me_bind_number.

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 covers both when to use it and what it does not do, plus a concrete prerequisite workflow: the country code is required, must not be guessed, and the agent should ask the user or call call2me_list_allowed_countries first. Nothing is left to inference.

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