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

StreamingOutputBase (base for FlowStreamingOutput/CrewStreamingOutput)
initialized self._result to None and treated None as "not set" when
raising RuntimeError("No result available"). This misfires whenever the
actual result legitimately is None -- e.g. a Flow whose final method
returns nothing -- even though streaming completed successfully with no
error.

The sibling StreamSessionBase class in the same file already solves this
correctly with a module-level _MISSING sentinel. Apply the same pattern
to StreamingOutputBase, and fix the one other call site that assumed
None-means-unset (CrewStreamingOutput.results' fallback to self._result).

Added a regression test confirming FlowStreamingOutput.result returns
None (rather than raising) when _set_result(None) was called. Confirmed
it fails against the pre-fix code and passes after. Ran the full
test_streaming.py suite (38 passed), ruff check/format, and mypy (clean).

Disclosure: I used an AI coding assistant (Claude) to help identify this
bug and draft the fix. I independently traced every other read site of
self._result in this file to catch a second place (CrewStreamingOutput
.results) that also assumed None-means-unset and would have silently
broken had I only patched the reported location.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: chuenchen309

lib/crewai/src/crewai/types/streaming.py

@@ -342,7 +342,7 @@ def __init__(
         async_iterator: AsyncIterator[StreamChunk] | None = None,
     ) -> None:
         """Initialize streaming output base."""
-        self._result: T | None = None
+        self._result: T | object = _MISSING
         self._completed: bool = False
         self._chunks: list[StreamChunk] = []
         self._error: Exception | None = None
@@ -370,9 +370,9 @@ def result(self) -> T:
             )
         if self._error is not None:
             raise self._error
-        if self._result is None:
+        if self._result is _MISSING:
             raise RuntimeError("No result available")
-        return self._result
+        return self._result  # type: ignore[return-value]
 
     @property
     def is_completed(self) -> bool:
@@ -553,8 +553,8 @@ def results(self) -> list[CrewOutput]:
             raise self._error
         if self._results is not None:
             return self._results
-        if self._result is not None:
-            return [self._result]
+        if self._result is not _MISSING:
+            return [self._result]  # type: ignore[list-item]
         raise RuntimeError("No results available")
 
     def _set_results(self, results: list[CrewOutput]) -> None:

lib/crewai/tests/test_streaming.py

@@ -179,6 +179,18 @@ def simple_gen() -> Generator[StreamChunk, None, None]:
         list(streaming)
         assert streaming.is_completed is True
 
+    def test_result_can_legitimately_be_none(self) -> None:
+        """Test that a flow result that is legitimately None (e.g. a final
+        method with no return value) does not raise, distinguishing 'never
+        set' from 'set to None'."""
+
+        def empty_gen() -> Generator[StreamChunk, None, None]:
+            yield StreamChunk(content="test")
+
+        streaming = FlowStreamingOutput(sync_iterator=empty_gen())
+        streaming._set_result(None)
+        assert streaming.result is None
+
 
 class TestCrewKickoffStreaming:
     """Tests for Crew(stream=True).kickoff() method."""