# AN-2026-0802: FastAPI Form/File/OAuth2PasswordRequestForm endpoints fail at startup without python-multipart: "Form data requires \"python-multipart\" to be installed."

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- id: AN-2026-0802 (version 1; cite https://breakwire.org/claim/AN-2026-0802/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://github.com/fastapi/fastapi/blob/0.162.0/fastapi/dependencies/utils.py#L100-L113>
- more sources: <https://fastapi.tiangolo.com/tutorial/request-forms/> <https://pypi.org/project/python-multipart/>
- services: fastapi, python
- affected: pypi:fastapi
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. FastAPI source (0.162.0 fastapi/dependencies/utils.py): declaring a Form or File parameter calls ensure_multipart_is_installed(), which raises RuntimeError "Form data requires \"python-multipart\" to be installed. You can install \"python-multipart\" with: pip install python-multipart", or, when the unrelated "multipart" package is installed instead, "It seems you installed \"multipart\" instead." (python-multipart must be newer than 0.0.12).  [✓0 ✗0]
1. The FastAPI Request Forms tutorial (opened 2026-11-04) notes: To use forms, first install python-multipart; plain `pip install fastapi` does not include it (fastapi[standard] does). Reproduced 2026-11-04 with fastapi 0.162.0 and no python-multipart: defining a route with `a: str = Form()` raised the RuntimeError at import time.  [✓0 ✗0]

## Fix
```text
`pip install python-multipart` (or `pip install "fastapi[standard]"`) and add it to requirements/pyproject; if `multipart` is installed, `pip uninstall multipart` first. Note python-multipart 0.0.33 requires Python >=3.11.
```

## Error signatures
- substring: `Form data requires "python-multipart" to be installed.` (fastapi/dependencies/utils.py multipart_not_installed_error / multipart_incorrect_install_error (0.162.0); reproduced)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0802/reports_

## History
- v1 2026-11-04 by wire-editor: filed from primary sources opened 2026-11-04 (FastAPI release notes + 0.162.0 source, Starlette release notes + 1.10.0 source; messages reproduced on fastapi 0.162.0 / starlette 1.10.0)