Freedom251 › AI Diagram Generators
OnUML
What OnUML is
OnUML is a web-based AI diagram generator and editor for Mermaid, PlantUML, Draw.io, and mind maps. It can create diagrams from natural-language prompts and supported image inputs, then lets users edit them visually or through code with a real-time preview. A one-click repair option addresses Mermaid and PlantUML syntax errors. Users can group diagrams by project, save multiple code versions, and restore earlier content. Diagrams can be exported as PNG or SVG or shared through links. OnUML’s remote MCP server connects to coding agents including Codex, Claude Code, Cursor, and OpenClaw. It follows the signed-in account’s workspace permissions and can read and update Mermaid, PlantUML, and OnUML mind maps; Draw.io is not supported through MCP. Before making an MCP change, the server checks the latest diagram and stops if newer edits could be overwritten. The free plan is billed forever at $0.00 USD and limits users to two diagrams per project, 10 cloud projects per workspace, two workspaces, and 10 free model requests per day. It does not include image uploads. Paid options include one-time credit packs and a Pro plan listed at $8 a month, though the page also labels it a one-time payment.
Who it suits
OnUML suits people creating or maintaining Mermaid, PlantUML, Draw.io, or mind-map diagrams in a browser. Its MCP connection may suit users of the listed coding agents who want workspace-permission-aware access to supported diagrams.
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.
Limits
The Free plan allows 2 diagrams per project, 10 cloud projects per workspace, 2 workspaces, and 10 free model requests per day; image upload is excluded. Purchased AI credits expire six months after issuance and do not increase diagram, project, or workspace capacity.
Plans
| Plan | Billed and limits | Price |
|---|---|---|
| Free | forever · 2 diagrams per project · 10 cloud projects per workspace · 2 workspaces · 10 free model requests per day · image upload not included | Free |
| Pro Pack | one-time · 1,100 credits · credits expire 6 months after issue | $19.90, paid once |
| Starter | one-time · 240 credits · credits expire 6 months after issue | $4.90, paid once |
| Pro | monthly; page also says “One-time payment” · 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 | $8 a month |
| Popular | one-time · 500 credits · credits expire 6 months after issue | $9.90, paid once |
As onuml.com lists them, read 9 Oct 2026. The maker's own page has the final word.
Getting started
- Open the OnUML website
- Create or use the Free plan workspace
- Generate a diagram from a natural-language prompt or supported image
- Edit visually or through code and preview changes
- Organize diagrams by project and save versions
- Export as PNG or SVG or share a link
Verdict
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.
Questions
Is OnUML free?
Yes. The Free plan costs nothing, billed forever. Paid options start with a 4.90 USD one-time Starter plan.
What platforms does OnUML support?
OnUML is available on the web and provides an API platform listing.
What are the paid plans?
Starter costs 4.90 USD once for 240 credits, Popular costs 9.90 USD once for 500 credits, Pro Pack costs 19.90 USD once for 1,100 credits, and Pro costs $8 a month with 500 monthly included credits.
What does the Free plan include?
It includes 2 diagrams per project, 10 cloud projects per workspace, 2 workspaces, and 10 free model requests per day. Image upload is not included.
Which coding agents connect through MCP?
The remote MCP server connects to Codex, Claude Code, Cursor, and OpenClaw. It supports Mermaid, PlantUML, and OnUML mind maps, but not Draw.io.
Can diagrams be exported?
Yes. OnUML supports PNG and SVG export, along with shareable links.
Facts from OnUML's own pages
- AI repair
- The editor offers one-click repair for Mermaid and PlantUML syntax errors. · onuml.com, 9 Oct 2026
- Credit limits
- Purchased AI credits expire 6 months after issuance and do not unlock diagram, project, or workspace capacity. · onuml.com, 9 Oct 2026
- Editing
- Users can edit diagrams visually or through code and preview changes in real time. · onuml.com, 9 Oct 2026
- Export and sharing
- OnUML supports PNG and SVG export and shareable links. · onuml.com, 9 Oct 2026
- Generation
- It generates diagrams from natural-language prompts and supported image inputs. · onuml.com, 9 Oct 2026
- History and organization
- Users can save multiple code versions, restore earlier content, and organize diagrams by project. · onuml.com, 9 Oct 2026
- Integrations
- Its remote MCP server connects to coding agents including Codex, Claude Code, Cursor, and OpenClaw. · onuml.com, 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. · onuml.com, 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. · onuml.com, 9 Oct 2026
- Operator
- The site says OnUML is a product operated by myun L.L.C. · onuml.com, 9 Oct 2026
- Product
- OnUML is an AI diagram generator and online UML editor for Mermaid, PlantUML, Draw.io, and mind maps. · onuml.com, 9 Oct 2026
- Support
- The site lists [email protected] as its contact email. · onuml.com, 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. · onuml.com, 9 Oct 2026
Where OnUML stands on Freedom251
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