set_video_thumbnail

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

Annotations already convey readOnly=false, idempotent=true, and destructive=false. The description adds the upload-scope requirement and clarifies that the thumbnail is derived from the video itself. It does not, however, disclose whether an existing custom thumbnail is overwritten or how invalid timecodes are handled, but the annotation coverage lowers the burden.

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, well-structured sentence that front-loads the core action and source, followed by the essential permission requirement. There is no redundant content or 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 two-parameter mutation tool with no output schema, the description is largely complete: it states the operation, the source of the thumbnail, and the required scope. Minor gaps such as overwrite behavior and response details are not critical for an agent to select and invoke the tool correctly.

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%, and both parameters are already clearly documented: video_id is the target video and time_seconds is the frame position. The description's 'timecode' wording aligns with the schema but does not add meaningful parameter-level detail beyond what the schema provides, so baseline 3 is appropriate.

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: 'Set a video's thumbnail', and adds the distinctive source 'from a timecode in the video itself.' This clearly differentiates it from general video update tools and other folder/showcase operations in the sibling list.

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 useful prerequisite ('Needs the upload scope'), which helps the agent know whether it is permitted to call the tool. However, it does not explicitly state when to choose this tool over alternatives such as update_video, nor does it provide exclusion conditions or use-case guidance beyond the obvious.

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