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

Annotations already declare destructiveHint=true and idempotentHint=true, but the description adds critical behavior not in annotations: phone is mandatory on every update, omitted fields are preserved, and passing group_ids replaces the contact's groups. That replacement semantics is the key risk for an agent to understand.

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 sentences, front-loaded with the tool purpose and immediately followed by the two most important behavioral rules. No wasted text.

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 destructive update tool with no output schema, the description covers the key operational risks: required phone and group replacement. It could mention what happens with an empty group_ids array or error conditions, but it is largely complete.

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?

With 8 parameters and 63% schema description coverage, the description compensates on the most important ones: it explains the mandatory phone parameter and the replace-all behavior of group_ids. Names like firstname/lastname lack schema descriptions, but their purpose is obvious from naming.

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 a specific verb and resource ("Update a contact") that clearly distinguishes it from sibling tools like create_contact or get_contact. It does not explicitly name sibling alternatives, but the update scope 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?

It gives clear usage context: phone is required on every update, omitted fields remain unchanged, and group_ids causes replacement. It lacks explicit when-to-use versus when-not-to-use guidance relative to siblings, but the operational rules are clear.

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