Look up contacts

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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds the behavioral limitation that @username lookup is impossible and enumerates the returned contact attributes, which is useful beyond the annotations.

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 sentences with no filler; the output fields are front-loaded and the username limitation is stated compactly. The first sentence is a fragment, but it is still efficient.

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 read-only, two-parameter lookup, the description plus schema covers inputs, accepted id formats, count limits, and returned fields. No output schema exists, but the description enumerates the key return values; it doesn't discuss errors or pagination, but those are not critical for this simple lookup.

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%, with accountId and contactIds both described, including the accepted formats E.164, digits, or `lid:<digits>`. The description reinforces the 1-50 count and the exclusion of usernames but adds no new parameter-level detail 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 names the resource (contacts) and the returned fields (about text, WhatsApp username, business name, device count), and the title supplies the verb 'look up'. It is clear but does not explicitly contrast with sibling tools such as check_numbers, so it stops short of full differentiation.

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 explicitly tells the agent to look contacts up by number or `lid:` id and explains that usernames cannot be searched because a linked device cannot resolve an @username. This is clear context and a positive/negative usage rule, though it doesn't name alternative tools.

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