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

Annotations already declare readOnlyHint=false, openWorldHint=true and idempotentHint=false, so the write/external nature is partly covered. The description does add real behavior — an outbound WhatsApp message to the stored number — but is silent on error behavior for unregistered numbers, rate limits, and whether re-invoking invalidates a prior code.

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 sentence with zero filler, front-loading the audience constraint before the action. Nothing can be removed without losing meaning.

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?

With one fully documented parameter, annotations present and no output schema, the core is covered. However, for a multi-step authentication flow whose siblings include complete_login and start_registration, the description omits the sequencing an agent needs to chain calls correctly.

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% with a single required 'phone' parameter whose description already gives format and example. The description only restates that the number must be the registered one, adding no format or validation detail beyond the schema, 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?

States a specific action and audience: it initiates login for existing customers by sending a WhatsApp code to the registered phone. The phrase 'per chi è già cliente' implicitly separates it from start_registration, but it never names the sibling it completes into (complete_login).

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 qualifier 'per chi è già cliente' implies the usage condition (existing customers vs. new registrations), but there is no explicit when-to-use statement, no mention of the required follow-up tool, and no exclusions or prerequisites such as the number having to exist in the system.

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