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

Annotations already declare readOnlyHint=false and destructiveHint=true; the description reinforces this with 'WRITES live', making the live mutation unmistakable. It adds context not in the annotations — the Pro plan gate and the minimum source-audience size — but says nothing about reversibility, propagation delay for lookalike builds, or failure modes.

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?

Four short clauses, front-loaded with the action and its constraint; nothing is redundant. The fragments 'Pro.' and 'WRITES live.' are terse to the point of being cryptic, costing a little readability.

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 5-parameter mutation with no output schema and only 40% parameter coverage, the description covers the essentials an agent needs to avoid a failed call (Pro plan, 100-person source, mutation warning). It stops short of explaining what is returned (e.g., new audience id), how long creation takes, or the meaning of meta_account_id.

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 40%, so the description needs to compensate. It usefully defines ratio ('1 = the closest 1% of the country') and ties source_audience_id to a minimum-size requirement, but leaves meta_account_id, the optional name/default-naming behavior, and valid ratio range largely unaddressed in prose.

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 ('Create a lookalike audience') and pins the source to 'one of your custom audiences', which separates it from create_website_audience. It doesn't explicitly name that sibling, but the source constraint is enough for an agent to disambiguate.

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 supplies a real prerequisite (source audience of at least ~100 people) and flags the plan requirement ('Pro'), which helps an agent decide whether the call can succeed. However, there is no explicit when-to-use/when-not guidance and no alternative tool is named for building an audience from a website or pixel instead.

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