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

Annotations already declare readOnlyHint and openWorldHint, so the bar is lower, and the description still adds genuinely useful facts beyond them: the per-call cost of 20 points, the fixed output format (JPG), the 'largest available' resolution guarantee, and the restriction to public videos. It does not explain what happens on private/deleted videos or where the image is delivered, which keeps it out of 5 territory.

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 plus a bracketed cost note, with the core action front-loaded; the Korean mirror is a reasonable cost for a bilingual service rather than filler. Slightly redundant across languages, but no wasted explanatory prose.

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 one-parameter, non-destructive download tool with annotations covering safety, the description supplies the essentials: source restriction, format, resolution, and cost. The remaining gap is the return channel (URL vs binary payload) with no output schema to cover it, but that is a minor omission.

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 the single 'url' parameter is fully documented there (YouTube URL or 11-character video ID), so the schema does the heavy lifting and the description adds nothing about the parameter. Baseline 3 for a single fully-documented parameter.

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?

Specific verb+resource with useful scoping: 'Download the largest thumbnail of a public YouTube video as a JPG image' tells the agent what is produced, at which resolution, and from what kind of source. It does not name the closely related sibling extract_video_thumbnail, so an agent still has to infer which of the two to call, which keeps this at 4 rather than 5.

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 only implied: the 'public YouTube video' qualifier signals applicability and the 11-char video ID hints at accepted input, but there is no explicit when-to-use, when-not-to-use, or alternative (e.g. extract_video_thumbnail, download_youtube_video). Nothing is misleading, but the routing decision is left to inference.

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