Set a video's thumbnail from a timecode in the video itself

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

Annotations already declare readOnlyHint=false, idempotentHint=true, destructiveHint=false and openWorldHint=true, so the mutation/safety profile is covered. The description adds genuinely new context beyond those annotations: the required 'upload' OAuth scope, which an agent must know before calling.

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, zero filler, with the core action front-loaded and the auth prerequisite second. Every clause carries 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?

For a simple two-parameter mutation with full schema coverage and annotations covering idempotency and non-destructiveness, the description is nearly sufficient. It omits only what happens to a pre-existing custom thumbnail, which the destructive/idempotent hints largely imply.

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 documented (video_id, time_seconds with a 0 minimum). The phrase 'from a timecode in the video itself' reinforces the meaning of time_seconds but adds no format, unit, or bounds detail beyond the schema, 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?

The description states a specific verb (set) and resource (video's thumbnail) and adds the key qualifier that the frame is taken from a timecode inside the video itself, distinguishing it from an image-upload approach. It does not explicitly name a sibling, but no sibling tool competes for this capability, so the intent is unambiguous.

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?

Usage is implied by the verb+resource, and the description supplies one prerequisite (the upload scope), which is more than the MID calibration case that scored 2. However, it never states when to prefer this over alternatives or any preconditions on the video's state (e.g., processing status), so guidance remains thin.

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