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

Annotations only supply the safety profile (readOnly=false, idempotent=true, destructive=false); the description adds real behavior beyond them: the change persists for 'every later send from it, in the API and in mail apps,' and clearing falls back to the domain default name or the bare address. That downstream effect is exactly what an agent needs to reason about consequences.

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?

Three sentences, no filler, and the primary effect is front-loaded before the clearing details. Every clause carries information the agent cannot get from the structured fields alone.

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 2-parameter mutation with no output schema, the description covers effect scope, persistence, and the clearing fallback, while annotations cover idempotency and lack of destructiveness. It omits any permission/auth prerequisite, which is a minor but real gap for a write 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 coverage is 100%, so the two parameters are already documented, making 3 the baseline. The description earns the extra point by explaining the semantics of the null/empty-string value (fallback to domain default or bare address), which the schema only summarizes as 'clears 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?

States a specific verb+resource ('Set or clear the name a mailbox sends as') and immediately identifies the concrete effect ('the From display name recipients see'). No sibling tool (create_mailbox, update_draft, etc.) touches this field, so the agent can route unambiguously.

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 clear invocation context, including the trigger for the clearing path ('Pass null or "" to clear it') and what happens after clearing. It does not name a sibling or state when *not* to use it, so it falls short of the explicit routing seen at 5.

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