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

Annotations already declare this is a non-read-only write with idempotentHint=true and destructiveHint=false, so safety framing is covered. The description adds the meaningful compliance constraint about opt-in recipients and the existence prerequisite. It does not, however, explain duplicate handling despite idempotentHint, the 500-contact cap, or partial-failure behavior — all material for a batch write.

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?

Two compact sentences, front-loaded with the action and followed by the eligibility constraint; no filler. Slightly harmed by the ambiguous 'whatsapp_list contact_lists' reference.

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 3-parameter batch mutation with annotations but no output schema, the description covers purpose, prerequisite and consent, which is the core. It stops short of batch semantics (duplicates, max items, what a partial success returns), which an agent calling a 500-item write would want to know.

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 only 33%, so the description must compensate and it does partially by clarifying that phone should carry a country code and that name is optional. It adds nothing about list_id format or the brand_id disambiguation flow, leaving a real gap for a low-coverage 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+resource: adding contacts (phone with country code and optional name) to an existing list in contact_lists. It is distinguishable from siblings such as whatsapp_create_contact_list, though the phrase 'whatsapp_list contact_lists' is slightly muddled naming rather than a clean reference to the target resource.

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?

It implies the list must already exist ('una lista existente') and that only opted-in people may be added, which is useful gating. But it never explicitly routes the agent to whatsapp_create_contact_list when no list exists, nor states when this tool should be preferred over other contact-related tools.

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