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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds useful framing that this is a pre-spend estimate rather than a mutation, but says nothing about auth requirements, rate limits, or any quirks of the estimate call.

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?

Front-loaded with the purpose in the first sentence, followed by compact per-parameter notes. The em-dash parentheticals make it dense but not padded; a reader can extract the key facts quickly.

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 values need not be described. For a 13-parameter tool with 0% schema coverage, the description documents most audience-shaping fields and their code systems; only age range and optimization_goal defaults remain undocumented, a minor gap.

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 carries the load. It documents countries (ISO comma-separated), interest_ids (with source tool), genders (1/2 coding), relationship_statuses (3=married), and lists the demographics/behaviours fields that mirror create_meta_adset codes — substantial value. It omits age_min/age_max and optimization_goal semantics, so it is not fully compensating.

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 ('Returns the estimated reach') and resource ('for a targeting setup'), then clarifies the scope as 'audience size before spending'. This clearly distinguishes it from create_meta_adset (which builds) and meta_delivery_estimate (which is about delivery, not audience size).

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?

Explicitly tells the agent when to use it ('size the audience here BEFORE building the ad set') and points to search_meta_interests as the source of interest_ids. It does not name the closest alternative, meta_delivery_estimate, or state exclusions, but the usage context is clearly framed.

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