Set the custom thumbnail on a YouTube video

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

The description goes well beyond the annotations, revealing that the change is public and immediate, requires a verified YouTube channel, compresses images over 2MB, mandates Hermoso-hosted images (not just generated ones), and refuses external hosts. Annotations only say readOnlyHint=false, idempotentHint=true, destructiveHint=false; the description adds substantial behavioral context critical for safe invocation.

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?

Though lengthy, every sentence earns its place—covering purpose, constraints, prerequisites, and confirmation requirement. The main action is front-loaded, and the rationale for the tool (thumbnail impact on click-through) is concise and relevant. 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?

Given the tool's complexity (prerequisites, hosting constraints, public nature), the description covers all essential operational details. No output schema exists, but the tool's behavior is simple enough (success/failure) that the description is complete. An agent can call this correctly without further info.

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 params are documented. The description adds value by clarifying videoId as the ID returned by post_to_youtube and elaborating on imageUrl (what counts as Hermoso-hosted, the need to pass through upload_file first, and that external hosts are refused). This enriches the schema without redundancy.

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 the CUSTOM THUMBNAIL on a video already on the connected channel'. It clearly distinguishes itself from other tools by emphasizing it changes ONLY the thumbnail, leaving video, title, and privacy untouched. This prevents confusion with tools like update_youtube_video.

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?

Provides clear context: it's for videos already on the channel, and implies when to use it (when you want to control the thumbnail instead of YouTube auto-picking a frame). It also gives prerequisites (verified channel) and a usage caveat (obtain user consent). However, it doesn't explicitly name sibling alternatives or state when NOT to use this tool, so it falls short of a perfect 5.

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