Side by side

Lucid Train vs OnUML

Both stand on ai diagram generators. Which one you can start free with, which one lets you read the code, and what each costs once you pay.

Lucid Train

Free plan LinuxMacWindows

Lucid Train connects repository mapping with coding-agent workflows and offers both a free CLI and a native app. Its stdio-only MCP support and three-machine desktop license limit may matter to some teams.

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OnUML

Free plan APIWeb

OnUML offers diagram generation, editing, version recovery, and sharing, with MCP access for several coding agents. Review the free-plan capacity and credit expiry, and note that the Pro billing description is inconsistent about whether payment is monthly or one-time.

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The facts

	Lucid Train	OnUML
Free plan	Yes	Yes
Open source	Not published	Not published
Runs on	Linux, Mac, Windows	API, Web
Lowest paid plan	Lucid Train: $2.50 a month	See the plans
Plans published	2	5
Maker	Not published	Not published
Website	lucidtrain.com	onuml.com
On ai diagram generators	#20 of 32	#24 of 32

Lucid Train: what it does well

Diagrams export to SVG, PNG, and Markdown.
Edits are presented as diffs.
Supports local models and hosted model endpoints.
CLI is free, forever, and open source.

What to know first

MCP connections use stdio only.
Remote HTTP and SSE servers cannot connect directly.
Desktop license covers up to three machines.
Desktop sign-in is required for licensing.

OnUML: what it does well

Generates diagrams from prompts and supported image inputs.
Supports visual and code editing with live preview.
Can repair Mermaid and PlantUML syntax errors.
Exports PNG and SVG and offers shareable links.
MCP checks for newer edits before updating diagrams.

What to know first

Free plan allows only two diagrams per project.
Free plan excludes image uploads.
Draw.io is not supported through MCP.
Purchased AI credits expire six months after issuance.

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