Use a Mermaid-aware linter in your local workflow and CI to catch malformed source—including diagrams embedded in Markdown—before deployment. For diagrams where appearance matters, render them with Mermaid CLI and inspect the output too: valid syntax does not guarantee a readable diagram.
What should you check before deploying Mermaid diagrams?
Mermaid turns text definitions into diagrams and charts. A dependable check has two parts: validate the source files your project actually uses, then render important diagrams when the generated appearance is part of what you ship. These checks catch different problems, so rendering should complement—not replace—linting.
- Source validation: Find syntax problems and, depending on the linter, structural or semantic issues.
- Rendered output: Check whether labels, layout, clipping, and legibility work in the generated artifact.
Decide first where diagrams live. They may be standalone .mmd or .mermaid files, Mermaid code fences inside Markdown, or both. A check that only scans standalone files can miss diagrams in README files and documentation pages.
How do I check Mermaid syntax in Markdown?
Choose a checker that scans the formats and directories in your repository. Both Maid and @mermaid-lint/core document support for extracting Mermaid from Markdown as well as checking standalone Mermaid files. Maid also documents recursive directory scans.
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The tools offer different feedback. @mermaid-lint/core describes structural checks, configurable semantic warnings, and validation with a fast parser plus a Mermaid.js parser fallback described in its documentation as authoritative. It documents a Node.js requirement of version 22 or higher. Maid documents text and JSON reports, optional strict-label checking, and safe or heuristic autofix modes. Review proposed changes before accepting them, especially heuristic fixes.
Before making a checker part of your release path, confirm that its input paths cover the Markdown and standalone files in your project, and decide how your team wants warnings handled. Diagnostics and exit behavior determine whether a result can reliably affect CI.
How can I validate Mermaid diagrams in GitHub Actions?
Run the same source check locally and in CI. Local feedback lets authors fix problems while editing; a required pull-request check and a workflow on the deployment branch make the check repeatable. Configure the workflow to scan the repository’s actual diagram locations rather than relying on a sample path.
Maid’s README includes a GitHub Actions example using Node 22.x, checkout, and setup-node. It documents exit code 1 when errors are present, while warnings do not fail the check. Adapt the example’s paths, triggers, action versions, and runtime to your repository and its chosen tool. Treat the workflow as a starting point, not a universal configuration.
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- Run the linter locally. Confirm its report and warning/error behavior suit your team’s review process.
- Add the check to CI. Make it a required pull-request status and run it on pushes to the deployment branch if that matches your release workflow.
- Verify failure behavior. Confirm that an error makes the job fail, and that warnings are handled as intended.
How do I render Mermaid diagrams in CI?
Use Mermaid CLI when you need generated diagrams as part of your check or deliverable. It converts Mermaid definitions into SVG, PNG, or PDF, offers basic support for code blocks in Markdown, and can be installed through npm or run in Docker or Podman.
Rendering complements a linter: a successful parse does not establish that a diagram is easy to read. Inspect the generated artifact for cramped or clipped content, label placement, layout, and legibility. Use this check for diagrams whose visual result matters, rather than treating a successful render as proof that the diagram communicates well.
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When should I use a browser preview?
The Mermaid Live Editor provides an editing-and-preview loop and can export PNG, SVG, or Markdown. It is useful for trying changes and getting quick visual feedback. A manual preview, however, does not show that every committed diagram in a repository has passed validation.
Mermaid Chart is a separate browser-based editor whose documentation describes collaboration and storage features. It may support diagram authoring, but it does not replace a repeatable check of repository content in CI.
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Which check belongs at each stage?
| Check | Best use | What it covers |
|---|---|---|
| Mermaid-aware linter | Local editing and CI | Source validation; coverage depends on supported inputs and configuration. Maid and @mermaid-lint/core document Markdown extraction as well as standalone-file support. |
| Mermaid CLI | Generating and checking rendered artifacts | Converts definitions to SVG, PNG, or PDF, with basic Markdown code-block support. |
| Mermaid Live Editor | Manual authoring and preview | Code-and-preview workflow and export; not a repository-wide deploy gate. |
For deployment confidence, make source validation repeatable in CI and add rendering where visual output is important. Keep manual preview as an authoring aid, not the only check.
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