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

Beyond the annotations (which only say destructiveHint=true, non-read-only), the description discloses the cost per call (1 credit), that every call renders a new portrait, the merge semantics of partial updates, and the side effect that changing language clears language_accent. It also gives an operational constraint (show the returned previewUrl, never substitute your own image). Nothing here contradicts the annotations.

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?

Front-loaded with purpose and tightly packed, but the single dense paragraph stacks several nested conditionals (auto-cast vs partial vs first actor vs voice-only) plus the previewUrl instruction, making it heavier to parse than the ideal. Every sentence does carry information, so no real waste.

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?

There is no output schema, and the description compensates by naming the return artifact (previewUrl) and how to use it. With 1 required parameter, a nested object, enum constraints, and a credit-consuming render, all the information an agent needs to call this correctly is present.

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 75% and the description adds semantics the schema cannot express: omitting archetype triggers auto-casting, archetype behaves as a merge patch on an existing actor, and a first actor requires all fields except facialFeatures. It also scopes language/language_accent to voice changes rather than recasting, which is not derivable from the enum alone.

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?

Opens with a specific verb+resource: 'Casts the on-camera narrator and renders their portrait', and immediately names the sibling it differs from (set_narrator_voice) and the field that selects between them. An agent can distinguish it from add_subject, set_product, and set_narrator_voice without opening any schema.

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?

Explicit conditional routing: omit archetype for an audience-fitted auto-cast, pass it to design someone, and use set_narrator_voice instead when only language/accent should change 'which doesn't recast'. It also states the create-vs-update rule (first actor needs every field except facialFeatures) and that partial fields merge, so the agent knows exactly when and how to invoke it.

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