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

Annotations only declare the generic write profile (readOnlyHint=false, idempotentHint=false, openWorldHint=true); the description adds genuinely new behavior — contacts are auto opted-in to WhatsApp notifications, and duplicate numbers return a 409 conflict. The consent warning flags a real, non-obvious side effect an agent would otherwise miss.

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, front-loaded with the WRITE marker and the core action, then compliance caveat, then the error contract. Every sentence carries unique information; the trailing endpoint path is the only borderline item and it usefully anchors the call.

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 2-param, non-destructive write with no output schema, the description covers action, side effects, and the duplicate-error path. It does not describe the success payload (contact ID and what fields come back), which matters since no output schema exists.

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 50%: phone_number already documents the country-code format and + handling in the schema, so the description merely restates it. The name parameter's constraints (1–200 chars) are covered by the schema but not explained in prose, so the description adds little semantic value beyond structured fields — baseline 3.

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?

Opens with an explicit verb+resource+mode ('WRITE: add a contact'), which cleanly separates it from the read siblings (find_contact, get_contact) and from update_contact. It never names a sibling or states an exclusion, so it stops just short of the top mark.

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?

Gives a concrete precondition ('only add people who agreed to be messaged') and warns that duplicates yield a 409, which implies checking existence first. It does not explicitly route the agent to find_contact/get_contact or tell it to switch to update_contact for existing numbers, so the guidance is contextual rather than prescriptive.

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