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

Annotations already declare readOnly/idempotent/non-destructive, and the description adds genuinely new operational facts: it is free, consumes no credits, makes no provider call, and the prompt is baked in server-side. That is exactly the kind of cost/side-effect context an agent needs before invoking.

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, front-loaded with what is returned, then the no-prompt constraint, then cost behavior. Every clause carries information; nothing is filler.

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 and no parameters, the description must explain return contents, which it does (demo clip, poster image, credit price at two durations), and it covers cost and side effects. An agent has everything needed to call it 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?

The tool takes zero parameters, so the baseline is 4. The description correctly implies no input is required and explains that only a photo and duration matter downstream, adding mild clarifying value.

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 precise verb+resource (list the 20 one-photo video effects) and enumerates the returned fields (demo clip, poster image, credit price for 5s and 10s). It also distinguishes itself from the sibling render_effect by noting there is no prompt to write, only a photo and a duration.

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?

"Call this before render_effect" gives explicit sequencing guidance and the 'no prompt to write' note clarifies the intended workflow. It does not, however, address when not to use it or how it relates to the other browse/list siblings (browse_library, list_models, get_template).

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