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

Annotations already declare readOnlyHint=true. The description adds behavioral context beyond annotations by stating that thumbnails are extracted at regular intervals, the result is a ZIP file, and each call costs 10 points. This does not contradict the annotations.

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 core behavior is front-loaded and the cost note is compact. The bilingual repetition of the same sentence is slightly redundant for an AI agent, but it remains acceptably concise for a mixed-language tool listing.

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 tool with one required parameter, fully documented optional parameters, a clearly stated ZIP output, and read-only annotations, the description is largely complete. It lacks only explicit usage-alternative guidance, but that does not prevent an agent from invoking the tool correctly.

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%, so count, width, and video_url are already fully documented with ranges and defaults. The description adds no parameter-level detail beyond what the schema provides, so the baseline of 3 is appropriate.

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 clearly states the operation (extract thumbnail images), the resource (a video), and the return type (a ZIP file). It is unambiguous, but it does not explicitly distinguish this tool from sibling media tools such as video_to_mp3 or download_youtube_video, so it stops short of 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 intended use is implied: use this tool when you need preview thumbnails extracted at regular intervals from a video. However, there are no explicit when-to-use or when-not-to-use instructions, and no alternative tools are named, leaving the agent to infer the appropriate selection context.

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