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

Annotations already declare readOnlyHint=false and destructiveHint=true, so the safety profile is covered. The description adds valuable auth-flow context, but it never discloses the concrete side effect of the flagged destructive behavior — namely that the previously primary phone is demoted or replaced — which is exactly the fact an agent needs before invoking it.

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 short blocks: the action first, then the access requirement. Nothing is wasted and the core action is front-loaded, though the authentication paragraph is somewhat boilerplate-heavy relative to the one-line operation.

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 values need not be explained, and the auth prerequisite is covered. What remains missing for a destructive mutation is the effect on the existing primary phone and any rollback/reversibility note, leaving the agent without the full consequence picture.

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 100% and the single 'id' parameter is already documented as 'Phone record ID to set as primary'. The description adds no format, provenance, or validation detail beyond the schema, so the baseline 3 applies.

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 states a specific verb and resource ('Set a phone number as the primary phone for the account'), making it trivially distinguishable from set_primary_email and set_primary_profile by resource. It is clear but never names a sibling tool for contrast, so it stops short of a 5.

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 second paragraph gives an access precondition (needs a Proof account, authenticate via /mcp or start_login) which is genuinely useful routing context. However, it gives no guidance on how to obtain the required phone-record id (e.g. via list_phones or start_add_phone), nor any when-to-use vs. when-not framing against the sibling setters.

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