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

No annotations are provided, so the description carries the full behavioral burden. It discloses key traits: 'Returns original timed caption text,' 'URLs expire,' and 'Only approved TikTok CDN URLs are accepted.' These are important operational constraints. However, it does not describe error handling or whether the response is paginated.

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 sentences, front-loaded with the core action. The negative constraints are compact and each sentence adds distinct value without redundancy.

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?

Given a single required parameter, no output schema, and no annotations, the description is nearly complete: it explains what is returned, what is not done, and key limitations (URL expiration, CDN restriction). It could mention that the output is a WebVTT file or how the result is structured, but otherwise it is thorough.

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 the subtitle_url parameter fully. The description reinforces the source of the URL (subtitleInfos) and the expiration/acceptance constraints, which adds contextual meaning beyond the schema. Baseline 3 is appropriate when schema does the heavy lifting.

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 and resource ('Fetch an existing TikTok WebVTT subtitle file') and explicitly distinguishes what it does NOT do (does not look up a video, transcribe audio, translate, or generate missing captions), clearly separating it from video_transcript and video_detail.

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?

Provides explicit prerequisites: 'Supply subtitle_url from video.subtitleInfos[].Url in native video data or subtitleInfos from video_detail.' It names the sibling tool (video_detail) as the source and states the condition under which this tool is appropriate.

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