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

Annotations indicate destructiveHint=true and readOnlyHint=false, implying mutation. The description goes beyond annotations by clearly stating the cost implications: cloning and image URL are free, while prompt generation spends 1 credit. It also reveals that library presets can be used directly, avoiding unnecessary clones, and that the return value is a characterId. This adds critical context for an agent deciding whether to call the tool.

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?

The description is dense but organized: it starts with the core purpose, then lists the three modes in a clear structure, and ends with a usage caveat. It is a bit long but each sentence adds value; the only minor inefficiency is repeating 'saved for FREE' which could be tightened.

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?

Given the tool's complexity (three alternative inputs, cost implications, integration with other tools), the description is comprehensive. It covers prerequisites (import_asset_from_url), return value (characterId), and downstream usage (create_video). With no output schema, the description adequately explains the return value, and the annotations cover mutation/safety, so nothing essential 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 schema already covers all parameters with descriptions, but the description adds significant meaning by explaining the parameter relationships (e.g., fromLibraryId defaults name/voice, imageUrl requires hosting, prompt requires a 9:16 portrait and costs a credit). It clarifies that name is optional in cloning context, which is not fully explicit in the schema.

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 clearly states the tool creates a saved character/actor for UGC video, distinguishes it from sibling tools like create_character_swap and list_character_library, and specifies the three input modes. It also explains the output (characterId) and its downstream use in create_video.

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?

Provides explicit when-to-use guidance: it details three mutually exclusive input methods (fromLibraryId, imageUrl, prompt), notes when to use import_asset_from_url for raw bytes, and warns against unnecessary cloning of library presets unless the user wants an editable copy, correctly routing to alternatives.

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