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

Annotations already cover the safety profile (readOnlyHint=true, openWorldHint=true), so the lower bar applies. The description adds useful non-obvious context beyond those annotations: the video must be public, subtitles covered include both manual and auto-generated, and a per-call cost of 20 points. It does not describe return format, but that is acceptable for a simple read.

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?

Three short sentences, front-loaded with the purpose, then scope, then cost. The Korean sentence repeats the English content, which is redundant for a bilingual audience but not wasteful enough to harm usability. Nothing else is superfluous.

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 read tool with full annotation coverage and no output schema, the description supplies the essentials: scope, subtitle types, and cost. It could say more about how to use the result versus youtube_subtitle, but nothing critical to issuing a correct call is missing.

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?

Only one parameter exists and schema description coverage is 100% ('유튜브 영상 URL 또는 11자리 영상 ID'), so the schema already documents the accepted input forms. The description adds no syntax or format detail beyond what the schema provides, making the baseline 3 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 states a specific verb and resource: 'List the manual and auto-generated subtitle languages available for a public YouTube video.' This clearly distinguishes the tool's output (a list of available subtitle languages) from the content-fetching intent implied by the sibling youtube_subtitle. It stops short of explicitly naming that sibling, so it is clear but not perfectly differentiated.

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: it applies to 'a public YouTube video,' which is a real constraint, but there is no explicit when-to-use/when-not statement or reference to alternatives such as youtube_subtitle for retrieving subtitle content. An agent can infer the context but must open the sibling set to know which tool to pick.

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