overlay

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

With no annotations, the description carries the full burden. It usefully discloses that a video overlay starts at 'start' and holds its last frame if it ends early, and that the output keeps the base video's audio and length. However, it does not mention overwrite behavior, error conditions, or file replacement semantics.

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 tightly written sentences with the core purpose front-loaded. The second sentence adds behavioral detail that earns its place, and there is no redundant or filler content.

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 10-parameter tool with 100% schema coverage and no output schema, the description covers the core operation and important playback behavior. It stops short of explaining overwrite semantics or output path handling, but the schema fills those gaps adequately.

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 100%, so the schema already documents all ten parameters clearly. The description adds minimal parameter meaning beyond the schema, only restating that the overlay is placed at a pixel position and can be scaled and time-limited, which maps to x/y, width/height, and start/end.

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 states a specific verb ('Composite') and resource ('image or video onto a base video at a pixel position'), with examples of use cases (logo, watermark, picture-in-picture). It distinguishes the operation from most siblings by focusing on compositing, though it does not explicitly name alternatives like title_card or burn_subtitles.

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 through examples (logo, watermark, picture-in-picture) and mentions scaling and time-limiting, but it does not state when to use this tool versus alternatives such as burn_subtitles or title_card, nor does it provide explicit exclusions.

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