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

Annotations already mark the tool as read-only and non-destructive. The description adds useful behavioral context beyond annotations: it notes that stills are free (no credit cost), that the output is a viewable PNG image, and that the time parameter defaults to 0. This provides cost and output-format transparency without contradicting the annotations.

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 description is front-loaded with the core purpose and then elaborates on usage and cost. It consists of five sentences, all of which contribute value (purpose, why, when, cost, default). It is slightly longer than the minimal effective description but remains focused and free of fluff.

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?

Given the annotations, full schema descriptions, and the presence of an output schema, the description is complete. It clearly communicates the tool's purpose, when to use it, cost implications, and default behavior. The `project_id` parameter is adequately covered by the schema, and return value details are implied by the output schema, so no additional explanatory burden falls on the description.

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 is 3. The description only redundantly mentions the `time` parameter ('Pass `time` (seconds) to choose the frame; defaults to 0') which is already fully described in the schema. No additional meaning is added for `project_id` or `time` beyond what the schema provides.

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?

The description clearly states the tool's function: 'Render a single frame of the current project to a PNG and return it as an image you can look at.' This uses a specific verb ('render') and resource ('single frame of the current project'), and distinguishes it from sibling tools by emphasizing that it produces a visual image for inspection.

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 description provides explicit guidance: 'This is how you check your work — in chat there is no other way to see what a composition actually looks like. Use it liberally: after composing, after edits, and at different times to inspect motion.' It also contrasts with render_video by noting 'Stills are FREE (credits are only spent by render_video),' clearly indicating when to use this tool versus the alternative.

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