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

Annotations already declare readOnlyHint=false and destructiveHint=true, so the mutation risk is flagged structurally. The description adds real value beyond that by spelling out the authentication requirement and the exact remediation path (start_login, then retry). It does not explain the destructive side effect — that setting a new primary presumably demotes the previous primary number.

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 sentences, purpose front-loaded ahead of the access requirement, with no filler. The second sentence is somewhat dense but every clause (auth requirement, two remediation routes, retry instruction) carries information.

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 single-parameter mutation with annotations covering the safety profile and no output schema, the description is nearly complete — it covers purpose and the auth gate. The one meaningful omission is the side effect on the existing primary phone, which matters for a destructive-flagged tool.

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 syntax, format, or sourcing 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.' This clearly distinguishes it from siblings like set_primary_email and set_primary_profile by the resource acted upon. It does not, however, explicitly name a sibling or contrast itself with them.

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 a genuine prerequisite ('needs a Proof account') and points at the start_login alternative for obtaining one, which is useful routing. But it offers no guidance on when to prefer this tool over other phone-related tools (start_add_phone, list_phones, remove_phone) or when not to call it.

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