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

Annotations only declare readOnlyHint=true; the description adds the substantive behavior on top of that: no cost to call, per-1K-character pricing for each voice family, and the language restriction (English-only for premium). That is exactly the context an agent needs to choose between families.

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?

'Free.' is front-loaded, then purpose, then Args, then Returns — a clean, scannable structure. The pricing figures are somewhat duplicated with what tool_cost_estimate would report, but they serve the up-front cost comparison.

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, the inline Returns block correctly documents the response shape including the premium/english_only flags. For a one-parameter read tool this is close to fully self-contained, with only the default-behavior detail left implicit.

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 0%, so the description must carry the parameter, and it does: it names the enum values, marks the field optional, and states the default ('both'). The only gap is that it doesn't spell out that 'both' yields both result arrays, though the Returns block makes that inferable.

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 ('The voices generate_voiceover can speak') and distinguishes the two families it returns (standard Gemini vs Premium MiniMax). An agent immediately knows this is a catalog lookup feeding the voiceover tool, not a generator itself.

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?

Establishes that the call is 'Free' and points to tool_cost_estimate for exact pricing, which is clear routing guidance. It never states an explicit when-not or a required ordering relative to generate_voiceover, so it stops short of the top score.

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