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

With no annotations supplied, the description carries the full burden and does so: it discloses that the new picture is not live until update_video re-renders it, that regeneration is quota-metered (10 free, then image allowance/Image Pack), and that Free plan users are blocked. These are exactly the behavioral facts an agent cannot infer from the schema.

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?

Purpose, sequencing rule and a clearly labelled COST block, all front-loaded with no filler. The cost paragraph is dense (three plan tiers in one sentence) and could be tightened, but every clause carries decision-relevant information.

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 mutation tool with no output schema, the description covers what changes (scene's picture is replaced, saved immediately), what does not (the rendered video), the cost gate, and the excluded video types. Nothing material is missing for correct invocation.

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; however the description adds real meaning by framing 'prompt' as optional and user-worded and by noting that omitting it regenerates the scene's own picture fresh, which clarifies the default behavior beyond the property description. The scene/video_id pairing is also contextualized within the multi-scene workflow.

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 and resource (generate a new picture for one scene) plus the scope constraint ('finished multi scene or whiteboard video'). Explicitly rules out Single Scene videos, podcasts and 3D films, and implicitly distinguishes itself from set_scene_image and update_video, both of which are named.

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?

Gives an explicit ordering rule ('make every picture change first and then call update_video once'), names the free alternative (set_scene_image with the user's own picture), and states the plan-based eligibility condition. The agent knows both when to call it and when to route elsewhere.

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