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

Annotations already declare readOnly, idempotent, and non-destructive, so the safety profile is covered. The description adds real domain context beyond that: what counts as a contact, that it counts once per billing cycle even for returning contacts, that it is per-number not per-organization, and that it resets at cycle_end. It does not cover error cases or behavior for unknown aliases, keeping it below 5.

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?

Front-loaded with the outcome the caller wants, followed by the counting rule and then scoping/reset facts. Every sentence carries information, though the 'new or returning / later cycle' explanation is a touch verbose for what it conveys.

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 described, and annotations plus a fully documented single parameter reduce the description's burden. It supplies the quota semantics and reset behavior an agent needs to interpret results, leaving only minor gaps such as what happens if the number alias is invalid.

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% for the single number_alias parameter, including accepted forms, the list_numbers pointer, and the 'main' default, so the baseline is 3. The description's phrase 'this number's' reinforces scoping but adds no syntax or selection guidance beyond the schema.

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 with scope: 'Check how much of this number's contact allowance is left.' It also distinguishes itself from the adjacent quota concept by noting 'Like the message quota, this is per number rather than per organization,' so an agent can tell it apart from get_message_quota and get_usage.

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 purpose implies when to call it (checking remaining allowance for a number), but there is no explicit 'use this when…' or exclusion of alternatives such as get_usage, get_message_quota, or get_number. The comparison to message quota clarifies scope, not routing.

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