render-html MCP tool

A developer has built a "render-html-mcp" tool, an MCP server that pops up a standalone desktop browser window to display HTML or Markdown content. The Python-based server uses the FastMCP framework and supports both Chromium-family browsers (with a chrome-less app mode) and Firefox as a fallback, launching each window with a unique user-data directory to avoid collisions. The tool returns immediately after spawning the browser process, which remains open until the user closes it.

| Give this to your LLM of choice. | | | Don't like python? ask it to build it in any language you prefer. | | | Note: I am running this on Linux, but same thing can be done for Windows or macOS, too. | | | ---- | | | Build a "render-html-mcp" — MCP Server for Rendering HTML in Desktop Popup Windows | | | Overview | | | Create a Python MCP server using the mcp package's FastMCP framework that pops up a standalone desktop browser window displaying HTML/markdown content and returns immediately. The window stays open until the user closes it. The browser process is fully detached start new session=True, stdio to /dev/null so it outlives the MCP call. | | | Project Structure | | | render-html-mcp/ | | | ├── pyproject.toml | | | ├── README.md | | | └── src/ | | | └── renderhtmlmcp/ | | | ├── init .py empty | | | ├── server.py all MCP logic | | | └── cli.py thin CLI entry point | | | pyproject.toml | | | - Build system: setuptools =61.0 | | | - Package name: render-html-mcp, version 0.1.0 | | | - requires-python: =3.10 | | | - Dependencies: mcp =1.0.0, markdown =3.5, Pygments =2.17 | | | - Console script: render-html-mcp = renderhtmlmcp.cli:cli | | | - Setuptools packages.find: where = "src" | | | - Ruff config: target-version = "py310", line-length = 100, lint select "E", "F", "W", "I", "UP" | | | cli.py | | | A minimal entry point. The cli function imports and calls run server from server.py, returns 0. Has an if name == " main " guard that calls cli via SystemExit. | | | server.py — Core Logic | | | MCP Instance | | | from mcp.server.fastmcp import FastMCP | | | mcp = FastMCP "render-html-mcp" | | | Temp Directory | | | The render directory defaults to $RENDER HTML MCP DIR if set, otherwise <tempfile.gettempdir /render-html-mcp/ i.e., /tmp/render-html-mcp . Store it in RENDER DIR as a Path. | | | Browser Discovery | | | - Chromium-family preference supports --app= for chrome-less standalone windows : | | | google-chrome, google-chrome-stable, chromium, chromium-browser, brave-browser, microsoft-edge | | | - Firefox fallback uses --new-window, shows full chrome : | | | firefox, firefox-esr | | | - Use shutil.which to find the first available on PATH. Return binary path, mode where mode is "chromium-app" or "firefox-window", or None. | | | Command Building | | | - chromium-app mode: browser, "--app={url}", "--window-size={width},{height}", "--user-data-dir={ RENDER DIR/profile/<uuid4-hex }", "--no-first-run", "--no-default-browser-check" | | | - Each window gets a unique --user-data-dir under RENDER DIR/profile/ so it never collides with the user's running browser or single-instance lock. | | | - firefox-window mode: browser, "--new-window", url | | | Launch Shared by All Tools | | | The launch url url, width, height function: | | | 1. Calls pick browser . If None, return {"ok": False, "error": "No supported browser found on PATH..."}. | | | 2. Checks os.environ for DISPLAY or WAYLAND DISPLAY. If neither set, return {"ok": False, "error": "No DISPLAY or WAYLAND DISPLAY set..."}. | | | 3. Builds command via build command . | | | 4. Spawns with subprocess.Popen cmd, stdin=DEVNULL, stdout=DEVNULL, stderr=DEVNULL, start new session=True, close fds=True . | | | 5. Returns {"ok": True, "pid": proc.pid, "browser": browser, "mode": mode}. | | | Markdown CSS GitHub-ish Typography | | | Inline a self-contained CSS block MARKDOWN CSS with: | | | - color-scheme: light dark on :root | | | - System font stack -apple-system, BlinkMacSystemFont, "Segoe UI", system-ui, sans-serif | | | - max-width: 860px, centered with margin: 2rem auto, padding: 0 1.5rem, line-height: 1.6 | | | - Light theme: color: 1f2328, background: ffffff, links 0969da, code blocks f6f8fa | | | - Dark theme via @media prefers-color-scheme: dark : color: e6edf3, background: 0d1117, links 4493f8, code blocks 161b22, blockquote 8b949e, borders 30363d, striped table rows 161b22 | | | - Headings: font-weight: 600, line-height: 1.25, margin-top: 1.6em, margin-bottom: 0.6em. H1/H2 have border-bottom: 1px solid currentColor + padding-bottom: .3em. H1 at 2em, H2 at 1.5em, both with slight opacity. | | | - Code: ui-monospace, SFMono-Regular, Menlo, Consolas, monospace, 0.92em, padded, rounded, 0.9em inside <pre | | | - Blockquote: border-left: 4px solid, padding: 0 1em | | | - Table: border-collapse: collapse, borders on th/td, striped rows, display: block with overflow-x: auto | | | - Images: max-width: 100% | | | - Lists: padding-left: 2em, li + li margin top 0.25em | | | - Horizontal rule: border-top, margin: 2em 0 | | | Markdown Rendering | | | render markdown md, title converts markdown to a full HTML document: | | | - Uses markdown.markdown with extensions: extra, sane lists, codehilite, toc, admonition | | | - codehilite config: guess lang=False, noclasses=True, pygments style="default" | | | - Sanitizes title: replace & → &, < → <, → | | | - Wraps in < doctype html with <html lang="en" , <meta charset="utf-8" , title, inline <style with MARKDOWN CSS, then <body with the rendered content | | | HTML Fragment Wrapping | | | wrap html html, title : | | | - If html.lstrip starts with < doctype or <html case-insensitive , return as-is | | | - Otherwise wrap in a minimal document with < doctype html , <html lang="en" , charset meta, sanitized title, and a minimal body style: font-family:system-ui,sans-serif;margin:1.5rem;line-height:1.5 | | | Four MCP Tools | | | 1. render html html: str, title: str = "render-html-mcp", width: int = 900, height: int = 700 - dict | | | - Wraps the HTML string via wrap html , writes to < RENDER DIR /render-<uuid4 .html | | | - Launches browser with the file URI file path.as uri | | | - On success, adds "path" absolute path to the written file to the result | | | - Returns {ok, path, pid, browser, mode} or {ok: False, error: ...} | | | 2. render html from file file path: str, width: int = 900, height: int = 700 - dict | | | - Expands ~ and resolves to absolute path via Path file path .expanduser .resolve | | | - Validates: file exists is file and is readable os.access os.R OK | | | - Opens the file directly no copy via its URI | | | - Returns {ok, path, pid, browser, mode} or {ok: False, error: ...} | | | 3. render html from markdown markdown: str, title: str = "render-html-mcp", width: int = 900, height: int = 700 - dict | | | - Converts markdown via render markdown , writes to < RENDER DIR /render-md-<uuid4 .html | | | - Launches browser with the file URI | | | - Returns {ok, path, pid, browser, mode} or {ok: False, error: ...} | | | 4. render html from markdown file file path: str, title: str | None = None, width: int = 900, height: int = 700 - dict | | | - Expands and resolves path; validates existence and readability | | | - Reads markdown content | | | - Default title: title if provided, otherwise the file's .stem | | | - Writes rendered HTML to < RENDER DIR /render-md-<uuid4 .html | | | - Returns {ok, path, source, pid, browser, mode} — note "source" is the original .md path | | | - The original markdown file is never modified | | | run server | | | At module level, define run server - None which calls mcp.run . | | | Key Design Decisions to Preserve | | | 1. Detached process: start new session=True + stdio to DEVNULL + close fds=True ensures the browser window survives the MCP server process ending | | | 2. Per-window user-data-dir: Unique --user-data-dir UUID-based prevents single-instance locking with the user's normal browser | | | 3. No JS rendering engine: This is NOT headless; it relies on the user's desktop browser requires DISPLAY or WAYLAND DISPLAY | | | 4. Fragment detection: Checks for full document vs fragment to avoid double-wrapping | | | 5. No garbage collection: Temp files and chrome profiles are NOT cleaned up. Document this limitation in the README. |