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

With no annotations supplied, the description does real work: it discloses the output format (PNG), the return value (list of image URLs), and hard limits (up to 20 timestamps, 1-20 frames). It does not disclose what happens if both or neither selector is given, why count defaults to 0 in the schema while the text says 1-20, or any cost/processing behavior for large videos.

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?

Three short sentences, all load-bearing, with the core action front-loaded and the parameter contract second. The mid-sentence line break around the times_s example slightly hurts readability but no sentence is wasted.

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 3-parameter tool with no annotations and no output schema, the definition covers action, both optional parameters with bounds, mutual exclusivity, and the return shape. The remaining gap is edge-case behavior (default count=0 semantics, missing-selector errors), which is minor but real.

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 description coverage is 0%, so the description must carry parameter meaning and largely does: it defines times_s as a list of seconds, count as N evenly spaced frames, gives numeric bounds for each, and states they are mutually exclusive. Only url is left unexplained, and that is self-evident.

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 a specific verb+resource (grab still frames) plus output format (PNG images) and return type (list of image URLs). It is clearly distinguishable from siblings like video_trim or video_info, though it never explicitly names or contrasts a sibling, so it stops short of 5.

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?

Offers use cases ("useful for thumbnails or for looking at a video") and a parameter-selection rule ("Either times_s ... or count"), which is implied guidance on how to call it. There is no explicit when-not guidance and no routing to alternatives such as video_info for metadata, so usage remains inferential.

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