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

No annotations are provided, so the description carries the burden. It discloses the access requirement (Proof account) and the authentication prerequisite, which is useful. However, it doesn't disclose whether this is a mutating operation, whether it affects other emails, or what the response looks like. The mutation is implied by 'set' but not explicitly stated.

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?

The description is concise and front-loaded with the core purpose. The access/authentication note is useful but slightly verbose with the parenthetical example. Still, every sentence earns its place.

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 tool with no output schema, the description covers the main prerequisite (authentication) and the parameter is fully documented in the schema. However, it doesn't mention what happens after setting primary email (e.g., whether the old primary is demoted) or any error conditions, which would be useful for a mutating 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 description coverage is 100%, so the schema already documents the 'id' parameter as 'Email record ID to set as primary.' The description adds no additional meaning beyond the schema, so baseline 3 is appropriate.

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 an email address as the primary email for the account.' This clearly distinguishes it from sibling tools like set_primary_phone and set_primary_profile, though it doesn't explicitly name those siblings.

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 description provides clear context on when to use the tool: after authentication, and it mentions the alternative start_login for opening a session. It doesn't explicitly state when not to use it or name alternatives like list_emails, but the access requirement and workflow are clear.

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