tiangolo/fastapi · error · RuntimeError

Form data requires "python-multipart" to be installed. You…

Error message

Form data requires "python-multipart" to be installed. 
You can install "python-multipart" with: 

pip install python-multipart

What it means

RuntimeError raised by ensure_multipart_is_installed when neither python-multipart nor the (wrong) multipart package is importable at all. It triggers as soon as a route uses Form()/File() and FastAPI needs a multipart parser to decode the request body. The error message tells you to install python-multipart.

Solutions

Install python-multipart: pip install python-multipart.
Install the standard extra: pip install "fastapi[standard]".
Pin python-multipart in requirements.txt / pyproject.toml so it is always present.
Restart the server/process after installing so the new import is picked up.

Example fix

# before
pip install fastapi   # no multipart parser
@app.post("/login")
def login(username: str = Form()): ...
# -> RuntimeError: Form data requires "python-multipart" to be installed.

# after
pip install python-multipart
@app.post("/login")
def login(username: str = Form()): ...

Defensive patterns

Strategy: fallback

Validate before calling

# Detect a missing multipart parser before relying on Form/File
import importlib.util, sys
if importlib.util.find_spec("python_multipart") is None:
    print("python-multipart is not installed; Form/File routes will fail")
    sys.exit(1)

Try / catch

try:
    from python_multipart import __version__  # noqa: F401
except ImportError:
    raise RuntimeError("Install python-multipart to use Form/File endpoints")

Prevention

Add python-multipart to requirements whenever you use Form() or File().
Install fastapi[standard] in dev/CI to get the parser automatically.
Restart the process after installing so the import resolves.
Write a smoke test that posts a form to confirm multipart parsing works.

When it happens

Trigger: Any request that requires multipart parsing (a Form or File parameter) hits a route whose environment lacks python-multipart entirely, so ensure_multipart_is_installed() falls through both import attempts and raises.

Common situations: Slim fastapi install (pip install fastapi) without the standard extra; CI image that omitted python-multipart; deploying with a trimmed requirements list; adding a Form endpoint to an existing app without reinstalling deps.

Related errors

AI-assisted analysis of tiangolo/fastapi@3e8d1526d8 (2026-08-11). Data as JSON: /api/errors/3ccff8d0c7715291. Report an issue: GitHub.

Appendix: source

Thrown at fastapi/dependencies/utils.py:129

            # __version__ is available in both multiparts, and can be mocked
            from multipart import (  # type: ignore[no-redef,import-untyped]
                __version__,
            )

            assert __version__
            try:
                # parse_options_header is only available in the right multipart
                from multipart.multipart import (  # type: ignore[import-untyped]
                    parse_options_header,
                )

                assert parse_options_header
            except ImportError:
                logger.error(multipart_incorrect_install_error)
                raise RuntimeError(multipart_incorrect_install_error) from None
        except ImportError:
            logger.error(multipart_not_installed_error)
            raise RuntimeError(multipart_not_installed_error) from None


def get_parameterless_sub_dependant(*, depends: params.Depends, path: str) -> Dependant:
    assert callable(depends.dependency), (
        "A parameter-less dependency must have a callable dependency"
    )
    own_oauth_scopes: list[str] = []
    if isinstance(depends, params.Security) and depends.scopes:
        own_oauth_scopes.extend(depends.scopes)
    return get_dependant(
        path=path,
        call=depends.dependency,
        scope=depends.scope,
        own_oauth_scopes=own_oauth_scopes,
    )


def _get_flat_body_params(dependant: Dependant) -> list[ModelField]:

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