diff --git a/storey/flow.py b/storey/flow.py
index 6c5091d2..5c405cde 100644
--- a/storey/flow.py
+++ b/storey/flow.py
@@ -1728,6 +1728,20 @@ def __init__(self, runnable_name: str, data: Any, runtime: float, timestamp: dat
         self.timestamp = timestamp
 
 
+class _StreamingResult:
+    """Wraps a streaming generator with timing metadata for model monitoring."""
+
+    def __init__(
+        self,
+        runnable_name: str,
+        generator: Generator | AsyncGenerator,
+        timestamp: datetime.datetime,
+    ):
+        self.runnable_name = runnable_name
+        self.generator = generator
+        self.timestamp = timestamp
+
+
 class ParallelExecutionMechanisms(str, enum.Enum):
     process_pool = "process_pool"
     dedicated_process = "dedicated_process"
@@ -1840,9 +1854,9 @@ def _run(self, body: Any, path: str, origin_name: Optional[str] = None) -> Any:
         start = time.monotonic()
         try:
             result = self.run(body, path, origin_name)
-            # Return generator directly for streaming support
+            # Return streaming result with timing metadata for streaming support
             if _is_generator(result):
-                return result
+                return _StreamingResult(origin_name or self.name, result, timestamp)
             body = result
         except Exception as e:
             if self._raise_exception:
@@ -1858,9 +1872,9 @@ async def _async_run(self, body: Any, path: str, origin_name: Optional[str] = No
         try:
             result = self.run_async(body, path, origin_name)
 
-            # Return generator directly for streaming support
+            # Return streaming result with timing metadata for streaming support
             if _is_generator(result):
-                return result
+                return _StreamingResult(origin_name or self.name, result, timestamp)
 
             # Await if coroutine
             if asyncio.iscoroutine(result):
@@ -1902,7 +1916,10 @@ def _streaming_run_wrapper(
     sending each chunk through the multiprocessing queue.
     """
     try:
-        for chunk in runnable._run(input, path, origin_name):
+        result = runnable._run(input, path, origin_name)
+        # Unwrap _StreamingResult to get the generator
+        generator = result.generator if isinstance(result, _StreamingResult) else result
+        for chunk in generator:
             queue.put(("chunk", chunk))
         queue.put(("done", None))
     except Exception as e:
@@ -2291,15 +2308,33 @@ async def _do(self, event):
         # Check for streaming response (only when a single runnable is selected)
         if len(runnables) == 1 and results:
             result = results[0]
-            # Check if the result is a generator (streaming response)
-            if _is_generator(result):
+            # Check if the result is a streaming result (contains generator with timing metadata)
+            if isinstance(result, _StreamingResult):
+                # Set timing metadata on the event before emitting chunks
+                # For streaming, microsec is None since we don't have total runtime
+                metadata = {
+                    "microsec": None,
+                    "when": result.timestamp.isoformat(sep=" ", timespec="microseconds"),
+                }
+                self.set_event_metadata(event, metadata)
+                await self._emit_streaming_chunks(event, result.generator)
+                return None
+            # Handle raw generator from process-based streaming (no timing info from subprocess)
+            elif _is_generator(result):
+                # Use current timestamp as fallback for process-based streaming
+                timestamp = datetime.datetime.now(tz=datetime.timezone.utc)
+                metadata = {
+                    "microsec": None,
+                    "when": timestamp.isoformat(sep=" ", timespec="microseconds"),
+                }
+                self.set_event_metadata(event, metadata)
                 await self._emit_streaming_chunks(event, result)
                 return None
 
         # Non-streaming path
-        # Check if any results are generators (not allowed with multiple runnables)
+        # Check if any results are streaming (not allowed with multiple runnables)
         for result in results:
-            if _is_generator(result):
+            if isinstance(result, _StreamingResult):
                 raise StreamingError(
                     "Streaming is not supported when multiple runnables are selected. "
                     "Streaming runnables must be the only runnable selected for an event."
diff --git a/storey/steps/collector.py b/storey/steps/collector.py
index 3f0b1ba4..76a67d57 100644
--- a/storey/steps/collector.py
+++ b/storey/steps/collector.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 #
 import copy
+import datetime
 from collections import defaultdict
 
 from ..dtypes import StreamCompletion, _termination_obj
@@ -46,6 +47,31 @@ def __init__(self, expected_completions: int = 1, **kwargs):
             lambda: {"chunks": [], "completions": 0, "first_event": None}
         )
 
+    def _calculate_streaming_duration(self, event):
+        """
+        Calculate total streaming duration and update event metadata with microsec.
+
+        Uses the 'when' timestamp from the first chunk's metadata (set by ParallelExecution)
+        to calculate total elapsed time from stream start to completion.
+
+        Streaming is only supported with a single selected runnable, so metadata is always
+        flat (top-level 'when' and 'microsec'), never nested under model names.
+        """
+        if not hasattr(event, "_metadata") or not event._metadata:
+            return
+
+        when_str = event._metadata.get("when")
+        if not when_str:
+            return
+
+        try:
+            start_time = datetime.datetime.fromisoformat(when_str)
+            now = datetime.datetime.now(tz=datetime.timezone.utc)
+            event._metadata["microsec"] = int((now - start_time).total_seconds() * 1_000_000)
+        except (ValueError, TypeError) as exc:
+            if self.logger:
+                self.logger.warning(f"Failed to calculate streaming duration from 'when' timestamp '{when_str}': {exc}")
+
     async def _do(self, event):
         if event is _termination_obj:
             return await self._do_downstream(_termination_obj)
@@ -73,6 +99,10 @@ async def _do(self, event):
                     del collected_event.streaming_step
                 if hasattr(collected_event, "chunk_id"):
                     del collected_event.chunk_id
+
+                # Calculate total streaming duration (microsec) if timing metadata exists
+                self._calculate_streaming_duration(collected_event)
+
                 await self._do_downstream(collected_event)
 
                 # Clean up
diff --git a/tests/test_streaming.py b/tests/test_streaming.py
index 84dee94b..35667ebe 100644
--- a/tests/test_streaming.py
+++ b/tests/test_streaming.py
@@ -1084,6 +1084,32 @@ def stream_chunks(x):
 
         asyncio.run(_test())
 
+    def test_streaming_with_multiple_runnables_raises_error(self):
+        """Test that streaming raises an error when multiple runnables are selected."""
+        streaming = StreamingRunnable(name="streamer")
+        non_streaming = NonStreamingRunnable(name="non_streamer")
+
+        controller = build_flow(
+            [
+                SyncEmitSource(),
+                ParallelExecution(
+                    runnables=[streaming, non_streaming],
+                    execution_mechanism_by_runnable_name={
+                        "streamer": ParallelExecutionMechanisms.naive,
+                        "non_streamer": ParallelExecutionMechanisms.naive,
+                    },
+                ),
+                Reduce([], lambda acc, x: acc + [x]),
+            ]
+        ).run()
+
+        try:
+            controller.emit("test")
+        finally:
+            controller.terminate()
+            with pytest.raises(StreamingError, match="Streaming is not supported when multiple runnables are selected"):
+                controller.await_termination()
+
 
 class TestStreamingWithIntermediateSteps:
     """Tests for streaming through intermediate non-streaming steps."""
@@ -1338,6 +1364,44 @@ def test_parallel_execution_streaming_error_propagation(self, execution_mechanis
             with pytest.raises(expected_error, match="Simulated streaming error"):
                 controller.await_termination()
 
+    def test_parallel_execution_streaming_single_runnable_sets_metadata(self):
+        """Test that streaming ParallelExecution with single runnable sets timing metadata.
+
+        This mirrors the non-streaming behavior where _metadata includes 'when' and 'microsec'.
+        After Collector aggregates chunks, the collected event should have timing metadata.
+        The 'microsec' field should contain the total streaming duration calculated by Collector.
+        """
+        runnable = StreamingRunnable(name="streamer")
+        controller = build_flow(
+            [
+                SyncEmitSource(),
+                ParallelExecution(
+                    runnables=[runnable],
+                    execution_mechanism_by_runnable_name={"streamer": ParallelExecutionMechanisms.naive},
+                ),
+                Collector(),
+                Reduce([], lambda acc, x: acc + [x], full_event=True),
+            ]
+        ).run()
+
+        try:
+            controller.emit("test")
+        finally:
+            controller.terminate()
+            result = controller.await_termination()
+
+        assert len(result) == 1
+        event = result[0]
+        assert hasattr(event, "_metadata"), "Expected event to have _metadata attribute"
+        metadata = event._metadata
+        assert "when" in metadata, "Expected _metadata to include 'when' field"
+        assert "microsec" in metadata, "Expected _metadata to include 'microsec' field"
+        # Verify 'when' is a valid ISO timestamp string
+        assert isinstance(metadata["when"], str), "Expected 'when' to be a string"
+        # Verify 'microsec' is a positive integer (total streaming duration calculated by Collector)
+        assert isinstance(metadata["microsec"], int), "Expected 'microsec' to be an integer"
+        assert metadata["microsec"] >= 0, "Expected 'microsec' to be non-negative"
+
 
 class TestStreamingGraphSplits:
     """Tests for streaming through branching graph topologies."""