Set Thumbnail

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

Annotations already declare destructiveHint=true, idempotentHint=true and readOnlyHint=false, so the safety profile is covered structurally. The description adds two things the annotations cannot: the verification prerequisite and the 50-unit quota cost, both genuinely actionable. It doesn't say the previous thumbnail is replaced, but the destructive annotation implies that.

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?

Three short, front-loaded statements with zero filler: the action first, then the prerequisite, then the quota. Every sentence carries distinct information.

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?

An output schema exists, so return values need not be described, and the annotations cover the mutation semantics. Adding the auth prerequisite and quota cost makes it nearly complete for a destructive write; only the replacement behavior of the existing thumbnail is left to inference.

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 includes format constraints ('JPEG or PNG, at most 2 MB'), 'Video ID or URL' and the multi-channel selector semantics. With the schema already doing the heavy lifting, the description contributes nothing beyond it, so the baseline 3 applies.

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?

States a specific verb and resource ('Set a video's custom thumbnail'), which is enough to tell it apart from set_channel_banner and set_watermark. It implicitly distinguishes from the similarly-named sibling set_playlist_thumbnail by scoping to a 'video', though it never names the sibling explicitly.

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?

Gives a concrete prerequisite ('The channel must be verified'), which is useful context for when the call will succeed. It stops short of stating when to use this tool versus alternatives like set_playlist_thumbnail or update_video, so usage is only implied.

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