In Python 3.14, annotation evaluation is stricter than before, so when FastAPI inspects the annotation Foo | None, Python attempts to resolve Foo immediately and raises a NameError because the symbol is missing at runtime.

This is not exactly true. Python 3.14 can handle this case using FORWARDREF. And from my understanding FastAPI doesn't require the type of "foo" when building the dependency graph
In python 3.13, I used "Foo | None" instead of Foo | None and everything worked well. FastAPI doesn't need to resolve "Foo"

Newer Python versions do improve handling of forward references, but that doesn’t remove the requirement that referenced names must exist once a framework actually resolves annotations. FastAPI doesn’t use annotations only for static typing... it inspects and resolves them at runtime to build the dependency graph, perform validation, and generate OpenAPI schemas. When that resolution happens (e.g. via typing.get_type_hints or similar), the annotation is expected to map to a real runtime object unless the framework explicitly keeps it unresolved.

If a type is defined only under if TYPE_CHECKING:, it never exists at runtime, so when FastAPI reaches the point where it needs a concrete type instead of a string, there’s nothing to resolve and a NameError is expected. from __future__ import annotations can postpone evaluation. Still, it doesn’t change this underlying requirement.

This is what I found. You can go through it once... maybe that will clear most of your doubts:

• http://fastapi.tiangolo.com/tutorial/dependencies/sub-dependencies/#use-the-dependency
• get_type_hints() always raises on forward references python/typing#508

In python 3.13, I used "Foo | None" instead of Foo | None and everything worked well. FastAPI doesn't need to resolve "Foo"

That worked in Python 3.13 because "Foo | None" is a string annotation and FastAPI didn’t hit a code path that required resolving it at runtime. It doesn’t mean FastAPI never needs to resolve Foo. Depending on dependency inspection, validation, or OpenAPI generation, FastAPI may later resolve annotations (e.g. via typing.get_type_hints), at which point Foo must exist at runtime. So this was an implementation detail that happened to work, not a guarantee that Foo never needs to be resolved.

For many reasons (circular imports, costly imports) I can not remove if TYPE_CHECKING in this specific case

I agree that it's not always possible to avoid if TYPE_CHECKING:..

In python 3.13, I used "Foo | None" instead of Foo | None and everything worked well. FastAPI doesn't need to resolve "Foo"

I just tried converting type annotations to strings and running code on Python 3.14 and it works:

from typing import TYPE_CHECKING
from fastapi import FastAPI, Depends

if TYPE_CHECKING:

    class Foo: ...


app = FastAPI()


def get_foo() -> "Foo | None":
    return None


@app.get("/")
def read_root(foo: "Foo | None" = Depends(get_foo)):
    return {"Hello": "World"}

(fastapi) yurii@yurii-laptop:~/code/fastapi$ python --version
Python 3.14.2
(fastapi) yurii@yurii-laptop:~/code/fastapi$ fastapi dev -e t:app

   FastAPI   Starting development server 🚀
 
             Searching for package file structure from directories with __init__.py files
             Importing from /home/yurii/code/fastapi
 
      code   Importing the FastAPI app object from the module with the following code:
 
             from t import app
 
       app   Using import string: t:app
 
    server   Server started at http://127.0.0.1:8000
    server   Documentation at http://127.0.0.1:8000/docs
 
       tip   Running in development mode, for production use: fastapi run
 
             Logs:
 
      INFO   Will watch for changes in these directories: ['/home/yurii/code/fastapi']
      INFO   Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
      INFO   Started reloader process [23579] using WatchFiles
      INFO   Started server process [23594]
      INFO   Waiting for application startup.
      INFO   Application startup complete.
      INFO   127.0.0.1:50464 - "GET / HTTP/1.1" 200

Sure, thank you for taking the time to test it.