video_thumbnail

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

With no annotations, the description must carry the full behavioral burden. It clearly indicates a non-destructive extraction ('Grab a single frame') and specifies timestamp format, but it does not mention failure modes, path requirements, or output handling. The read-only nature is implied rather than explicit.

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?

Two short clauses with no filler; the main action is front-loaded and the timestamp format is a necessary detail. It could be slightly expanded with output or destination information but remains efficient.

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 two-parameter tool with an output schema, the description is nearly adequate, but it lacks usage context versus sibling video tools and doesn't state behavior on invalid timestamps or paths. Output result is presumably covered by the schema, so that is not penalized.

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 schema has no property descriptions (0% coverage), so the description is the only explanation. It adds that timestamp uses HH:MM:SS and that the frame comes from 'a video', implying video_path is the source. This partially compensates for the missing schema descriptions but doesn't explain timestamp defaults or path format expectations.

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 the verb 'Grab', the resource 'a video', and the output 'PNG thumbnail', plus the timestamp format. This clearly distinguishes it from siblings like video_to_gif and video_trim, which produce animated or trimmed video output rather than a still frame.

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 no guidance on when to use this tool over alternatives such as video_to_gif or video_trim. An agent must infer from the name alone that this is for single-frame extraction, which is risky when multiple video-related tools are present.

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