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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, so the safety profile is covered. The description adds real behavioral context beyond that: prices are in EUR, requiresAddress means the owner must have a saved address in that country, and other plans need billing details saved — prerequisites an agent could not infer from annotations.

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 tight sentences, front-loaded with what is returned, then the two field semantics that matter for downstream decisions. No filler or repetition of the title.

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, so return values need not be explained, and the description still clarifies the two non-obvious fields (requiresAddress, canSendSms). For a one-parameter read-only lookup this is nearly complete; only explicit when-to-call guidance 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 countryid parameter is fully documented in the schema, so the description carries no burden here. It adds no filtering syntax or behavior for countryid, which is acceptable given the schema does the work.

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 names a specific resource set — countries, rental plans and prices in EUR for phone numbers — so an agent knows exactly what it retrieves. It does not explicitly contrast itself with siblings like get_balance or list_my_numbers, but the mention of plan_id for buy_number gives a partial anchor.

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?

Usage is only implied: the description says to use a plan's id as plan_id in buy_number, which hints this is a pre-purchase lookup, but it never states 'call this before buying' or when to skip it. No exclusions or alternative tools are named.

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