Fast Markdown Preview

A live-preview plugin for Sublime Text 4. It renders your Markdown to styled HTML in a browser window that updates as you type.

You type in Sublime, the browser preview refreshes within ~50 ms. Switch to another Markdown tab and the preview follows it.

It works by running a small Go server on localhost that parses Markdown with goldmark and streams the rendered HTML to the browser over Server-Sent Events. The browser patches only the changed DOM nodes with morphdom, so updates are fast and your scroll position is preserved.

Features

• Instant updates. The preview refreshes within ~50 ms of your last keystroke.
• Follows the active view. Switch tabs or open another .md file and the preview tracks it automatically.
• Full Markdown support. GFM tables, strikethrough, task lists, definition lists, footnotes, typographer (smart quotes and dashes), autolinks, emoji shortcodes, auto heading IDs, and syntax-highlighted code blocks.
• Light and dark themes. Follows your OS preference, or force one with ?theme=light / ?theme=dark.
• Zero config. The server binary is auto-downloaded from GitHub Releases on first use, and the port is auto-selected.
• Low resource usage. A single ~13 MB static Go binary. SSE uses no bandwidth when idle, and the server auto-shuts down after inactivity.
• Cross-platform. Linux, macOS, and Windows, both amd64 and arm64.
• No dependencies. The plugin uses only the Python standard library, and the browser UI is a single self-contained HTML file.

Installation

Clone this repository into your Sublime Text Packages/ directory:

# macOS
cd ~/Library/Application\ Support/Sublime\ Text/Packages/

# Linux
cd ~/.config/sublime-text/Packages/

# Windows (in PowerShell)
cd $env:APPDATA\Sublime Text\Packages\

git clone https://github.com/ipmanlk/fast-markdown-preview.git FastMarkdownPreview

Restart Sublime Text. The server binary is downloaded automatically on first preview.

If you see a SyntaxError about type annotations in the ST console, make sure .python-version is present in the package directory. It tells ST4 to use its Python 3.8 host instead of the legacy 3.3 host.

See docs/installation.md for more detail.

Usage

1. Open a Markdown (.md) file.
2. Run Fast Markdown Preview: Toggle Preview from the Command Palette, or press Ctrl+Alt+M (Windows/Linux) / Cmd+Alt+M (macOS).
3. Your default browser opens with the rendered preview.
4. Type. The preview updates as you go.
5. Switch to another Markdown tab. The preview follows it.

Commands

Command	Palette entry	Default key
Toggle preview	FMP: Toggle Preview	Ctrl+Alt+M (Win/Linux), Cmd+Alt+M (macOS)
Reopen preview	FMP: Reopen Preview
Settings	FMP: Settings

Configuration

Settings live in FastMarkdownPreview.sublime-settings. Open them via Preferences > Package Settings > Fast Markdown Preview > Settings.

{
    // Port for the preview server. 0 = auto-select a free port.
    "server_port": 0,

    // Milliseconds to wait after the last keystroke before sending markdown.
    "debounce_ms": 50,

    // Automatically open the browser when the preview is toggled on.
    "auto_open_browser": true,

    // Override path to a custom server binary. If empty, auto-download.
    "server_binary_path": "",

    // Preview theme: "auto" (follow OS), "light", or "dark".
    "theme": "auto",

    // Idle timeout in minutes. Server auto-shuts down after this long with no
    // connected browsers and no updates. 0 = never (not recommended).
    "idle_timeout_minutes": 5,

    // Follow the active Markdown view: switching tabs updates the preview.
    "follow_active_view": true,

    // GitHub repo for binary downloads (owner/name).
    "github_repo": "ipmanlk/fast-markdown-preview"
}

How it works

Sublime Text 4 (Python plugin)
  |  on_modified_async (debounced ~50ms) + on_activated_async (follow view)
  |  HTTP POST /update  (localhost)
  v
Go server (single static binary)
  |  goldmark parse -> HTML
  |  SSE broadcast to all connected browsers
  v
Browser UI (single HTML page, embedded in the binary)
  |  EventSource -> morphdom DOM diff (only changed nodes are repainted)

• The plugin spawns the Go server as a subprocess on first preview. The server binds to 127.0.0.1 only, never to the network, and picks a free port.
• The port is written to a well-known file so multiple Sublime windows share one server.
• The server auto-shuts down after 5 minutes (configurable) with no connected browsers and no updates, so no orphaned processes are left behind.
• If Sublime crashes, the idle timeout cleans up the server.

For the details behind these decisions, see docs/development.md.

Development

Prerequisites: Go 1.22+ and Python 3.8+ (with pytest for the tests).

make build      # build the server into server/bin/
make test       # run Go + Python tests
make run        # build and run the server on a free port
make fmt        # format Go code
make lint       # vet Go code
make install    # copy the plugin into your ST Packages dir

For the full development guide, see docs/development.md. Other docs:

• Installation
• Usage
• Settings
• Troubleshooting

License

MIT