视频关键帧截图

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

Annotations only declare readOnlyHint and openWorldHint. The description adds real behavioral detail beyond that: the default sampling positions, the DASH-extraction fallback to Bilibili's thumbnail sprite, and the no-ffmpeg behavior of returning the entire sprite. These degradation paths are exactly the failure-mode information an agent cannot get from 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?

Three tight sentences: purpose first, then defaults/input formats, then fallback behavior. Each sentence carries distinct information and nothing is padding.

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 an 8-parameter extraction tool with no output schema, the description covers the return medium, default sampling, accepted inputs, and fallback modes well. It does not detail issues like rate limits or oversized multi-image returns, leaving a small gap, but it is largely self-sufficient.

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%, so the baseline is 3. The description still adds value by clarifying the accepted `times` syntax (seconds, time like 1:23, or percentage) and pinning the default frame positions, which reinforces and clarifies the schema entries rather than merely repeating them.

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 ('截取视频关键帧并作为图片返回') and the return medium (images) that distinguishes it from text-returning siblings like video_transcript. An agent can tell what it produces without opening the schema.

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 explains how to use the tool (default 15%/50%/85%, accepted time formats, fallback behavior) but never says when to prefer it over an alternative such as video_transcript. Usage is implied rather than contrasted, so it is only minimum-viable on this dimension.

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