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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds genuine context beyond that: the exact content of the response (formats, engines, per-duration credit costs, allowed option values) and the fact that the call costs nothing ('Free'), which is not visible in any structured field.

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?

A single sentence that front-loads the primary purpose and packs the return contents into one parenthetical list, closing with the cost note. No filler, no repetition of the title.

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?

With no output schema and no parameters, the description carries the full burden of explaining what comes back, and it does so concretely (formats, two engines with credit costs per duration, option values). Nothing an agent needs in order to decide to call this before create_ad is missing.

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?

The tool takes zero parameters, so the baseline is 4. The description correctly spends no words on inputs and instead describes outputs — the right allocation for a no-arg enumeration tool.

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 ('Lists') and a precisely scoped resource (Ad Studio formats), then enumerates the categories returned: formats, the two engines with credit costs per duration, and the ratio/language/caption/music options create_ad accepts. That framing also separates it from the sibling create_ad, which consumes rather than enumerates these values.

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?

Usage is only implied: naming the options 'create_ad accepts' signals this is the discovery step before create_ad, and 'Free' implicitly contrasts with paid operations like estimate_cost. There is no explicit when-to-use statement and no exclusions, so the agent must infer the sequencing itself.

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