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

Beyond the annotations (readOnlyHint, openWorldHint), the description adds useful behavioral details: it returns MP4 format, fails for non-public videos, and mentions a per-call cost of 3 points. This enriches the agent's understanding of side effects and failure modes without contradicting the 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 description is concise, with the core action front-loaded in the first sentence. It includes a bilingual repetition for clarity and a cost note, but every sentence serves a purpose. No unnecessary filler.

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 simple tool with one parameter and no output schema, the description adequately informs the agent of what to expect ('return it as an MP4 file') and key constraints (public availability, failure on private/blocked). Minor gaps exist on how the file is returned (e.g., URL vs. binary), but overall it is sufficient for a low-complexity tool.

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 input schema already fully describes the single parameter 'url' with an example (100% schema coverage). The description adds no additional parameter semantics, so the baseline score of 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?

The description clearly states the tool's function: 'Download a publicly available YouTube video and return it as an MP4 file.' It identifies the specific verb (download), resource (YouTube video), and output format (MP4), distinguishing it from sibling tools like crawl_youtube or google_search.

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 implies usage context ('publicly available') and notes failure for private/blocked posts, but does not explicitly state when to prefer this tool over alternatives or provide exclusions. It lacks explicit guidance such as 'Use crawl_youtube for metadata extraction' but the name and context make the primary use clear.

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