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

Annotations already declare readOnly, idempotent, non-destructive, so the safety profile is covered. The description adds genuine value beyond that by disclosing the breadth of returned data (custom fields, communication preferences, sites, status) and the underlying endpoint, which no structured field provides since there is no output schema.

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?

A single front-loaded sentence with the purpose first, followed by the returned-field inventory and endpoint. It is efficient, though the trailing 'API: GET /addressbook/contacts/{id}' is arguably redundant for an agent that already has the tool name and schema.

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 one-parameter read tool with annotations covering safety and no output schema, the description compensates well by listing what comes back. It stops short of describing the shape of those fields (nesting, formats) or error behavior for an unknown id, so it is strong but not fully 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?

Schema description coverage is 100% for the single contact_id parameter, so the schema already documents type, range and meaning. 'By id' adds no syntax or format detail beyond what the schema states; baseline 3 is appropriate.

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 and resource ('Get one Address Book contact by id') and enumerates the returned fields, which distinguishes it from list_contacts and get_child. An agent can identify this as the single-record lookup without opening the schema.

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?

'By id' implies this is a lookup requiring an already-known contact id, which is useful implied usage. However, there is no explicit when-to-use guidance, no mention of list_contacts as the way to obtain ids, and no exclusions relative to sibling retrieval tools.

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