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

Annotations already declare readOnlyHint/idempotentHint true, so the safety profile is covered; the description still adds real behavior: frames are captured only "once its elements have arrived", and the plan's end mark is drawn into the image, which changes what the agent will see. It does not discuss render latency, cost, or whether a stale render can be returned, which keeps it off a 5.

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 default case and the alternatives are front-loaded, which is right, but the middle sentence is a compressed run-on ("...a transition from its last frame before to its first frame after (join; eight frames, frames gives 4 to 12)") that needs re-reading. Dense without being well-structured at the clause level.

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?

With no output schema, the description must convey what comes back, and calling it a contact sheet that is "the only way to see the result" does that adequately for a 6-parameter read tool. The watermark disclosure is a genuinely useful completion detail; only render-freshness/pagination behavior is left unstated.

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 a 3 is the floor, but the description earns more by explaining the mode-dependent meaning of `frames` (2–8 instants for one shot, 4–12 for a join) and the default one-frame-per-shot behavior when `shots` is omitted. The `moments` and `format` parameters, however, are only echoed from the schema text.

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?

Opens with a concrete image of the output ("a contact sheet of the film as a picture: one frame per shot by default") and enumerates the distinct modes (single shot, motion sampling, chosen moments, transition join). It also names the sibling it is not ("For one frame at full size, use still_film"), so an agent can separate it from still_film and show_film without opening a 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?

Explicitly states when to call it ("Look at it after each round of changes: it is the only way to see the result") and where to go instead for the full-size single-frame case ("use still_film"). Both the trigger and the alternative are named, leaving nothing to inference.

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