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

Annotations already declare readOnlyHint=true and idempotentHint=true, so safety is covered. The description adds the search semantics (by name/address) but largely repeats the schema's parameter description. It does not disclose return format, result limits, or behavior on no match, though these are less critical given simple read-only nature.

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?

The description is two sentences with no fluff. The first sentence immediately states the action and target, and the second sentence adds a concrete use case. Every word earns its place.

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 is a simple read-only lookup with one parameter and clear annotations. The description covers what and when, which is sufficient for the low complexity. However, without an output schema, it does not describe what the tool returns (e.g., a list of contacts or a single match), leaving some ambiguity.

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%, and the schema already describes the query parameter as 'A name, part of a name, or an address.' The description restates the same information without adding extra meaning or examples, so it provides no additional value beyond the schema.

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 ('Look someone up') and clearly identifies the resource ('address book') and the search criteria (name, part of a name, or address). It distinguishes itself from sibling tools like contacts_list and contacts_add/delete by focusing on lookup rather than enumeration or modification.

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 provides explicit usage context: 'Use this before sending mail to a person named rather than addressed.' This tells the agent when to invoke the tool. However, it does not explicitly name alternative tools or state when not to use it, so while the context is clear, it lacks explicit exclusions.

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