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

Annotations already flag destructiveHint=true and readOnlyHint=false, so the safety profile is covered; the description earns credit for going further with refusal semantics on name conflict (refuses rather than overwriting) and the privacy rule that return values mask the number and must not be repeated in chat. It does not cover failure modes beyond name conflicts or any permission requirements.

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, front-loaded with the action and scope, then the confirmation prerequisite, then the failure and privacy rules. Every sentence carries a distinct and useful constraint with no filler.

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 destructive 4-parameter mutation with full schema coverage, full annotations, and an output schema, the description supplies exactly the missing pieces: confirmation gating, atomicity, conflict refusal, and output masking. Nothing an agent needs in order to call it correctly is absent.

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% and every property carries its own description, so the baseline is 3. The description reinforces the E.164 format and atomicity but adds no syntax, constraint, or interaction detail beyond what the schema already documents.

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 and resource ('Edit a saved contact's name and/or E.164 phone number') with the atomic scope qualifier, which is more precise than the title. It does not name or differentiate against the obvious siblings save_contact and delete_contact, so the agent must infer the boundary itself.

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 clear prerequisite sequence: confirm the existing contact and the proposed change with the user before invoking. It stops short of stating when to prefer this over save_contact (create) or delete_contact (remove), so alternatives are left implicit.

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