fix(bedrock): consume orphaned task exception on stream cancellation

When a consumer cancels, breaks from, or times out on BedrockModel.stream,
the internal asyncio.Task wrapping asyncio.to_thread is never awaited. If
boto3 eventually raises, asyncio emits 'Task exception was never retrieved'.

Add a done-callback on the unhappy path that retrieves the exception,
silencing the warning without interrupting the background thread.

Resolves: #2266

Author: AnnasMazhar

src/strands/models/bedrock.py

@@ -64,6 +64,12 @@ def _clear_unsupported_count_tokens_cache() -> None:
     _UNSUPPORTED_COUNT_TOKENS_MODELS.clear()
 
 
+def _suppress_task_exception(task: "asyncio.Task[None]") -> None:
+    """Consume exception from orphaned stream task to silence 'never retrieved' warning."""
+    if not task.cancelled():
+        task.exception()
+
+
 T = TypeVar("T", bound=BaseModel)
 
 DEFAULT_READ_TIMEOUT = 120
@@ -898,14 +904,17 @@ def callback(event: StreamEvent | None = None) -> None:
         thread = asyncio.to_thread(self._stream, callback, messages, tool_specs, system_prompt_content, tool_choice)
         task = asyncio.create_task(thread)
 
-        while True:
-            event = await queue.get()
-            if event is None:
-                break
-
-            yield event
-
-        await task
+        try:
+            while True:
+                event = await queue.get()
+                if event is None:
+                    break
+
+                yield event
+            await task
+        except BaseException:
+            task.add_done_callback(_suppress_task_exception)
+            raise
 
     def _stream(
         self,

tests/strands/models/test_bedrock.py

@@ -1,3 +1,4 @@
+import asyncio
 import copy
 import logging
 import os
@@ -20,6 +21,7 @@
     DEFAULT_BEDROCK_REGION,
     DEFAULT_READ_TIMEOUT,
     _clear_unsupported_count_tokens_cache,
+    _suppress_task_exception,
 )
 from strands.types.exceptions import ContextWindowOverflowException, ModelThrottledException
 from strands.types.tools import ToolSpec
@@ -3495,3 +3497,102 @@ async def test_skip_native_api_when_use_native_token_count_false(self, bedrock_c
         bedrock_client.count_tokens.assert_not_called()
         assert isinstance(result, int)
         assert result >= 0
+
+
+@pytest.mark.asyncio
+async def test_suppress_task_exception(bedrock_client, model, messages):
+    """_suppress_task_exception consumes exception from a failed task without re-raising."""
+
+    async def fail() -> None:
+        raise RuntimeError("inner task failure")
+
+    task = asyncio.create_task(fail())
+    await asyncio.sleep(0)  # let the task complete with exception
+
+    assert task.done()
+    assert task.exception() is not None
+
+    # Calling the helper should not raise — it simply retrieves the exception
+    _suppress_task_exception(task)
+
+
+@pytest.mark.asyncio
+async def test_suppress_task_exception_skips_cancelled():
+    """_suppress_task_exception is a no-op for cancelled tasks."""
+
+    async def hang() -> None:
+        await asyncio.sleep(999)
+
+    task = asyncio.create_task(hang())
+    task.cancel()
+    with pytest.raises(asyncio.CancelledError):
+        await task
+
+    # Should not raise — cancelled tasks are skipped
+    _suppress_task_exception(task)
+
+
+@pytest.mark.asyncio
+async def test_stream_break_does_not_leak_task_exception(bedrock_client, model, messages, caplog, alist):
+    """Breaking from an async-for on BedrockModel.stream must not leak the inner task's exception."""
+    caplog.set_level(logging.WARNING, logger="asyncio")
+
+    # Mock converse_stream to yield one event then raise — simulates e.g. ReadTimeoutError
+    # in the boto3 thread *after* the consumer has disconnected.
+
+    def stream_with_error():
+        yield {"messageStart": {"role": "assistant"}}
+        raise RuntimeError("simulated boto3 timeout after consumer disconnect")
+
+    bedrock_client.converse_stream.return_value = {"stream": stream_with_error()}
+
+    stream = model.stream(messages)
+    collected: list = []
+    async for event in stream:
+        collected.append(event)
+        break  # disconnect before the generator raises
+
+    # Let the event loop process the done-callback and the thread task
+    await asyncio.sleep(0.01)
+
+    # Verify we got the event before breaking
+    assert len(collected) == 1
+
+    # The critical assertion: no "Task exception was never retrieved" warning
+    assert "Task exception was never retrieved" not in caplog.text
+    # Also ensure no exception propagates to consumer
+    assert "exception was never retrieved" not in caplog.text.lower()
+
+
+@pytest.mark.asyncio
+async def test_stream_timeout_cancellation_does_not_leak(
+    bedrock_client,
+    model,
+    messages,
+    caplog,
+):
+    """Applying asyncio.wait_for on BedrockModel.stream must not leak the inner task's exception."""
+    caplog.set_level(logging.WARNING, logger="asyncio")
+
+    # Make converse_stream yield slowly so wait_for fires first
+    import time
+
+    def slow_stream():
+        time.sleep(0.05)  # simulate a slow network call
+        yield {"messageStart": {"role": "assistant"}}
+        time.sleep(0.05)
+        raise RuntimeError("boto3 timeout after consumer disconnected")
+
+    bedrock_client.converse_stream.return_value = {"stream": slow_stream()}
+
+    stream = model.stream(messages)
+    with pytest.raises(TimeoutError):
+        # Very short timeout — fires before the slow stream finishes
+        await asyncio.wait_for(stream.__anext__(), timeout=0.001)
+
+    # Let event loop settle
+    await asyncio.sleep(0.01)
+
+    # Critical: no orphaned-task warning
+    assert "Task exception was never retrieved" not in caplog.text
+    assert "exception was never retrieved" not in caplog.text.lower()