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

Annotations already disclose this is a non-read-only, non-destructive, non-idempotent operation. The description adds that a frame is selected from the video and becomes the thumbnail, which clarifies the mutation target, but it does not explain overwrite behavior, permissions, or other state effects.

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 a single front-loaded sentence with no redundant or wasted wording. It communicates the core action immediately.

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 simple two-parameter tool with annotations and an output schema, the description is minimally adequate. However, it does not clarify relevant context such as how this differs from thumbnail upload tools or whether selecting a frame replaces an existing thumbnail.

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%, so the schema already documents both parameters. The description adds no additional parameter meaning. Notably, the schema's asset_id description says the asset 'needs to be deleted,' which is misleading for this operation, and the description does not correct that error.

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 states a specific verb and resource: selecting a video frame to use as a thumbnail. It is understandable and differentiates itself from upload-based siblings by focusing on frame selection rather than custom image upload, though it does not explicitly name those alternatives.

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 intended usage is implied: use this when choosing a thumbnail from an existing video frame. There is no explicit when-to-use guidance versus siblings like thumbnail_upload or thumbnail_upload_live, nor are prerequisites or exclusions stated.

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