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

Adds real behavior beyond the annotations: the return shape ({ compositionId, format, etag }), that format defaults to the project's compositionFormat, that the first composition becomes the project's active default (a side effect), and that authored graphics ride as locked clips carried verbatim. It does not cover permission/auth requirements, but annotations already carry the mutation/idempotency profile.

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?

Front-loaded with purpose, then returns, then defaults, then the authoring/render workflow, then sibling routing — good ordering. It is dense with stacked parentheticals, which costs some readability, but no sentence 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?

For a two-parameter create tool with no output schema, the description supplies the return shape, the default behavior, the side effect, and the correct follow-up calls. Nothing an agent needs to invoke it correctly is missing.

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, and the description earns above it by explaining the default-fill semantics of format ('defaults to the project's compositionFormat'), which the schema itself does not state. It maps the format sub-fields (aspectRatio/fps/resolution) but adds no detail on the platform field.

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 ('Create a NEW composition') and immediately defines the domain term ('a new video ... inside a project — a project is a workspace that holds many videos'). It also names the sibling it is not (derive_composition for reframed siblings), so an agent can distinguish it without opening schemas.

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 explicit downstream routing: author with apply_composition + compose({fromComposition: true}), render via create_render with the compositionId, and use derive_composition for a different aspect ratio. Both the 'use this when' and the 'use sibling instead when' cases are stated, not inferred.

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