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

The description discloses key behavioral traits beyond annotations: the masked area is regenerated across the whole clip while the rest is preserved, the operation takes a few minutes, and the result is either an mp4 link or a generation id to poll with fattly_video_status. This adds meaningful async and output behavior context that annotations do not provide.

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 a single dense paragraph with no wasted words. It front-loads the purpose, then gives mask semantics, prerequisites, and output behavior in order, so an agent can extract key information quickly.

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?

Given there is no output schema, the description compensates by explaining what is returned (mp4 link or generation id) and how to check status. It also covers the required input format, mask polarity, latency, and what happens to the video. This is sufficient for an agent to select and invoke the tool correctly.

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, but the description adds useful extra meaning: prompt examples clarify what instructions should look like, and 'Source video AND mask image must both be fal URLs (get them with fattly_upload_image)' reinforces how parameters must be supplied. It does not deeply describe resolution or duration semantics, but those are already covered by the schema.

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 opens with a specific operation: 'edits a REGION of a video using a mask + a text instruction.' It includes concrete examples ('make the car burn', 'remove the person'), making the tool's purpose immediately clear and distinct from whole-video generation and image-only tools among the siblings.

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?

It gives clear context: use it when you have a source video and a matching mask and want a region changed by a prompt. It also points to fattly_upload_image for obtaining fal URLs and fattly_video_status for checking a generation id, though it does not explicitly name when-not-to-use alternatives such as fattly_generate_video or fattly_inpaint.

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