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

Goes well beyond the annotations: discloses that the call is free and instant, that 'values' are merged rather than replaced, that 'narration: null' removes the field, that HTML edits cause variables to be re-read from the markup, and the required scopes demos:read/demos:write. These are exactly the mutation side effects an agent needs and that readOnly/idempotent/destructive hints cannot convey.

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?

A single front-loaded sentence opens with the core action and the 'free, instant' property, then enumerates fields in a scannable list, followed by a short action-routing line and a one-line requirements footer. Heavy parenthetical nesting, 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 9-parameter mutation tool with no output schema, the definition covers constraints, merge behavior, downstream staleness, prerequisites, and tool routing. An agent has everything needed to invoke it without further discovery calls.

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 only 22% with 9 parameters including a nested transition object, so the description must compensate — and it does, explaining semantics for every meaningful field: value merging and 'asset:<key>' image encoding, duration bounds tied to narration length, null narration removal, HTML size cap and authoring rules from list_video_blocks. This is meaning the schema does not carry.

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 concrete verb and resource ('Changes one scene directly') and enumerates exactly which fields are mutable: values, label, intent, duration, transition, narration, HTML. It also disambiguates from the closest sibling by saying to use write_scene for look/motion changes, so an agent can pick correctly 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?

Names the alternative tool and the exact condition that selects it ('To change how a scene looks or moves, ask write_scene instead of writing HTML'), plus a follow-up requirement ('Changing the script makes its narration stale: then call generate_video_voiceovers'). It also tells the agent where to source ids and current values (get_video).

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