fix: Fix appending None to a list in State when the source key is missing

[sjrl]

Related Issues

• fixes Agent state corrupted with None entries when outputs_to_state source key absent from tool result #10981

Proposed Changes:

Fix ToolInvoker._merge_tool_outputs silently appending None to list-typed state when a tool's outputs_to_state source key is absent from the tool result. This is a common scenario with PipelineTool wrapping a pipeline that has conditional branches where not all outputs are always produced even if defined in outputs_to_state. The mapping is now skipped entirely when the source key is not present in the result dict.

How did you test it?

Added a new unit test based on what was described in the issue

Notes for the reviewer

I decided not to update merge_lists since the fix in ToolInvoker prevents None from reaching merge_lists. It's also possible that None is an intended to be passed to merge_lists which is still allowed if the source key is present in the tool dict and the value it points to is None.

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes.
• I have added unit tests and updated the docstrings.
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I have documented my code.
• I have added a release note file, following the contributors guidelines.
• I have run pre-commit hooks and fixed any issue.

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
haystack-docs	Ignored		Mar 31, 2026 7:14am

[coveralls]

Pull Request Test Coverage Report for Build 23785192769

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.001%) to 92.9%

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Files with Coverage Reduction	New Missed Lines	%
components/tools/tool_invoker.py	4	97.11%

Totals
Change from base Build 23754030523:	0.001%
Covered Lines:	15872
Relevant Lines:	17085

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💛 - Coveralls

[julian-risch]

Looks good to me! 👍 Explanation of what caused the issue and how we fixed is thorough. This PR unblocks the 2.27 release. Thanks for addressing the issue so quickly.