EdMd is a small, fast WYSIWYG Markdown editor for Windows. It's a WPF window hosting a
single WebView2 that renders the whole UI (tab strip, toolbar, editor, footer) as HTML/CSS/JS,
with the Toast UI Editor doing the editing. The C# side only
owns the window and the disk I/O the browser sandbox can't do. Built for authoring prompts,
skills, CLAUDE.md files, PRs, and specs — with a live token/word/char counter.
Multi-file, single instance: open several documents in tabs (Ctrl+T new, Ctrl+W close,
middle-click to close, or open/select many files at once). Double-clicking a .md in Explorer
opens it as a new tab in the running window instead of launching a second copy.
Works offline, stays offline. Everything ships with the app — the editor and the diagram renderer are vendored, not loaded from a CDN — and the page's Content-Security-Policy allows no remote origin at all, so opening a document never tells anyone you opened it. See Security & design.
In Browser
Writing
- WYSIWYG by default, with a Raw toggle for the markdown source — and a Preview tab next to it when you want to see the rendered result while editing the source.
- Mermaid diagrams. A
```mermaidblock is shown as the diagram in WYSIWYG (click it to edit the source behind it), as markdown in Raw, and rendered in the preview. Flowcharts, sequence, gitGraph, pie — whatever Mermaid supports; a broken diagram gets a plain error box instead of a crash. - Paste or drag in an image. In a saved document the bytes are written to a sibling
assets/folder and linked relatively (), so the.mdstays portable and renders on GitHub. Identical images are stored once. An unsaved document embeds the image inline as a data URI instead, so nothing is written next to a file that doesn't exist yet. - Find & replace with live highlighting, case and regex options.
- Live counter in the footer:
~tokens · words · chars— handy when writing for an LLM.
Documents
- Tabs, multi-select open, and single-instance file associations (above).
- Session restore & crash recovery. Reopening picks up the tabs you had, including unsaved buffers.
- Export to PDF or HTML — a clean, self-contained, print-friendly document. Diagrams are included as pictures.
- Copy as Markdown / plain text — the whole document to the clipboard in one click, ready to paste into a chat.
- Open in Browser — hands the current document to Chrome/Edge running the same editor.
- Encoding and line endings (UTF-8/BOM, CRLF/LF) are preserved on save, writes are atomic, and an external change to an open file is detected before it can be overwritten.
Appearance
- Twelve themes (light and dark), zoom, and an adjustable reading-column width. The native title bar follows the theme.
Grab the latest build from the Releases page. Two options:
Registers .md / .markdown file associations, adds a Start-menu entry, and supports
in-place upgrades.
Add-AppxPackage .\EdMd-<version>.msixInterim trust step. Releases are currently signed with a self-signed certificate, so Windows needs to trust it once before the MSIX will install. Download
edmd-codesign.cerfrom the same release and run, in an elevated PowerShell:Import-Certificate -FilePath .\edmd-codesign.cer -CertStoreLocation Cert:\LocalMachine\TrustedPeople Import-Certificate -FilePath .\edmd-codesign.cer -CertStoreLocation Cert:\LocalMachine\RootThis step goes away if builds move to a publicly-trusted certificate.
Download EdMd-<version>-win-x64.exe and run it. It's self-contained (no .NET needed).
No file associations, and SmartScreen may show "Windows protected your PC" → More info →
Run anyway until the download builds reputation.
Prefer a zip? EdMd-<version>-win-x64.zip has the same portable exe plus LICENSE.txt —
extract it anywhere and run EdMd.exe. A zipped download also sidesteps some browser/AV
"can't download .exe" friction.
EdMd needs the Microsoft Edge WebView2 Evergreen Runtime, which is already present on most Windows 10/11 machines. If it's missing, EdMd shows a prompt with the download link.
Prerequisites: the .NET 10 SDK, the WebView2
Runtime, and (only for the MSIX) the Windows 10/11 SDK (MakeAppx.exe + SignTool.exe).
cd src/EdMd
dotnet restore
dotnet run # build + launchRun the tests (xUnit — covers the security-sensitive C# logic):
dotnet test # from repo root or src/EdMd.TestsPortable single-file exe:
dotnet publish src/EdMd/EdMd.csproj -c Release -r win-x64 --self-contained true -p:PublishSingleFile=trueMSIX installer — see src/EdMd.Installer/README.md.
The same UI also runs as a plain web app in Chrome/Edge (File System Access API for open/save). Serve it locally:
pwsh -File src/EdMd/serve.ps1 # http://localhost:8080, opens ChromePull requests welcome — branch, run dotnet test, open a PR against main (CI must pass).
See CONTRIBUTING.md for the full workflow and the guardrails that keep
the app safe.
Releases are tag-driven — full process and versioning policy in RELEASING.md.
In short, push a version tag and CI does the rest:
git tag v1.2.3 && git push origin v1.2.3The Release workflow runs on windows-latest: it tests,
publishes the portable exe, builds the MSIX via src/EdMd.Installer/build.ps1, signs, and
attaches everything to a GitHub Release. workflow_dispatch does a dry build (artifacts on
the run, no Release).
Releases are signed with a self-signed CN=EdMd certificate (CI reads the key from the
secrets SELFSIGN_PFX_BASE64 / SELFSIGN_PFX_PASSWORD), which is why the MSIX needs the
one-time trust step above.
The private key (certs/*.pfx) is never committed (see .gitignore); CI
reads it from a secret. See certs/README.md and, for the full release
process, RELEASING.md.
A markdown file you open is untrusted input, and the WebView2 rendering it can also write to disk — so the boundary between them is where the design effort went:
- Origin-pinned WebView2. The UI is served from a virtual host; navigation anywhere else is cancelled and opened in your normal browser instead, and messages from any other origin are ignored before they can touch disk.
- CSP behind Toast's sanitizer. No inline script, no
unsafe-eval, and no remote origin — including images, so a document can't phone home with a tracking pixel. - No network. The app makes no requests of its own; the browser engine it embeds is started
with background networking, component updates, crash reporting and telemetry switched off.
One caveat, measured and documented in
CLAUDE.md: the WebView2 runtime still makes a Windows-account probe of its own that no in-process setting reaches. - Careful writes. Atomic saves, preserved encoding/line endings, an external-change guard, and a size- and type-limited path for pasted images that can't escape the document's folder.
- Token-gated loopback for the "Open in Browser" handoff, so nothing else on localhost can read your document.
Details in CLAUDE.md and
docs/production-readiness-review.md.
MIT.