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

Annotations already declare readOnlyHint and openWorldHint. The description adds value beyond them by disclosing the cost (30 points per call) and the constraint to public videos, plus the dependency on youtube_subtitle_list. It doesn't describe the returned file/payload structure, but the safety profile is already covered.

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?

Front-loaded purpose sentence, then a prerequisite and a cost note; no wasted framing. The bilingual duplication adds length but serves the Korean-facing audience and mirrors the schema language.

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 read-only download tool with no output schema, the description covers purpose, formats, prerequisite, and cost. Return-value details are not strictly required, leaving only minor gaps such as error behavior for videos lacking subtitles.

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 the schema already documents url, lang, type, and format in detail, including the translated-subtitle caveat. The description adds nothing beyond the schema, so the baseline 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?

States a specific verb (download) and resource (subtitles of a public YouTube video) and enumerates the output formats (VTT, SRT, plain text). It also routes the agent to the sibling youtube_subtitle_list for language discovery, distinguishing it from download_youtube_video and youtube_metadata.

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?

Gives a clear prerequisite workflow: check available languages via youtube_subtitle_list before calling. This is the key 'when/how' guidance for this tool, though it does not state explicit exclusions (e.g., videos without subtitles).

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