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

Annotations already declare destructiveHint=true and idempotentHint=true, so the safety profile is covered. The description adds value beyond them by disclosing the force_transfer behavior — that it takes a username currently held by a different number in the same business — which is the genuinely risky side effect an agent needs warned about. It stops short of saying what happens to the previous holder's username or whether eligibility checks are required.

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 dense sentences with no filler. The core action is front-loaded, followed by validation rules and then the escaping-hatch parameter, which is the right ordering for an agent deciding whether it can call the tool at all.

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 mutation tool with no output schema, the description covers the action, validation constraints, and the destructive transfer path, and the annotations carry the idempotency/destructiveness signal. Remaining gaps — rate limits, username availability, and whether the displaced number loses its username — are minor but real for a destructive operation.

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 only 50% (username and transfer_action have no schema descriptions), so the description must compensate and largely does: it spells out the username character set and structural rules (must contain a letter; no leading, trailing, or consecutive periods) and explains the non-obvious force_transfer enum value. It adds nothing further for profile_id and phone_number_id, but those are already documented in 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?

The description gives a precise verb pair plus resource and scope: "Claim or change the WhatsApp username of a phone number." This is unambiguous against the sibling verbs (whatsapp_username_get, whatsapp_username_delete, whatsapp_username_suggestions), which all act on the same resource with different operations.

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 only routing guidance given is conditional and parameter-scoped: pass transfer_action: force_transfer "to move a username already held by another number in the same business." There is no guidance on when to prefer this tool over whatsapp_username_get (to check availability) or whatsapp_username_suggestions (to find a valid name), so the usage context is implied rather than stated.

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