OnUML
OnUML is an online UML editor and AI diagram generator for Mermaid, PlantUML, Draw.io, and mind maps.
What OnUML is
OnUML is an online UML editor and AI diagram generator for Mermaid, PlantUML, Draw.io, and mind maps. It can create diagrams from natural-language prompts and supported image inputs, then lets users refine them visually or in code with a real-time preview. A one-click repair option addresses Mermaid and PlantUML syntax errors. Projects can hold organized diagrams and multiple code versions, which users can restore. Finished diagrams can be exported as PNG or SVG or shared with a link.
OnUML also provides a remote MCP server for coding agents including Codex, Claude Code, Cursor, and OpenClaw. It follows workspace permissions and can read or update Mermaid, PlantUML, and OnUML mind maps; Draw.io is excluded from MCP. The free plan is free, billed forever, and limits diagrams, projects, workspaces, and daily model requests. Paid options include a 4.90 USD one-time Starter plan, an $8 a month Pro plan, a 9.90 USD one-time Popular plan, and a 19.90 USD one-time Pro Pack.
Good
- Generates diagrams from prompts and supported images
- Visual and code editing with real-time preview
- Exports PNG and SVG
- Saves versions and organizes diagrams by project
- Free plan includes 10 daily model requests
Watch out
- Free plan limits diagrams, projects, and workspaces
- Free plan excludes image uploads
- Purchased AI credits expire after six months
- MCP does not support Draw.io
Who it suits
OnUML suits people who want to create or edit supported diagrams in a browser, including through code or AI prompts. Its MCP server may also suit users of the listed coding agents who want diagram access within workspace permissions.
Verdict
OnUML combines prompt-based diagram creation with visual and code editing, version history, and export options. Check the free-plan limits and whether its MCP support covers the diagram types you use.
Plans and prices
From OnUML's own pricing page, checked 9 Oct 2026.
| Plan | Price | What it gives you |
|---|---|---|
| Free | Free | 2 diagrams per project · 10 cloud projects per workspace · 2 workspaces · 10 free model requests per day · image upload not included · forever |
| Starter | $4.90, paid once | 240 credits · credits expire 6 months after issue · one-time |
| Pro | $8 a month | 500 monthly included credits · unlimited diagrams per project · unlimited cloud projects per workspace · unlimited workspaces · image upload included · no daily limit for free model requests · monthly; page also says “One-time payment” |
| Popular | $9.90, paid once | 500 credits · credits expire 6 months after issue · one-time |
| Pro Pack | $19.90, paid once | 1,100 credits · credits expire 6 months after issue · one-time |
The facts
Each one read from OnUML's own pages, with the date it was checked.
What it does
- AI repair
- The editor offers one-click repair for Mermaid and PlantUML syntax errors.checked 9 Oct 2026
- Credit limits
- Purchased AI credits expire 6 months after issuance and do not unlock diagram, project, or workspace capacity.checked 9 Oct 2026
- Editing
- Users can edit diagrams visually or through code and preview changes in real time.checked 9 Oct 2026
- Export and sharing
- OnUML supports PNG and SVG export and shareable links.checked 9 Oct 2026
- Generation
- It generates diagrams from natural-language prompts and supported image inputs.checked 9 Oct 2026
- History and organization
- Users can save multiple code versions, restore earlier content, and organize diagrams by project.checked 9 Oct 2026
- Integrations
- Its remote MCP server connects to coding agents including Codex, Claude Code, Cursor, and OpenClaw.checked 9 Oct 2026
- MCP access
- The MCP server follows the signed-in account’s workspace permissions and supports reading and updating Mermaid, PlantUML, and OnUML mind maps; Draw.io is not supported through MCP.checked 9 Oct 2026
- MCP safeguards
- OnUML says MCP updates check the latest diagram before changing it and stop if newer edits would be overwritten; retries are designed not to duplicate project or diagram creation.checked 9 Oct 2026
- Operator
- The site says OnUML is a product operated by myun L.L.C.checked 9 Oct 2026
- Product
- OnUML is an AI diagram generator and online UML editor for Mermaid, PlantUML, Draw.io, and mind maps.checked 9 Oct 2026
- Support
- The site lists [email protected] as its contact email.checked 9 Oct 2026
- Usage limits
- The free plan allows 2 diagrams per project, 10 cloud projects per workspace, 2 workspaces, and 10 free model requests per day.checked 9 Oct 2026
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