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

Annotations only declare this is a non-read-only, destructive, closed-world write. The description goes well beyond that: it discloses that the write lands in the same address book used by the web app (cross-surface side effect), that the result masks the phone number, and that the agent must not echo the full number in chat (a privacy constraint on downstream behavior). That is exactly the kind of context annotations cannot carry.

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?

Four short sentences, purpose front-loaded, then format constraint, cross-surface note, and output privacy rule. No filler, though the phone-format sentence and the schema's own E.164 description are somewhat redundant.

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-value explanation is not required, and the confirmation workflow plus privacy behavior are covered. It leaves one gap for a destructive write: what happens when the contact already exists (overwrite vs duplicate), which matters given destructiveHint=true.

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%, so the baseline is 3. The description adds a concrete E.164 example (+85291234567) and reinforces the ordering semantics of `user_confirmed` ('Set user_confirmed only after agreement'), which is slightly richer than the schema's own wording, though it partly restates it.

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?

Clear specific verb+resource: 'Add or update one contact.' An agent immediately knows this writes to an address book. However it never distinguishes itself from the sibling `update_contact` (or `delete_contact`), which is a real ambiguity for a save/update tool.

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 gives one strong usage rule — confirm name and number with the user first, and set `user_confirmed` only after agreement — which is genuine when-to-use guidance. But there is no routing guidance against the overlapping `update_contact` sibling, so the agent is left to infer which of the two to call.

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