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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the bar is lower. The description still adds genuinely useful operational context beyond them: the API-key requirement and the fact that the text limit is tied to the caller's plan, which affects downstream calls. It does not discuss pagination or caching, but for a static listing this is solid.

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 front-loaded sentence with no filler, leading with the resource name and then the concrete contents. The trailing 'Needs an API key' would read better as its own short sentence, but nothing is wasted.

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 description carries the burden of explaining what is returned, and it does so by enumerating the voice attributes and the plan-dependent text limit. Auth requirement is also stated. For a zero-parameter read tool this is close to complete, lacking only an explicit tie-in to when to call it before synthesis.

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 and the schema is an empty object, so there is nothing for the description to disambiguate. Baseline 4 applies; the description correctly refrains from inventing parameter detail.

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 resource (Text to Speech voices) and enumerates exactly what comes back: the four named voices with language, accent and gender, plus the plan's text limit. This clearly distinguishes it from the sibling text_to_speech_synthesize (listing metadata vs. generating audio), though it never names that sibling 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?

Usage is only implied — an agent can infer this is a pre-flight lookup to discover valid voice names and the character limit before calling a synthesis tool. There is no explicit when-to-use statement, no when-not, and no named alternative, so the routing has to be inferred from the sibling list.

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