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

Beyond readOnlyHint=true, the description discloses non-obvious behavior: AI-analyzed posts have a permanent copy while others return expiring CDN links, and TikTok links often block non-browser downloads. It also states the no-residue policy for local downloads, which an agent could not infer from annotations.

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 purpose is front-loaded in the first sentence, with the expiry/CDN caveats and cleanup policy following in a logical order. It is on the longer side and the temp-folder directive is slightly verbose, but each sentence carries actionable information.

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?

With no output schema, the description carries the return-value burden and does so by explaining the URL, thumbnail, and the 'permanent_copy' source distinction. It could be clearer about the exact response shape, but the critical variability an agent needs to handle is covered.

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 coverage is 100% and the single post_id parameter is already documented with its source, so the description's contribution is limited to implying that the ID refers to a library post. That matches the baseline 3 for a well-covered schema.

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 first clause names a specific verb and resource ('Get a downloadable URL for a library post's video file') and adds thumbnail coverage. The 'library post' scope plus the note that post_id comes from list_viral_posts/list_analyzed_posts effectively separates it from siblings like get_asset_download_url and get_my_account_post_download_url without opening schemas.

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?

It gives explicit use cases ('to save it or send it to another service like a Drive') and scopes the tool to library posts, which implicitly excludes account/asset variants. It does not explicitly say when not to use it or name the alternative sibling tools, so it stops short of a 5.

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