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

Beyond annotations (which only say non-readOnly, non-destructive, non-idempotent, closed-world), the description discloses the side effect of the initial track structure and the auto-expanding spare tracks, which an agent would otherwise not anticipate before adding clips.

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 short sentences, front-loaded with the action and return value, then the format guidance, then the post-creation state. No filler and nothing repeated from structured fields.

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 no-output-schema mutation tool, it covers the return value (id), the initial state, and the parameters. The one gap is whether the newly created timeline becomes the active/open one — relevant given the set_active_timeline and open_timeline siblings.

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 baseline applies; every parameter including token and projectId is already documented in the schema. The description's '1080×1920 @30' example adds a little intuition about how width/height/fps combine, but no constraints or defaults beyond what the schema already states.

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 ('Creates a new timeline in the project') plus the return value (its id), which is more than the title alone. It does not distinguish itself from sibling creation tools such as duplicate_timeline or create_motion_composition, so an agent must infer the boundary.

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 format hint ('Set fps, width and height for the target format... vertical short') implies the creation scenario, but there is no explicit when-to-use/when-not guidance and no named alternative such as duplicate_timeline for copying an existing timeline.

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