Merge pull request #20 from panel-extensions/fix/screenshot-browser-install

Install Chromium via `pls install-browser` (pixi/uv/pip)

Author: ahuang11

docs/tutorials/installation.md

@@ -27,6 +27,12 @@ terminal. By the end, `pls --version` will print the installed version.
     pixi add --pypi "panel-live-server[pydata]"
     ```
 
+    Install the browser the `screenshot` tool needs:
+
+    ```bash
+    pixi run pls install-browser
+    ```
+
     Find the `pls` path:
 
     **macOS / Linux:**
@@ -51,6 +57,12 @@ terminal. By the end, `pls --version` will print the installed version.
     uv tool install "panel-live-server[pydata]"
     ```
 
+    Install the browser the `screenshot` tool needs:
+
+    ```bash
+    pls install-browser
+    ```
+
     Find the `pls` path:
 
     **macOS / Linux:**
@@ -98,6 +110,12 @@ terminal. By the end, `pls --version` will print the installed version.
     pip install "panel-live-server[pydata]"
     ```
 
+    Install the browser the `screenshot` tool needs:
+
+    ```bash
+    pls install-browser
+    ```
+
     Find the `pls` path:
 
     **macOS / Linux:**

docs/tutorials/mcp-server.md

@@ -187,12 +187,17 @@ The server runs in an isolated uv tool environment. Install missing packages as
 
 ### `screenshot` fails with a Playwright error
 
-The `screenshot` tool needs the Chromium browser that Playwright manages. Install it once with:
+The `screenshot` tool needs the Chromium browser that Playwright manages, which is
+not installed automatically. Install it once with:
 
 ```bash
-playwright install chromium
+pls install-browser
 ```
 
+This downloads Chromium into the same environment that runs `pls`. See
+[Installation → Enable the screenshot tool](installation.md#enable-the-screenshot-tool)
+for the per-installer command.
+
 ---
 
 ## What You've Learned

src/panel_live_server/cli.py

@@ -271,6 +271,29 @@ def list_packages(
         typer.echo(f"{name:<{name_width}}  {version}")
 
 
+@app.command(name="install-browser")
+def install_browser() -> None:
+    """Download the Chromium browser the `screenshot` MCP tool needs.
+
+    Playwright ships its browser binary separately from the Python package, so a
+    `pip` or `uv` install does not fetch it automatically. Run this once after
+    installing (pixi users get it via `pixi run postinstall`). It lands in the
+    same environment that runs `pls`.
+    """
+    from panel_live_server.screenshot import install_browser as _install_browser
+
+    typer.echo("Installing Chromium for the screenshot tool (one-time)...")
+    code = _install_browser()
+    if code == 0:
+        typer.echo("Done — the screenshot tool is ready.")
+    else:
+        typer.echo(
+            "Browser install failed. Try manually: python -m playwright install chromium",
+            err=True,
+        )
+        raise typer.Exit(code)
+
+
 def main() -> None:
     """Entry point for the pls command."""
     app()

src/panel_live_server/screenshot.py

@@ -12,6 +12,8 @@
 
 import asyncio
 import logging
+import os
+import sys
 
 logger = logging.getLogger(__name__)
 
@@ -20,7 +22,46 @@ class PlaywrightUnavailableError(RuntimeError):
     """Raised when Playwright or its browser is not installed/launchable."""
 
 
-_INSTALL_HINT = "Playwright Chromium is not installed. Run:\n" "    playwright install chromium"
+_INSTALL_HINT = "Playwright's Chromium browser is not installed. Run:\n    pls install-browser"
+
+
+def install_browser() -> int:
+    """Download the headless Chromium browser the screenshot tool needs.
+
+    Playwright ships its browser binary separately from the Python package, so
+    ``pip``/``uv`` installs do not fetch it automatically. This shells out to
+    ``<this-interpreter> -m playwright install chromium`` so the browser always
+    lands in the same environment that is running ``pls`` — avoiding the common
+    trap where the binary is installed under a different interpreter.
+
+    Returns
+    -------
+    int
+        The installer subprocess exit code (``0`` on success).
+    """
+    import subprocess
+
+    return subprocess.run([sys.executable, "-m", "playwright", "install", "chromium"]).returncode
+
+
+def is_browser_installed() -> bool:
+    """Return ``True`` if the Chromium binary Playwright needs is present.
+
+    This is a cheap check — it does not launch a browser. It uses Playwright's
+    sync API, so call it from a worker thread (e.g. ``asyncio.to_thread``), not
+    directly inside a running event loop.
+    """
+    try:
+        from playwright.sync_api import sync_playwright
+    except ImportError:
+        return False
+    try:
+        with sync_playwright() as p:
+            path = p.chromium.executable_path
+        return bool(path) and os.path.exists(path)
+    except Exception:
+        return False
+
 
 # Best-effort wait for Panel/Bokeh content to mount before capturing.
 _CONTENT_SELECTOR = "canvas, .bk-Row, .bk-Column, .bk, .markdown, table, img, svg"

src/panel_live_server/server.py

@@ -262,6 +262,19 @@ async def app_lifespan(app):
     else:
         logger.warning("Panel Live Server failed to start - show tool will not work")
 
+    # Warn early if the screenshot browser is missing, so users find out now
+    # rather than mid-screenshot. The check uses Playwright's sync API, so run
+    # it in a worker thread to stay off the event loop.
+    try:
+        from panel_live_server.screenshot import is_browser_installed
+
+        if not await asyncio.to_thread(is_browser_installed):
+            msg = "The `screenshot` tool needs Chromium, which is not installed. Run `pls install-browser` to enable it."
+            print(f"\n  {msg}\n", file=sys.stderr, flush=True)  # noqa: T201
+            logger.warning(msg)
+    except Exception:
+        logger.debug("Could not check screenshot browser availability", exc_info=True)
+
     try:
         yield
     finally: