set_thumbnail

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

With no annotations, the description carries the behavioral burden and does meaningful work: it discloses a quota cost of ~50 units, an authorization requirement, and file constraints. It could further disclose whether the thumbnail replaces an existing one and what the response or failure modes are, but the key operational behavior is present.

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 three short, purposeful sentences with the action first. The quota cost, prerequisite, and file constraints each earn their place with no fluff or repetition.

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 no annotations and no output schema, the description covers prerequisites, quota, and accepted image inputs. It is slightly incomplete around whether image_path is local and what the effect on an existing thumbnail is, but not enough to hinder correct invocation.

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 0%, so the description must compensate; it adds concrete semantics for the image value (format, size, recommended dimensions) but does not explain video_id provenance or whether image_path is a local file path versus URL. The provided constraints clearly attach to image_path by context, making this partial rather than absent.

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 uses a clear verb and resource ('Set a custom thumbnail') and is supported by the video_id/image_path arguments, so an agent can infer the target is a video thumbnail. It does not explicitly contrast itself with siblings like update_video, so it misses full differentiation.

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 states a concrete precondition ('verified channel with custom thumbnails enabled') and technical constraints (JPEG/PNG, under 2MB, 1280x720 recommended), giving clear context for when this tool is applicable. It does not name alternative tools or when-not-to-use conditions.

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