get_thumbnail_url

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

With no annotations, the description carries the full burden of behavioral disclosure. It does not mention whether the URL is always valid, whether any network request occurs, or what happens for invalid video IDs. This lack of detail leaves the agent uncertain about edge cases.

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 a single declarative sentence and an Args block that adds semantic context. It is well-structured, though the Args block partially duplicates the schema.

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 utility, the description covers the basic purpose and parameter meaning. However, it omits any details about the return value's format or possible failure modes, which is significant given the absence of an output schema and annotations.

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 description explicitly labels video_id as 'The YouTube video ID', which adds semantic meaning beyond the schema's plain string type. This clarifies the expected input, though it lacks format constraints like typical ID length.

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: it returns the thumbnail URL for a YouTube video ID. This is a specific verb+resource and distinguishes it from sibling tools like get_watch_url and get_normalized_url.

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 provides no guidance on when to use this tool versus the sibling URL utilities. It simply states the operation without context or exclusions.

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