First Check

• I added a very descriptive title here.
• I used the GitHub search to find a similar question and didn't find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google "How to X in FastAPI" and didn't find any information.
• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to FastAPI but toPydantic.
• I already checked if it is not related to FastAPI but toSwagger UI.
• I already checked if it is not related to FastAPI but toReDoc.

Commit to Help

• I commit to help with one of those options 👆

Example Code

fromtypingimportAny,Dict,List,UnionfromfastapiimportFastAPI,Bodyfromstarlette.requestsimportRequestfromstarlette.responsesimportResponseapp=FastAPI()@app.post("/any")# fails with 422asyncdefhandle(request:Any):print(f"got{request}")returnrequest@app.post("/raw")# worksasyncdefhandle(request:Request):print(f"got{request}")returnResponse(content=awaitrequest.body())@app.post("/dict")# worksasyncdefhandle(request:Dict):print(f"got{request}")returnrequest@app.post("/list")# fails with 422asyncdefhandle(request:List):print(f"got{request}")returnrequest@app.post("/anybody")# worksasyncdefhandle(request:Any=Body(None)):print(f"got{request}")returnrequest

Description

I have what I think to be a bug in FastAPI, or is at least an opportunity to make something clearer in the docs.

We are creating a service that basically proxies requests from the outside world to our existing services. Some of those services basically pass requests along to other services. We would like the outer services to have no opinion on the content that is getting passed along, such that request validation happens at the service that actually cares about the request. This reduces duplication and risk of subtle bugs.

To us the obvious way to do this was to declare a method parameter ofAny. But that doesn't work. Rummaging around, I discovered that we could, as shown in the example code, just get the body and pass it along. But that's not convenient if we want to, say, log some of the JSON. We can declare a request as as Dict and have it work, but List doesn't. And rummaging through Stack Overflow, I found that Any does work if it's done asAny = Body(None), which even after some study is still mysterious to me.

If people agree this is a bug, should I file it as an issue?

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.122.0

Pydantic Version

2.12.3

Python Version

Python 3.13.5

Additional Context

No response