Set Thumbnail

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

With annotations already declaring destructiveHint=true and idempotentHint=true, the bar is lower, and the description still adds two substantive facts: a verification prerequisite (an auth/setup requirement not in any structured field) and a 'Quota: 50 units' cost disclosure. It doesn't cover whether an existing thumbnail is replaced or error behavior, so not a 5.

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?

Two short sentences: the action first, then prerequisite and quota on separate lines. Every clause carries information and nothing is buried.

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?

A rich output schema exists and annotations cover the mutation/idempotency profile, so the description needn't explain return values. Prerequisite and quota are supplied; only the overwrite/error semantics are left implicit, which is a minor gap for a 3-param tool.

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% — the image format/size limit, video ID-or-URL, and channel selection are all documented in the schema. The description adds no parameter meaning beyond that, so the baseline of 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+resource: setting a video's custom thumbnail. The resource ('video's') implicitly separates it from siblings like set_playlist_thumbnail and set_channel_banner, but the description never explicitly names 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?

Provides one real prerequisite — 'The channel must be verified' — which is actionable context an agent needs before calling. However, there is no guidance on when to use this over the sibling thumbnail/banner tools, nor on what happens to an existing thumbnail.

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