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

Annotations already provide readOnlyHint=true, but the description goes further by disclosing two critical runtime behaviors: AI-analyzed posts yield a permanent copy via source 'permanent_copy', while other URLs may be temporary CDN links, and TikTok CDN links often refuse non-browser downloads. This is valuable context beyond the structured annotation.

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 description is two tight sentences. The first covers what the tool does and a motivating use case; the second covers the key caveat about permanent vs expiring URLs. Every sentence earns its place, and critical technical details are front-loaded.

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-only tool with no output schema, the description is complete enough: it names the resource, the use case, what is included in the result (video plus thumbnail), and the important behavioral caveats. An agent can decide when to call it and what to expect.

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?

The single post_id parameter is fully documented in the schema with its UUID type and origin from list_viral_posts / list_analyzed_posts. The description reinforces the relevance of post_id but adds no new parameter-level semantics, so the baseline 3 for high schema coverage 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 uses a specific verb ('Get a downloadable URL') and a well-defined resource ('a library post's video file plus its thumbnail'). The phrase 'library post' clearly distinguishes this from sibling tools like get_asset_download_url and get_my_account_post_download_url, and the use case ('save it or send it to another service') reinforces the intent.

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 description clearly states why someone would use this tool: to save a video or send it to another service like Drive. It also warns about conditions under which the URL may not work well. However, it does not explicitly name alternatives or state when not to use this tool versus sibling download tools.

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