D2 vs Kroki vs PlantUML

D2, Kroki and PlantUML all let engineering teams keep diagrams as version-controlled plain text instead of drag-and-drop files, but they solve different parts…

Free / Open Source · From Free

Best for: Teams that want modern, git-friendly software architecture and infrastructure diagrams with automatic layout and minimal manual positioning.

Kroki

Free / Open Source · From Free

Best for: Teams that need one self-hosted API endpoint to render many different diagram syntaxes, including PlantUML and Mermaid, without installing and maintaining separate renderers.

PlantUML

Open Source · From Free

Best for: Teams standardized on classic UML documentation who want the broadest native diagram-type coverage and the largest ecosystem of editor and CI integrations.

At a Glance

	D2	Kroki	PlantUML
Primary category	Collaboration	Collaboration	Collaboration
Rating	Not documented	Not documented	Not documented
Pricing model	Free / Open Source	Free / Open Source	Open Source
Starting price	Free	Free	Free
Free plan	Yes	Yes	Yes
Free trial	Not documented	Not documented	Not documented
Platforms	Mac, Windows	Web	Not documented
Team collaboration	Not documented	Not documented	Not documented
AI features	Not documented	Not documented	Not documented
Public API	Yes	Yes	Yes

Standout Differences

D2 is purpose-built for architecture diagrams, not general UML

D2's shape library leans into cylinders, queues, clouds and infrastructure icons with selectable dagre and ELK layout engines, giving polished technical diagrams with less manual tuning than classic UML tools, though it uses its own language rather than a widely pre-integrated one like Mermaid.

Kroki trades its own syntax for universal coverage

Rather than defining a diagram language, Kroki exposes one HTTP API that renders over 20 existing syntaxes, including PlantUML, Mermaid, GraphViz, C4 and Excalidraw, which is valuable for teams that don't want to standardize on a single diagram language but do want a single rendering endpoint.

Kroki

PlantUML remains the deepest native UML tool, and a Kroki backend

Among native UML diagram types, PlantUML supports state, class, sequence, use case, activity, component and deployment diagrams out of the box, plus extensions like C4 and Gantt charts, and because it's also one of the syntaxes rendered by Kroki's unified API, it has reach both as a standalone tool and inside Kroki-based pipelines.

PlantUML, Kroki

All three are free with different open-source licenses

D2 is MPL-2.0, Kroki is MIT, and PlantUML is GPL - all are free to self-host with no paid tier documented for the core tool, though the license terms differ enough to matter for teams with strict open-source compliance policies.

D2, Kroki, PlantUML

Feature-by-Feature

Licensing and Deployment

Feature	D2	Kroki	PlantUML
Open-source, self-hostable core	Available	Available	Available
Free public hosted instance	Not documented	Available	Available

Diagram Language Coverage

Feature	D2	Kroki	PlantUML
Own dedicated diagram-description language	Available	Unavailable	Available
Single API rendering multiple diagram syntaxes	Unavailable	Available	Unavailable
Broad native UML diagram types (sequence, class, use case, state)	Limited	Available	Available

Layout, Styling and Integration

Feature	D2	Kroki	PlantUML
Selectable auto-layout engines	Available	Not documented	Limited
Built-in visual themes, including dark mode	Available	Not documented	Not documented
Editor and documentation-tool integrations	Available	Available	Available

Pricing Compared

Starting price reflects the lowest paid tier, not the full cost for every team size or usage level.

D2

D2 (Open Source) — Free N/A

Kroki

Free (Public API or Self-Hosted) — $0 N/A

PlantUML

PlantUML — Free N/A

Pros & Cons

D2

Pros

Completely free and open source (MPL-2.0)
Diagrams stored as plain text, ideal for Git version control and code review
Purpose-built shape library and layout for software architecture diagrams
Active open-source community and growing ecosystem of integrations

Cons

Newer and less pre-integrated into platforms like GitHub or Notion than Mermaid
Requires learning a diagram-specific syntax rather than pure drag-and-drop
Advanced visual and collaborative editing requires Terrastruct's separate paid app
Auto-layout engines can require manual tuning for very complex diagrams

Kroki

Pros

Free to use, both as a self-hosted service and via the public kroki.io API
One API covers dozens of diagram-as-code syntaxes instead of installing separate renderers
Docker-based self-hosting keeps proprietary diagrams off third-party servers
Wide ecosystem of plugins for documentation tools like Asciidoctor, MkDocs, and Confluence
Actively maintained open-source project with a permissive MIT license

Cons

No official SaaS support contract or SLA, since it is a community open-source project
Self-hosting the full set of diagram types requires running multiple companion containers
Complex diagrams can render slowly on the free public instance
No built-in visual, WYSIWYG diagram editor, since input is text-based markup
Documentation assumes familiarity with the underlying diagram syntaxes, such as PlantUML or Mermaid

PlantUML

Pros

Completely free and open source with no paid tiers
Diagrams-as-code approach fits naturally into version control and documentation-as-code workflows
Extremely broad diagram type support beyond just UML
Large ecosystem of editor, wiki and CI integrations
Long-running, mature project with an active community

Cons

Text-based syntax has a learning curve compared to drag-and-drop diagram tools
Fine-grained visual styling and layout control is more limited than dedicated design tools
Relies on Java and, for some diagram types, Graphviz, which adds setup steps for self-hosting
No official commercial support plans, so enterprise users rely on community help or third-party vendors

Use Cases

Choose D2: Teams that want modern, git-friendly software architecture and infrastructure diagrams with automatic layout and minimal manual positioning.

Choose Kroki: Teams that need one self-hosted API endpoint to render many different diagram syntaxes, including PlantUML and Mermaid, without installing and maintaining separate renderers.

Choose PlantUML: Teams standardized on classic UML documentation who want the broadest native diagram-type coverage and the largest ecosystem of editor and CI integrations.

D2

Software architecture documentation-as-code — Maintaining architecture diagrams as version-controlled text alongside source code.
Auto-generated database ER diagrams — Generating entity-relationship diagrams directly from a live database schema.
Technical diagrams in internal docs — Embedding rendered D2 diagrams into wikis and internal documentation sites.

Kroki

Docs-as-code diagramming — Embed version-controlled, text-based diagrams directly in Markdown or AsciiDoc documentation instead of maintaining binary image files.
Internal developer platforms — Self-host Kroki behind a firewall so architecture and sequence diagrams never leave internal infrastructure.
Automated diagram generation in CI/CD — Call the Kroki API from build pipelines to regenerate architecture or pipeline diagrams automatically whenever source text changes.

PlantUML

Software architecture documentation — Engineering teams document system architecture and sequence flows as version-controlled text alongside their code.
API and onboarding documentation — Technical writers embed generated diagrams directly into Markdown docs and wikis to explain system behavior.
Automated diagram generation in CI — Teams generate up-to-date architecture or dependency diagrams automatically as part of build pipelines.

Frequently Asked Questions

Can Kroki render diagrams written in D2 syntax?

Based on Kroki's documented list of 20-plus supported diagram types, which includes PlantUML, Mermaid, GraphViz, C4 and others, D2 is not listed among them, so teams standardized on D2 should keep using D2's own CLI or server rendering rather than expecting Kroki to render D2 files.

Is PlantUML one of the diagram types Kroki supports?

Yes, PlantUML is listed among Kroki's supported diagram languages, meaning a team can send PlantUML markup to Kroki's unified API and get a rendered diagram back without running a separate PlantUML server.

Which of these three is the simplest to self-host?

D2 ships as a single Go binary with no external runtime dependencies. Kroki self-hosts via Docker or Docker Compose, though rendering the full set of 20-plus diagram types means running additional companion service containers. PlantUML runs as a Java application or server and, for some diagram types, also depends on Graphviz being installed.

Do any of these tools have a paid tier?

Based on the provided pricing data, D2, Kroki and PlantUML are all free and open source with no paid tier for the core tool itself. Terrastruct, the company behind D2, does sell a separate commercial visual editor product, but that is distinct from the free D2 language and CLI.

Which tool requires the least setup to try immediately without installing anything?

Kroki is the most immediate option because of its free public instance at kroki.io, which can render diagrams via a simple HTTP request with no local installation. D2 and PlantUML both require downloading and running a local binary, server, or Docker image to render diagrams.

Which tool has the broadest native UML diagram support?

PlantUML goes deepest on classic UML: activity, component, state, class, sequence, use case and deployment diagrams are all natively supported, alongside extensions such as C4, Gantt charts and mind maps. D2 leans architecture-focused rather than UML-complete, and Kroki only reaches UML territory indirectly, by passing PlantUML markup through its API instead of implementing UML support of its own.

Read the full D2 review · Read the full Kroki review · Read the full PlantUML review

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