update_thumbnail

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

With no annotations, the description carries the burden of behavioral disclosure. It states the auth requirement and the return shape (data, error, successful), providing more transparency than many mutation tools. However, it does not mention whether the change is reversible, how it affects existing thumbnails, or any rate limits. This is moderate but not exhaustive.

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 well-structured with a clear first line stating the action, followed by permission, then parameter details, and return info. Every sentence adds necessary information without redundancy or fluff. It is appropriately sized for a simple tool with two parameters.

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 low complexity and minimal parameter count, the description covers the key aspects: prerequisites, parameter specifications, and return format. The output schema exists, so not explaining every return field is acceptable. Minor gaps like error handling or impact on existing thumbnails prevent a perfect score, but the description is complete enough for confident use.

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 has no parameter descriptions (0% coverage), so the description must compensate. It thoroughly explains both parameters: thumbnail_url includes format, size, and dimension constraints; video_id is clearly identified. This fully adds meaning beyond the raw schema and effectively guides usage.

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's verb and resource: 'Sets the custom thumbnail for a YouTube video using an image from thumbnailUrl.' It specifies the exact action and target, distinguishing it from siblings like update_video and upload_video. The purpose is unambiguous and specific.

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?

The description gives a clear context for use: updating a video thumbnail, with the prerequisite that the user must have permission to edit the video. Although it does not explicitly mention alternatives or exclusions, the context is strong enough to know when to apply this tool. No sibling comparisons are made, but the action is self-contained.

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