langchain-ai/langchain · error · ImportError

Install Pyppeteer to use the Pyppeteer method: `pip install

Error message

Install Pyppeteer to use the Pyppeteer method: `pip install pyppeteer`.

What it means

When draw_mermaid_png() is called with MermaidDrawMethod.PYPPETEER, rendering launches a headless Chromium via pyppeteer. If pyppeteer is not installed, _render_mermaid_using_pyppeteer raises this ImportError telling you the exact extra required.

Source

Thrown at libs/core/langchain_core/runnables/graph_mermaid.py:344

            f"Invalid draw method: {draw_method}. "
            f"Supported draw methods are: {supported_methods}"
        )
        raise ValueError(msg)

    return img_bytes


async def _render_mermaid_using_pyppeteer(
    mermaid_syntax: str,
    output_file_path: str | None = None,
    background_color: str | None = "white",
    padding: int = 10,
    device_scale_factor: int = 3,
) -> bytes:
    """Renders Mermaid graph using Pyppeteer."""
    if not _HAS_PYPPETEER:
        msg = "Install Pyppeteer to use the Pyppeteer method: `pip install pyppeteer`."
        raise ImportError(msg)

    browser = await launch()
    page = await browser.newPage()

    # Setup Mermaid JS
    await page.goto("about:blank")
    await page.addScriptTag(
        {"url": "https://cdn.jsdelivr.net/npm/mermaid/dist/mermaid.min.js"}
    )
    await page.evaluate(
        """() => {
                mermaid.initialize({startOnLoad:true});
            }"""
    )

    # Render SVG
    svg_code = await page.evaluate(
        """(mermaidGraph) => {

View on GitHub (pinned to e32fa9a52e)

Solutions

pip install pyppeteer, then run pyppeteer-install once to fetch Chromium
If pyppeteer fails to install/run (common on Python 3.10+), switch to the default draw_method=MermaidDrawMethod.API
For fully offline pipelines, render Mermaid with an external tool (e.g. mermaid-cli) instead

Example fix

# before
draw_mermaid_png(syntax, draw_method=MermaidDrawMethod.PYPPETEER) # ImportError
# after
pip install pyppeteer && pyppeteer-install
draw_mermaid_png(syntax, draw_method=MermaidDrawMethod.PYPPETEER)

Defensive patterns

Strategy: validation

Validate before calling

try:
    import pyppeteer  # noqa: F401
    ok = True
except ImportError:
    ok = False
if not ok:
    draw_mermaid_png(syntax)  # default API path

Try / catch

try:
    img = draw_mermaid_png(syntax, draw_method=MermaidDrawMethod.PYPPETEER)
except ImportError:
    img = draw_mermaid_png(syntax)  # fall back to API

Prevention

Prefer MermaidDrawMethod.API unless you must render offline
Run pyppeteer-install in your Dockerfile so Chromium is present before first use

When it happens

Trigger: draw_mermaid_png(syntax, draw_method=MermaidDrawMethod.PYPPETEER) in an environment without pyppeteer.

Common situations: Opting for local rendering to avoid the mermaid.ink network API but forgetting the optional dependency; pyppeteer is also unmaintained and its bundled Chromium download often breaks on modern Python versions.

Related errors

AI-assisted analysis of langchain-ai/langchain@e32fa9a52e (2026-08-14). Data as JSON: /api/errors/2c05caabcbea42ac. Report an issue: GitHub.