fix: serialize nested chat generators via component_to_dict in FallbackChatGenerator

[rautaditya2606]

Summary

Fixes #11846.

Proposed Changes

• Updated FallbackChatGenerator.to_dict() in haystack/components/generators/chat/fallback.py to use component_to_dict() instead of directly calling .to_dict() on nested chat generators.
• Added unit tests in test_fallback.py covering:
  • serialization/deserialization of mixed fallback chains (components with and without explicit to_dict() methods),
  • preservation of fallback ordering after a serialization round-trip,
  • successful round-trip execution after deserialization,
  • fail-loud behavior for non-serializable components.
• Added release notes describing the serialization fix.

How did you test it?

hatch run test:unit test/components/generators/chat/test_fallback.py

Result:

26 passed, 2 warnings

Notes for the reviewer

Root cause

FallbackChatGenerator serialized its nested chat generators using:

chat_generators=[
    gen.to_dict()
    for gen in self.chat_generators
    if hasattr(gen, "to_dict")
]

This bypassed Haystack's standard serialization helper (component_to_dict()), which supports components that do not implement explicit to_dict()/from_dict() methods by falling back to constructor-based serialization. As a result, otherwise serializable components were silently omitted from the serialized representation.

Reproducer

from haystack import component
from haystack.components.generators.chat.fallback import FallbackChatGenerator
from haystack.dataclasses import ChatMessage


@component
class CustomGeneratorWithoutSerDe:
    def __init__(self, text: str = "custom_ok"):
        self.text = text

    def run(self, messages, **kwargs):
        return {"replies": [ChatMessage.from_assistant(self.text)]}


original = FallbackChatGenerator(
    chat_generators=[CustomGeneratorWithoutSerDe()]
)

data = original.to_dict()

# Before fix:
# data["init_parameters"]["chat_generators"] == []

# After fix:
# data["init_parameters"]["chat_generators"] contains the serialized component.

Precedent

This change follows the same serialization pattern already used by Agent.to_dict(), which serializes nested components through component_to_dict().

Checklist

• Read contributors guidelines and code of conduct
• Updated related issue ([bug]: FallbackChatGenerator silently omits serializable chat generators without explicit to_dict() #11846)
• Added unit tests and updated docstrings
• Used a conventional commit type in the PR title
• Documented the code
• Added a release note (releasenotes/notes/fix_fallback_chat_generator_serialization-9498ee2bbe2971be.yaml)
• Ran pre-commit hooks and fixed any issues

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Pull request overview

Fixes FallbackChatGenerator nested-component serialization so custom chat generators that rely on Haystack’s default constructor-based SerDe are no longer silently dropped during to_dict() / from_dict() round-trips.

Changes:

• Updated FallbackChatGenerator.to_dict() to serialize nested generators via component_to_dict() (instead of filtering on hasattr(..., "to_dict")).
• Added unit tests covering mixed fallback chains, order preservation, round-trip execution, and fail-loud behavior for non-serializable components.
• Added a release note documenting the serialization fix.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
haystack/components/generators/chat/fallback.py	Uses component_to_dict() for nested generator serialization, preventing silent omission of otherwise-serializable components.
test/components/generators/chat/test_fallback.py	Adds regression tests for serialization/deserialization correctness and explicit failure on non-serializable init parameters.
releasenotes/notes/fix_fallback_chat_generator_serialization-9498ee2bbe2971be.yaml	Documents the behavioral fix in release notes.

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/components/generators/chat
fallback.py
Project Total

This report was generated by python-coverage-comment-action

[sjrl]

Please move the import to the top of the file

[rautaditya2606]

done, i've moved the import to the top of the file
thanks

[sjrl]

Looks good! Thanks for the contribution!