justcaptions

preview_style

Read-onlyIdempotent

Generate a caption PNG preview on a neutral backdrop to check caption style, size, position, and safe area before final export, without calling a paid API.

Instructions

Return a caption PNG preview with a neutral backdrop. Does not call a paid API.

Tool call template

This tool declares no required parameters. Add optional parameters from the input schema as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {},
    "name": "preview_style"
  }
}

Input Schema

Name	Required	Default
`size`	No	large
`text`	No	Make your next video stand out
`width`	No
`height`	No
`position`	No	bottom
`style_id`	No	word-highlight
`overrides`	No
`safe_area`	No	tiktok

Schema Changelog

Changes observed during successful MCP inspections.

First observedv1.4.0Oct 7, 2026

TDQS

B3.1/5.0

Behavior3/5

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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds the cost-side-effect fact ('does not call a paid API') and the output nature (caption PNG on a neutral backdrop), which is useful context, but omits output format details or any constraints.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Two tight sentences, front-loaded with the core action and followed by the key distinguishing trait. No filler; every clause carries information.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness2/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a tool with 8 undocumented parameters, no output schema, and zero schema description coverage, the description is too thin to call it correctly. It doesn't explain what the parameters control or what the returned PNG will depict beyond 'caption on neutral backdrop'.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters1/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 0% across 8 parameters, and the description mentions none of them. Opaque parameters like 'overrides', 'style_id', 'safe_area', and 'position' are left completely undefined, so the description does not compensate for the coverage gap at all.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a specific verb and resource: 'Return a caption PNG preview with a neutral backdrop.' An agent can tell it produces an image preview rather than a video render (caption_video) or a style listing (list_styles). It does not, however, explicitly name or differentiate itself from siblings like estimate_video or run_demo.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines3/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

'Does not call a paid API' implies this is a cheap, safe way to preview before invoking a paid render, which is a usable when-to-use signal. But it never states explicit conditions or names an alternative tool, leaving the routing inference to the agent.

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

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