langchain-ai/langchain · error · ValueError

Invalid draw method: {draw_method}. Supported draw methods a

Error message

Invalid draw method: {draw_method}. Supported draw methods are: {supported_methods}

What it means

draw_mermaid_png() dispatches on the draw_method argument and only accepts MermaidDrawMethod enum members (PYPPETEER, API). Any other value falls through to this ValueError listing the supported methods. Passing the method as a plain string fails the enum equality check and lands here.

Source

Thrown at libs/core/langchain_core/runnables/graph_mermaid.py:329

            )
        )
    elif draw_method == MermaidDrawMethod.API:
        img_bytes = _render_mermaid_using_api(
            mermaid_syntax,
            output_file_path=output_file_path,
            background_color=background_color,
            max_retries=max_retries,
            retry_delay=retry_delay,
            base_url=base_url,
            proxies=proxies,
        )
    else:
        supported_methods = ", ".join([m.value for m in MermaidDrawMethod])  # type: ignore[unreachable]
        msg = (
            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()

View on GitHub (pinned to e32fa9a52e)

Solutions

Pass the enum: from langchain_core.runnables.graph_mermaid import MermaidDrawMethod; draw_mermaid_png(syntax, draw_method=MermaidDrawMethod.API)
If you must accept strings from config, convert first: MermaidDrawMethod(str_value.lower())
Omit draw_method to use the default MermaidDrawMethod.API

Example fix

# before
draw_mermaid_png(syntax, draw_method="api")
# after
from langchain_core.runnables.graph_mermaid import MermaidDrawMethod
draw_mermaid_png(syntax, draw_method=MermaidDrawMethod.API)

Defensive patterns

Strategy: type-guard

Validate before calling

from langchain_core.runnables.graph_mermaid import MermaidDrawMethod
assert draw_method in set(MermaidDrawMethod)

Type guard

from typing import cast
from langchain_core.runnables.graph_mermaid import MermaidDrawMethod

def is_draw_method(v: object) -> bool:
    return isinstance(v, MermaidDrawMethod)

Try / catch

try:
    draw_mermaid_png(syntax, draw_method=method)
except ValueError as e:
    if "Invalid draw method" in str(e):
        draw_mermaid_png(syntax)  # default API method

Prevention

Always pass the MermaidDrawMethod enum, never a string
Type-hint your config as MermaidDrawMethod so mypy catches string values

When it happens

Trigger: Calling draw_mermaid_png(mermaid_syntax, draw_method="api") or draw_method="pyppeteer" (string instead of enum), or a custom/misspelled value.

Common situations: Users copying example code that quotes the method; upgrading code that used strings in older versions; IDE autocompleting the value instead of the enum member.

Related errors

AI-assisted analysis of langchain-ai/langchain@e32fa9a52e (2026-08-14). Data as JSON: /api/errors/8ee6537708bc3b9e. Report an issue: GitHub.