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

Annotations already mark destructiveHint=true, and the description adds precise behavioral detail: 'adds a member to the number's assigned_to list.' This goes beyond the annotation by specifying exactly what mutation occurs. It does not contradict any annotation. It does not clarify idempotency (e.g., what happens if the member is already assigned), which is a minor gap given idempotentHint=false.

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 plus a side-effect note, front-loaded with the core action. Every sentence adds value: the first states the primary purpose, the second explains parameter roles, and the side-effect note discloses the mutation. No fluff.

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?

The tool has 3 parameters with a nested object and an output schema. The description covers the main purpose, parameter semantics, and the side effect. It does not detail output or error cases, but the output schema exists, and the description sufficiently equips an agent to call the tool correctly. Missing idempotency details are minor.

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 schema fully documents all parameters. The description adds extra meaning by explaining the default for user_id ('usually self') and the relationship between user_id (owner context) and payload.assigned_to (target member). This is useful semantic context beyond what the schema provides.

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 uses the specific verb 'Assign' with a clear resource ('a fax number') and target ('to a corporate member'), and explicitly states the mechanism ('adds them to assigned_to'). It stands apart from the sibling list, which contains no other number-assignment mutation, so there is no ambiguity about what the tool does.

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 implies when to use it by stating the action and clarifying the roles of user_id and payload.assigned_to. It doesn't name alternatives, but none exist among siblings for this exact purpose. It also discloses the side effect, which helps the agent decide if this is the right tool for a destructive operation. Lacking an explicit 'when not to use' or alternative routing, but context is clear.

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