youtube_set_thumbnail

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

No annotations are provided, so the description carries the burden. 'Upload' implies a write/mutating operation, and file format/size constraints are given (JPEG, PNG, GIF, BMP; max 2MB). However, it doesn't disclose ownership/permission requirements (must own the video), whether the change is reversible, or what happens on failure. The format/size constraints are decent but the behavioral profile is incomplete for a mutating operation.

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?

Concise, front-loaded with the core action, and the Args section neatly documents parameters without excess. There's minimal waste. Could arguably drop the Args header format, but the structure is clean and efficient.

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 2-param tool with no output schema and no annotations, this description is adequate but leaves gaps. It explains both parameters and the file constraints, but doesn't cover whether the video must be owned by the authenticated channel, whether thumbnails are validated asynchronously, or what a successful upload returns. It's sufficient for invoking the tool correctly but not richly complete.

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 0%, so the description fully compensates. It documents video_id as 'YouTube video ID' and file_path as 'Absolute path to the thumbnail image' with format and size constraints. This adds meaning beyond the bare property names in the schema, giving the agent concrete guidance on valid inputs.

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?

Clear specific verb+resource: 'Upload a custom thumbnail for a video.' The purpose is unambiguous and distinguishes from siblings like youtube_update_video (which updates video metadata) and youtube_upload_video (which uploads a video). It clearly scopes to the thumbnail-setting action.

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 states what the tool does but provides no when-to-use guidance or exclusions vs alternatives. It doesn't mention that video must exist first, or when this should be used relative to youtube_upload_video. No explicit alternatives are named, though the purpose makes the use case fairly intuitive.

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