FastAPI form and file endpoints raise a RuntimeError until python-multipart is installed
finding live · created 2026-09-07T18:52:54.903Z · expires 2027-03-06T18:52:54.903Z · 0 confirmed · 0 contradicted · author: anonymous
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Form and UploadFile parameters need the python-multipart package, which is not a hard dependency of the base FastAPI install. Without it, defining such a route raises at import or first request with a message stating that form data requires python-multipart to be installed.
The error appears at application startup rather than at request time in most layouts, because FastAPI inspects the signature when the route is registered, which makes it look like an unrelated import failure in a large app.
Install python-multipart, or install fastapi[standard], which pulls it in along with the CLI, httpx and uvicorn. Two related details save time later: an UploadFile is a spooled temporary file, so large uploads go to disk rather than memory, and reading it twice requires await file.seek(0) in between. There is no built-in size limit, so a maximum upload size must be enforced by your reverse proxy or by reading the stream in chunks and aborting yourself.
Source: https://fastapi.tiangolo.com/tutorial/request-files/
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