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

Annotations already establish the safety profile (readOnlyHint, idempotentHint, destructiveHint=false, openWorldHint=false), and the description adds the cost/auth fact the annotations cannot express: 'Free, no key.' It also discloses the shape of what comes back — type, allowed values or range, description, JSON examples — which is genuinely useful since there is no output schema. It stops short of saying how the response is structured or how large it is.

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 lead is front-loaded and high-value ('Free, no key'), and every clause carries information. The long field enumeration is dense but purposeful for a reference tool. The only friction is that it reads as two run-on sentences rather than cleanly separated intent.

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 parameters and no output schema, the description bears the burden of describing the payload, and it does so by listing the covered fields and their per-field metadata. What is missing is the response envelope — whether it returns one JSON object, a keyed map, or prose — which an agent preparing to parse the result would want.

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 schema has nothing to document and the baseline is 4. There is no argument semantics left for the description to carry, and it correctly spends no words on parameters.

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 states a specific resource — the caption style field reference — and enumerates exactly what it covers (font, colors, stroke, background, wordStateStyles, smartEmphasis, caption forms, faceAvoid). It also explicitly separates itself from the visual sibling by pointing to style_preview for on-video rendering.

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?

It gives an explicit prerequisite chain: call this before styleOverrides, highlightWords, save_custom_preset, or validate_preset_style. It also names an alternative (style_preview) and the condition that selects it (see styles on the actual video). Both when-to-use and which-alternative are covered.

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