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

With no annotations, the description carries the full burden and does well: it discloses the 5-credit cost, the destructive consequence of using specPrompt (loses authenticity, discards the real frame), and the default behavior of 'prompt'. It does not mention auth requirements or rate limits, but the operational consequences are unusually well covered.

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?

Four dense sentences, each front-loaded with the highest-value guidance first (what it does and its cost), followed by parameter routing and the key pitfall. No filler.

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?

For a 4-param tool with no annotations and no output schema, the description covers cost, inputs, defaults, and failure modes thoroughly. The only gap is that it says nothing about what the call returns (an image URL, render status, etc.), which an agent might need to proceed.

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 100%, so all four parameters are already documented in the schema. The description reinforces the roles of sourceImageUrl, faceImageUrl, prompt, and specPrompt, but adds little beyond what the schema descriptions already state, so it sits at the baseline.

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+resource: swapping an avatar's face onto a real hook frame. It is instantly distinguishable from siblings like generate_image or generate_video because it describes an image-to-image face compositing operation.

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 routing rules: ALWAYS use sourceImageUrl + faceImageUrl, only override 'prompt' when the user gives custom instructions, and NEVER put user instructions in specPrompt. When-to-use and when-not-to-use are both spelled out.

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