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

Annotations already declare destructiveHint=true, idempotentHint=false and openWorldHint=true. The description adds value beyond them by disclosing the concrete side effects: the member is added to the number's assigned_to list and is notified via email and push — exactly the consequence an agent should know before invoking. It stops short of saying how to undo the assignment or what errors occur if the member is already assigned.

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, front-loaded blocks: the operation and parameter roles first, side effects second. No filler, no restatement of the title, and the most decision-relevant information (notifications) is not buried.

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?

With an output schema present, return values need no explanation, and annotations carry the safety profile. The description covers purpose, parameter roles, and side effects. Minor remaining gaps are failure modes (member already assigned, invalid number) and whether the assignment is reversible.

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 baseline would be 3. The description earns more by disambiguating the easily confused pair: user_id is the account/owner context (usually self for the admin), while payload.assigned_to is the member to assign TO. That distinction is the main semantic risk in this tool and is called out explicitly.

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 precise verb+resource: assigning a fax number to a corporate member, and names the underlying mechanic (adds them to assigned_to). This tells an agent it is an assignment operation rather than a generic number edit. It doesn't explicitly contrast with bulk-update-faxes, though the singular/member framing makes the distinction inferable.

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?

Usage is implied by the operation itself (assign a member to a number), and the note that user_id is 'usually self for the admin' gives practical guidance. However, there is no explicit when-to-use vs. the bulk-update-faxes sibling, nor any precondition such as needing the member to already exist or already own a number.

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