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

Annotations already cover the safety profile (readOnlyHint=false, destructiveHint=false, idempotentHint=false, openWorldHint=true), and the description adds real context beyond them: the creation side effect and the fact that `shared` discloses the contact to corporate members. That visibility consequence is exactly the kind of detail annotations cannot express.

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 short, front-loaded sentences covering purpose, input constraints, and side effects with no filler. The 'Side effects:' fragment is slightly clipped but still readable.

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 annotations, a full schema, and an output schema present, the description covers everything needed to call the tool correctly. Only minor gaps remain, such as duplicate-handling behavior or permission/plan prerequisites.

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 all nine parameters are already documented, including the group-names-not-IDs constraint. The description repeats the E.164 requirement and the groups rule, adding emphasis but no new semantic information, so 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?

States a specific verb (Create) and resource (contact in Fax.Plus address book), which clearly separates it from list-contacts, update-contact, and delete-contact. It stops short of explicitly naming the sibling boundary, but the operation is unambiguous.

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 gives the required fields and a format constraint (groups must be names, not IDs), which implies when the tool applies. It never states when to prefer it over update-contact, create-contact-group, or share-contacts, leaving the routing to inference.

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