mermaid-render

Render Mermaid → SVG, PNG and PDF, and Mermaid flowcharts → connected Visio VSDX on Linux, Windows, and macOS. Python uses the official Mermaid.js renderer inside Chromium Headless Shell, controlled through Playwright. Node.js, a browser installation, and Microsoft Visio are not required on the target machine when installing a platform-specific bundled wheel.

The converter is adapted from the MIT-licensed FBklyra/mermaid-to-visio. Mermaid itself is MIT licensed; Chromium and Playwright have their own third-party notices and licenses. Keep those notices with redistributed binaries.

Prebuilt offline wheels via GitHub Actions

The workflow .github/workflows/bundled-wheels.yml builds two distributions using uv:

• PyPI: a small universal Python wheel and source archive, without Mermaid.js or browser binaries. The first render automatically installs the pinned runtime support files.
• GitHub Releases: the exact PyPI artifacts plus four bundled wheels for Linux x86-64, Windows x86-64, macOS Apple Silicon, and macOS Intel. Bundled wheels contain Mermaid 12.1.0 and the Chromium Headless Shell matching Playwright 1.63.0, so rendering needs no runtime downloads.

A matching version tag (for example v0.9.0) attaches all six artifacts to a GitHub Release and publishes only the lightweight artifacts to PyPI through Trusted Publishing. Pull requests, pushes to main, and manual runs build and test without publishing.

Runtime support files

Each asset is resolved independently: first from valid, version-matched persistent per-user application data, then from its bundled location inside the installed package (mermaid_render/runtime and mermaid_render/browsers). Support directories are:

OS	Support directory
macOS	~/Library/Application Support/mermaid-render
Linux	$XDG_DATA_HOME/mermaid-render, default ~/.local/share/mermaid-render
Windows	%LOCALAPPDATA%\mermaid-render

These are support files, not temporary or cache files. Mermaid uses a versioned mermaid/<version> subdirectory; Chromium uses browsers/playwright-<version>-<os>-<architecture>. Upgrades keep versions separate. Downloads are staged and validated before installation, and concurrent processes share an installation lock. Rendering reuses existing files without downloading them again.

Missing assets install automatically on render. Optionally run mermaid-render setup beforehand. Setup always populates the user support directory: it copies valid bundled assets if available, otherwise downloads them. Existing valid support files are reused without being overwritten. Runtime paths are managed internally; the CLI and rendering API have no Mermaid or browser path overrides. If automatic setup fails, the error links directly to the GitHub Releases bundled wheels and explains how to install one for offline use.

Building from GitHub

1. Push this repository to GitHub (including .github/workflows/bundled-wheels.yml).
2. Select Actions → mermaid-render distributions → Run workflow.
3. Download your platform's wheel from the run artifacts, or get all four wheels from the GitHub Release created when you push a v* tag.

Install and use (after downloading your platform wheel)

uv tool install ./mermaid_render-*-py3-none-macosx_*_arm64.whl
mermaid-render diagram.mmd -o diagram.svg
mermaid-render diagram.mmd -o diagram.png --scale 2
mermaid-render diagram.mmd -o diagram.pdf
mermaid-render diagram.mmd -o diagram.vsdx

The example installs the macOS Apple Silicon wheel. Choose the wheel matching your operating system and architecture.

For the lightweight PyPI installation:

uv tool install mermaid-render
mermaid-render diagram.mmd -o diagram.vsdx

from mermaid_render import convert

source = """flowchart LR
  A[Input] --> B{Valid?}
  B -->|Yes| C[Process]
  B -->|No| D[Reject]
"""

convert(source, "diagram.svg")
convert(source, "diagram.png", scale=2, background="transparent")
convert(source, "diagram.pdf")
convert(source, "diagram.vsdx")

Visio VSDX generation creates editable flowchart shapes and native connection relationships, not generic disconnected SVG paths. Only flowcharts currently support connected VSDX export; other Mermaid diagram types can be exported as SVG, PNG or PDF. Connection behavior after editing/moving nodes in desktop Visio is not yet independently verified. Complex flowchart glyphs (including manual input and stacked documents/processes) retain their rendered outlines as native editable geometry.

VSDX uses right-angle connectors by default, with a small number of rectangular bends and native Visio rerouting enabled. Mermaid's attachment points are preserved, including connections along diamond edges. Each shape also retains its standard right/top/left/bottom connection points for adding new connections. Choose --visio-connectors straight for direct lines, or --visio-connectors mermaid to preserve Mermaid's detailed curves. Self-loops retain a visible loop even in straight mode. The Python equivalent is convert(source, "diagram.vsdx", visio_connectors="right-angle"); the lower-level build_connected_vsdx API uses connectors="right-angle". These options affect VSDX only.

Output formats

mermaid-render infers the format from the output extension; use -f/--format to choose explicitly (svg, png, pdf, visio/vsdx). Without an output filename or explicit format, SVG is the default; the CLI writes beside the input file with a .svg extension. The default Mermaid theme is redux-color; use --theme or the API’s theme= argument to override it. The API returns bytes and optionally writes the output file:

mermaid-render diagram.mmd -o diagram.svg
mermaid-render diagram.mmd -o diagram.png --scale 2 --background transparent
mermaid-render diagram.mmd -o diagram.pdf --background white
mermaid-render diagram.mmd -o diagram.vsdx
mermaid-render diagram.mmd -o diagram.vsdx --visio-connectors straight

• SVG: Original Mermaid-generated SVG markup, without rasterization.
• PNG: Chromium screenshot at the Mermaid SVG viewBox size. --scale 2 doubles pixel dimensions (96 to 192 pixels per CSS inch). --background accepts a CSS color or transparent.
• PDF: Chromium print-to-PDF with a tight single diagram-sized page, zero margins, and vector SVG content where supported. The --scale option applies only to PNG. PDF transparency is not guaranteed by Chromium; use SVG or PNG when an alpha channel is needed.
• VSDX: Native Visio shapes and connection relationships for Mermaid flowcharts. Other Mermaid types deliberately fail rather than writing disconnected shapes.

All four formats use the same official Mermaid.js runtime in headless Chromium. No Node.js process or installed system browser is used by the offline wheels.

Development using uv

uv sync
uv run pytest -q
uv run mermaid-render examples/example.mmd --format png -o diagram.png

uv sync creates a .venv from the committed lockfile. Rendering and browser tests automatically prepare missing runtime support files. The project uses Hatchling with a PEP 621 pyproject.toml and src/ layout.

Default builds are always lightweight, even if the checkout contains old bundled files:

uv build --out-dir dist/pypi
uv run python scripts/check_lightweight_dist.py dist/pypi

Local platform-specific wheel build

On a machine with internet access, with the same OS/architecture as your intended wheel:

uv sync
uv run python scripts/build_platform_wheel.py --outdir dist/bundled
uv run python scripts/test_wheel_install.py dist/bundled/*.whl

The platform tag is detected automatically. For Linux, install system browser libraries first (uv run python -m playwright install-deps chromium). The script downloads the pinned Mermaid distribution, installs the matching headless shell into temporary staging, writes a relative executable manifest, and builds a platform-specific wheel directly with uv build and a Hatchling hook. Staging is removed afterward and the source tree is never populated with runtime assets. The wheel retains native executable permissions and includes upstream license assets distributed in the payload.

Linux compatibility: Linux wheels are available through GitHub Releases only. They use the linux_x86_64 tag and bundle Chromium without vendoring its system libraries. Install compatible browser dependencies on the target system (python -m playwright install-deps chromium). The wheel is tested on Ubuntu 24.04 and is not audited for manylinux compatibility.

macOS caveat: The wheel tag derives its minimum version from the bundled Mach-O binaries using otool; there is no project-defined macOS minimum. CI tests on macOS 15, so older versions are not independently tested. If Gatekeeper imposes restrictions on downloaded unsigned binaries, local signing or organizational policy may be needed.

Configure PyPI publishing

Commit and push the release changes. Set project.version to the intended release version, then push the matching tag, for example:

git tag v0.9.0
git push origin v0.9.0

The workflow also runs builds on pull requests and main-branch pushes, but those runs do not publish to PyPI. If a publish run stops after a partial upload, rerun the failed job: uv checks PyPI and skips identical files already uploaded. Published versions cannot be overwritten; use a new version for changed artifacts.

After the first successful publication, on any supported operating system:

uv tool install mermaid-render
mermaid-render examples/example.mmd -o diagram.vsdx

Development/project structure

• src/mermaid_render/api.py — Python API and SVG/PNG/PDF rendering
• src/mermaid_render/assets.py — bundled lookup and persistent runtime support installation
• src/mermaid_render/graph_capture.js — Mermaid graph semantics and SVG geometry extraction
• src/mermaid_render/semantic.py — connected Visio shapes/edges with Glue formula references
• src/mermaid_render/vsdx.py — geometry-only VSDX exporter for arbitrary SVG
• scripts/build_platform_wheel.py — real platform wheel build with bundled Mermaid/Chromium
• hatch_build.py and scripts/wheel_platform.py — native wheel metadata and binary-derived platform tags
• scripts/test_wheel_install.py and scripts/smoke_offline.py — clean-venv install, offline SVG/Visio smoke test
• .github/workflows/bundled-wheels.yml — four-platform build/test/release workflow

License

MIT for the Python port and its source adaptation; see LICENSE. Mermaid and Chromium remain third-party projects with their own notices and copyright holders.