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

Beyond the annotations (non-read-only, non-idempotent, non-destructive), the description discloses the cost model (time-based billing tied to session duration), the auth requirement and where to obtain the key, and the payload returned. It does not mention concurrency limits or what happens on repeated calls despite idempotentHint=false.

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?

Two tight sentences: the primary effect and return values come first, followed by the billing/cleanup obligation and the auth prerequisite. No redundant restatement of the tool name or 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, the description usefully names the returned client_token and WebRTC info, and it covers auth, billing, and cleanup for a mutation-style tool with a nested object parameter. Error behavior and session concurrency limits are the remaining gaps.

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 100% and every parameter, including nested metadata and voice_id syntax, is documented in the schema. The description only reinforces the api_key requirement rather than adding parameter-level meaning, so the baseline of 3 applies.

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 names a specific verb and resource — reserves an avatar slot and returns a client_token plus WebRTC connection info. This is clearly distinguishable from siblings like get_session, list_sessions, and end_session.

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?

It states the lifecycle context explicitly: billing accrues while the session is open, so end_session must be called when finished, and an api_key issued in the admin console is required. It stops short of naming alternative entry points or conditions when-not to use it.

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