fix: prevent GeneratedAnswer.from_dict crash on empty all_messages

[i-anubhav-anand]

Related Issues

Self-found bug (no existing issue). GeneratedAnswer.from_dict crashes with IndexError when the serialized meta contains an empty all_messages list — which to_dict itself can emit, so a plain round-trip crashes.

Proposed Changes:

GeneratedAnswer.from_dict guarded the metadata branch with is not None:

if (all_messages := meta.get("all_messages")) is not None and isinstance(all_messages[0], dict):

An empty list [] is not None, so it passes the guard and the immediate all_messages[0] access raises IndexError: list index out of range. to_dict already uses the correct truthiness guard (if all_messages and isinstance(all_messages[0], ChatMessage)), which leaves an empty all_messages untouched in the output — so to_dict → from_dict round-trips into the crash.

The fix aligns from_dict with to_dict by using the same truthiness guard, so an empty all_messages list round-trips correctly instead of crashing.

Reproduction (before the fix):

from haystack.dataclasses.answer import GeneratedAnswer
a = GeneratedAnswer(data="ans", query="q", documents=[], meta={"all_messages": []})
GeneratedAnswer.from_dict(a.to_dict())  # IndexError: list index out of range

How did you test it?

Added two unit tests in test/dataclasses/test_answer.py:

• test_from_dict_with_empty_all_messages — deserializing meta={"all_messages": []} no longer raises.
• test_to_dict_from_dict_round_trip_with_empty_all_messages — full round-trip equality.

Both fail on main (IndexError) and pass with this change. Ran hatch run test:unit test/dataclasses/test_answer.py (20 passed), hatch run fmt (clean), and hatch run test:types haystack/dataclasses/answer.py (mypy clean). Added a release note under releasenotes/notes.

Notes for the reviewer

One-line behavior fix; the existing non-empty / ChatMessage / string all_messages paths are unchanged and still covered by the existing tests.

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have added unit tests and updated the docstrings.
• I've used a conventional commit type for my PR title (fix:).
• I have added a release note file.
• I have run pre-commit hooks / hatch run fmt and fixed any issue.

This PR was generated with the help of an AI assistant. I have reviewed the changes, reproduced the bug, and run the relevant tests locally.

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/dataclasses
answer.py
Project Total

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
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[davidsbatista]

Looks good! Thanks @i-anubhav-anand for the contribution