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

Annotations already cover read-only, idempotent and non-destructive behavior, and the description adds real value on top: the caveat that results are estimates, not measured performance, and the cost note (no Adrails credits). It omits latency, failure modes for invalid targeting, and any rate-limit context.

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 short sentences, purpose front-loaded, caveat second. The credits sentence is minor but relevant to call-cost decisions; nothing is redundant.

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?

No output schema exists, so the description should say more about the shape of an estimate (single reach figure vs range, breakdown by audience component). It also omits what happens with invalid or unserviceable targeting, leaving meaningful gaps for a nested-schema estimation tool.

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?

With 75% schema coverage, the baseline is 3. The description adds nothing about the nested input fields — notably the opaque genders enum (1/2) and the countries vs locations distinction — so the schema carries the full parameter burden.

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: 'Estimate Meta reach for proposed targeting.' An agent can distinguish this from sibling listing tools like targeting_options, though no sibling is named explicitly.

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?

Implies usage (pre-launch reach check for proposed targeting) via 'proposed targeting' and 'existing account audience service,' but never states when to use this versus targeting_options or planning_calculate, nor any exclusions or prerequisites.

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