fix(types): distinguish a legitimate None result from never-set in streaming

[chuenchen309]

AI-generated contribution disclosure

I used an AI coding assistant (Claude) to help identify this bug and draft the fix. I reviewed the diff, wrote/verified the regression test in both red and green states, traced every other read site of the affected field to check for related issues, and ran the full local test suite + ruff + mypy before opening this PR. Per CONTRIBUTING.md I understand this should carry the llm-generated label — as an external contributor I don't have permission to apply labels myself, so flagging it here explicitly; happy for a maintainer to add it.

Problem

StreamingOutputBase (the base class behind FlowStreamingOutput/CrewStreamingOutput) initializes self._result to None and treats None as "no result was ever set":

if self._result is None:
    raise RuntimeError("No result available")

This misfires whenever the actual streamed result legitimately is None — e.g. a Flow whose final method has no return value — even though streaming completed successfully with no error.

The sibling StreamSessionBase class in the same file already solves exactly this problem correctly, using a module-level _MISSING = object() sentinel instead of comparing to None.

Fix

Apply the same _MISSING sentinel pattern to StreamingOutputBase. While tracing every read site of self._result in the file to make sure I wasn't missing a related spot, I found one more: CrewStreamingOutput.results' fallback branch (if self._result is not None: return [self._result]) made the same None-means-unset assumption and needed the same fix, or it would have silently broken (returning [_MISSING]) once the default changed.

Testing

• Added test_result_can_legitimately_be_none to TestFlowStreamingOutput in tests/test_streaming.py, confirming .result returns None (rather than raising) after _set_result(None). Verified it fails against the pre-fix code and passes after.
• Ran the full tests/test_streaming.py suite (38 passed).
• ruff check/ruff format --check clean; mypy clean on the changed file.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: aa3744cb-431f-4c47-bbb9-0224f1547bc3

📥 Commits

Reviewing files that changed from the base of the PR and between e460c4a and 345b198.

📒 Files selected for processing (2)

• lib/crewai/src/crewai/types/streaming.py
• lib/crewai/tests/test_streaming.py

🚧 Files skipped from review as they are similar to previous changes (2)

• lib/crewai/src/crewai/types/streaming.py
• lib/crewai/tests/test_streaming.py

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📝 Walkthrough

Walkthrough

Streaming outputs now distinguish an unset result from a completed result set to None, and tests cover accessing a legitimate None flow result after streaming.

Changes

Streaming result state

Layer / File(s)	Summary
Sentinel-based result handling lib/crewai/src/crewai/types/streaming.py, lib/crewai/tests/test_streaming.py	StreamingOutputBase uses _MISSING for unset results, while result access and CrewStreamingOutput.results recognize explicitly set None values. A flow streaming test verifies that behavior.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main fix: distinguishing a real None streaming result from an unset result.
Description check	✅ Passed	The description accurately explains the bug, fix, and regression test and matches the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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