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

Annotations already declare readOnlyHint=true, idempotentHint=true and destructiveHint=false, so safety is covered. The description still adds non-obvious context: the estimate reflects only existing targeting and incurs no spend, which an agent would otherwise have to assume.

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 sentences, each carrying distinct information, with the 'WITHOUT spending' constraint emphasized early. It is dense but not padded; no filler sentences to trim.

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?

An output schema exists, so return formatting need not be described, and the safety profile is covered by annotations. For a two-parameter read-only estimator the description supplies the goal-override behavior and the intended pre-launch workflow, leaving little an agent would still need to guess.

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 0%, so the description must carry the load, and it largely does: optimization_goal is explained as an optional override with concrete example values (OFFSITE_CONVERSIONS, LINK_CLICKS), and adset_id's expected source is stated via 'from list_meta_adsets'.

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 states a specific verb and resource ('Estimates an ad set's reachable audience size and expected daily results') and scopes it to CURRENT targeting. It is clear enough to act on, though it never explicitly contrasts itself with the sibling meta_reach_estimate, which covers adjacent ground.

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?

Gives a clear context: 'Use it to sanity-check targeting before launching,' and clarifies it costs nothing, which is the key usage constraint. It does not name a when-not or point to a competing tool such as meta_reach_estimate.

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