Mermaid: The Diagramming Tool That Thinks Like a Developer

What it is and who it is for

Mermaid is an open-source JavaScript library that renders diagrams from plain text definitions written in a Markdown-inspired syntax. You describe a flowchart, sequence diagram, or class diagram using simple text, and Mermaid turns it into a rendered SVG. No dragging boxes around a canvas. No exporting PNGs from Visio. Just text that becomes a picture.

It is natively supported in GitHub Markdown, GitLab, Obsidian, Notion, VS Code, and a growing list of documentation platforms. If you write Markdown anywhere, there is a good chance Mermaid already works there.

This review focuses on the open-source library and its Markdown integration rather than the commercial Mermaid Chart platform, though I will touch on that briefly.

What is good

The core strength of Mermaid is that diagrams become part of your documentation rather than something bolted on beside it. A flowchart in a GitHub README updates when you update the text. A sequence diagram in an Obsidian note versions with the note. There is no separate file to maintain, no binary image to regenerate when the process changes. The diagram is the text and the text is the diagram.

The syntax is approachable. A basic flowchart looks like this:

graph TD;
    A[Start] --> B{Decision};
    B -->|Yes| C[Do the thing];
    B -->|No| D[Do something else];

Anyone who has written Markdown can pick this up in an afternoon. The learning curve is real but shallow, and the payoff is immediate.

The range of supported diagram types is impressive. Flowcharts, sequence diagrams, class diagrams, state diagrams, entity relationship diagrams, Gantt charts, user journey maps, Git graphs, and mind maps are all available. For most software documentation needs, Mermaid has you covered without reaching for a separate tool.

Then there is the version control angle. Because Mermaid diagrams are plain text, they diff properly in Git. You can see exactly what changed in a pull request. You can review a diagram update the same way you review a code change. For teams that care about traceability, and if you are working under any kind of quality standard you should, this is significant.

Why it matters for AI workflows

Here is where Mermaid becomes genuinely important for the way many of us work now.

If you are using AI tools to design software, plan systems, or build agentic workflows, you are constantly working with structures that benefit from visual representation. Data flows, decision trees, state machines, API call sequences, agent interaction patterns. These things are hard to hold in your head and harder still to communicate in prose.

Mermaid fits into AI-assisted workflows in two ways that matter.

First, AI models are very good at generating Mermaid syntax. Ask Claude or any capable model to describe a system architecture and request the output as a Mermaid diagram, and you will get something usable. The syntax is simple enough that models produce it reliably, and because it is text, you can iterate on it conversationally. "Add an error handling path from step three" is a meaningful instruction when the diagram is text you can both read.

Second, Mermaid diagrams serve as a shared language between you and the AI. When I am designing a new system, I often start by sketching the flow in Mermaid, pasting it into a conversation, and asking the model to critique the design or suggest improvements. The diagram becomes a reference point that both parties can reason about precisely. Try doing that with a screenshot of a Lucidchart canvas.

For teams building agentic AI systems specifically, where you need to map out which agent does what, what tools they call, and how information flows between them, Mermaid sequence diagrams and flowcharts are close to essential. The alternative is paragraphs of description that nobody reads twice.

What is not good

The styling options are limited compared to dedicated diagramming tools. You can apply themes and basic CSS-like styling, but if you need pixel-perfect control over how a diagram looks, Mermaid will frustrate you. This is a documentation tool, not a design tool. For most purposes that is fine, but there are occasions when you want something more polished for a client presentation or a board pack.

Complex diagrams can become unwieldy in text form. A flowchart with thirty nodes and multiple crossing paths is harder to read as source text than it is as a rendered image. The syntax scales well up to medium complexity, but beyond that point you start wrestling with layout rather than focusing on content. The automatic layout engine does its best, but "its best" is sometimes not where you would put things yourself.

Error messages when you get the syntax wrong are not always helpful. A misplaced semicolon or a bracket in the wrong place can produce a cryptic failure rather than a clear pointer to the problem. The Mermaid Live Editor at mermaid.live helps here, as you get immediate visual feedback, but it is an extra step in the workflow.

Documentation, while improving, can be patchy in places. Some diagram types are thoroughly documented with clear examples. Others feel like they were written for someone who already knows how the feature works. The community is active and Stack Overflow has good coverage, but the official docs could be more consistent.

Who is it for

Developers who write documentation in Markdown. That is the primary audience, and for them it is close to a no-brainer.

Beyond that, anyone who designs systems collaboratively will benefit. Product managers sketching user journeys, architects mapping service interactions, team leads documenting processes. If you work in a text-first environment like GitHub or Obsidian, Mermaid meets you where you already are.

For people using AI tools to build software or design workflows, Mermaid should be in your toolkit. The ability to generate, share, and iterate on diagrams as plain text makes it a natural fit for conversational AI development. It removes the friction between "I described this system to Claude" and "I have a diagram I can put in the documentation."

It is less suited to non-technical teams who want a visual canvas they can drag things around on. For that, tools like Miro or Lucidchart remain more appropriate. Mermaid assumes you are comfortable thinking in text, and if that is not your team's mode of working, it will feel like an obstacle rather than an aid.

Ethics and accessibility

Mermaid is open source under the MIT licence. The core library is free, the syntax is an open standard, and your diagrams are plain text files that you own and control. There is no vendor lock-in. If Mermaid disappeared tomorrow, your diagrams would still be readable text.

The commercial Mermaid Chart platform offers collaboration features, AI-assisted diagramming, and integrations with tools like Jira and Confluence. The free tier is generous enough for individual use, and the Pro tier at roughly $6.67 per user per month is reasonable for teams. But the important thing is that the commercial platform is built on top of the open-source library, not instead of it. You never need the paid product to use Mermaid.

On accessibility, rendered Mermaid diagrams are SVGs, which is better than raster images for screen readers and zoom. The underlying text is always available as an alternative representation. It is not perfect, complex diagrams still present challenges for assistive technology, but the text-first approach is inherently more accessible than image-only diagramming tools.

Verdict

Mermaid is one of those tools that, once you start using it, you wonder how you managed without it. It is not the prettiest diagramming tool. It will not replace a dedicated design application for polished deliverables. But for the daily work of documenting systems, communicating designs, and thinking visually while staying in a text-based workflow, it is exceptional.

For anyone working with AI tools to design and build software, Mermaid is particularly valuable. It bridges the gap between the conversational, text-based world of AI interaction and the visual world of system design. Your AI can read it, write it, and reason about it. Your documentation platform can render it. Your version control system can track it. That combination is hard to beat.

Open source, well supported, and quietly getting better with every release. Mermaid earns its place in the toolbox.