The Tool Desk
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What to check before a deployment
Mermaid turns text definitions into diagrams and charts. Those definitions may live in standalone .mmd or .mermaid files, or inside fenced code blocks in Markdown. A check that only looks for standalone files can therefore miss diagrams in documentation. Both @mermaid-lint/core and Maid document Markdown extraction or scanning as well as standalone-file support; Maid also documents recursive directory scans.
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- Source validation: Find malformed definitions and, depending on the tool and configuration, structural or semantic issues.
- Rendered-output review: Check whether the generated diagram is legible and communicates as intended.
Choose a linter for your repository
Match the checker to where diagrams are stored and how CI should report failures. The documented capabilities differ, so confirm the current package documentation before adopting a tool or relying on a particular option.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware match| Tool | Inputs and checks | CI and output details |
|---|---|---|
| @mermaid-lint/core | Documents extraction and validation for Markdown and .mmd files. Describes structural checks, configurable semantic warnings, a fast parser, and a Mermaid.js parser fallback it identifies as authoritative. |
Documents Node.js 22 or later. Check its documentation for the current command and reporting behavior. |
| Maid | Documents standalone files, Markdown fences, and recursive scans. Offers text or JSON reports, optional strict-label checking, and safe or heuristic autofix modes. | Documents exit code 1 when errors are present; warnings do not fail. Its README includes a GitHub Actions example using Node 22.x. |
| Mermaid CLI | Renders Mermaid definitions to SVG, PNG, or PDF, with basic support for code blocks in Markdown. | Can be installed from npm or run with Docker or Podman. It is a rendering tool, not a replacement for repository-wide source linting. |
Use the linter’s documented discovery behavior to make sure it reaches every relevant directory and Markdown document. If you enable autofix, inspect the changes before accepting them, especially when using a heuristic mode.
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How do I check Mermaid syntax in Markdown?
- Inventory the source locations. Include standalone Mermaid files and Markdown documents containing Mermaid fences, such as README files and documentation pages.
- Choose a checker that extracts or scans Markdown. Both @mermaid-lint/core and Maid document Markdown support; configure paths so the check covers the repository’s actual docs and source directories.
- Run it locally and read the diagnostics. Resolve errors before committing. Treat warnings according to your project’s policy; Maid documents that warnings alone do not produce its error exit code.
- Render diagrams whose appearance matters. A parser can accept source that still produces a confusing or hard-to-read diagram.
How can I validate Mermaid diagrams in GitHub Actions?
Run the same source check in CI that authors use locally, and make it a required pull-request check. Also run it on pushes to the branch used for deployment so invalid diagrams cannot pass unnoticed on the release path. Maid’s README provides a GitHub Actions example with checkout, setup-node, and Node 22.x; adapt its paths, triggers, action versions, and runtime to your repository rather than copying the example unchanged.
Ensure the job fails when the checker reports errors. For Maid, the documented behavior is exit code 1 when errors are present, while warnings do not fail. If warnings should block deployment in your project, configure and verify that policy using the tool’s current documentation rather than assuming its default does so.
How do I render Mermaid diagrams in CI?
Use Mermaid CLI when a generated SVG, PNG, or PDF is itself part of the build, or when you want to inspect rendered output as a separate quality check. The CLI documents those output formats and basic Markdown code-block support. Add rendering after source validation, then inspect the generated artifacts for layout, clipped content, label collisions, and legibility. Rendering complements linting; it does not prove that a diagram is clear to readers.
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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 quick authoring feedback, but previewing one diagram there does not validate all committed diagrams. Keep a repository-wide automated check in CI for deployment assurance. Mermaid Chart is a separate browser editor; it is not a substitute for that validation workflow.
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