fix: handle BaseException in trace spans to prevent span leaks on Key…

src/strands/agent/agent.py

@@ -841,7 +841,7 @@ async def stream_async(
 
                     self._end_agent_trace_span(response=result)
 
-                except Exception as e:
+                except BaseException as e:
                     self._end_agent_trace_span(error=e)
                     raise
 
@@ -1044,7 +1044,7 @@ def _start_agent_trace_span(self, messages: Messages) -> trace_api.Span:
     def _end_agent_trace_span(
         self,
         response: AgentResult | None = None,
-        error: Exception | None = None,
+        error: BaseException | None = None,
     ) -> None:
         """Ends a trace span for the agent.
 

src/strands/event_loop/event_loop.py

@@ -138,6 +138,7 @@ async def event_loop_cycle(
         custom_trace_attributes=agent.trace_attributes,
     )
     invocation_state["event_loop_cycle_span"] = cycle_span
+    model_events: AsyncGenerator[TypedEvent, None] | None = None
 
     with trace_api.use_span(cycle_span, end_on_exit=False):
         try:
@@ -153,15 +154,21 @@ async def event_loop_cycle(
                 model_events = _handle_model_execution(
                     agent, cycle_span, cycle_trace, invocation_state, tracer, structured_output_context
                 )
-                async for model_event in model_events:
-                    if not isinstance(model_event, ModelStopReason):
-                        yield model_event
+                try:
+                    async for model_event in model_events:
+                        if not isinstance(model_event, ModelStopReason):
+                            yield model_event
+                finally:
+                    await model_events.aclose()
 
                 stop_reason, message, *_ = model_event["stop"]
                 yield ModelMessageEvent(message=message)
         except Exception as e:
             tracer.end_span_with_error(cycle_span, str(e), e)
             raise
+        except BaseException as e:
+            tracer.end_span_with_error(cycle_span, str(e), e)
+            raise
 
         try:
             if stop_reason == "max_tokens":
@@ -238,6 +245,9 @@ async def event_loop_cycle(
             yield ForceStopEvent(reason=e)
             logger.exception("cycle failed")
             raise EventLoopException(e, invocation_state["request_state"]) from e
+        except BaseException as e:
+            tracer.end_span_with_error(cycle_span, str(e), e)
+            raise
 
 
 async def recurse_event_loop(
@@ -323,6 +333,7 @@ async def _handle_model_execution(
             system_prompt=agent.system_prompt,
             system_prompt_content=agent._system_prompt_content,
         )
+        streamed_events: AsyncGenerator[TypedEvent, None] | None = None
         with trace_api.use_span(model_invoke_span, end_on_exit=False):
             try:
                 await agent.hooks.invoke_callbacks_async(
@@ -338,18 +349,22 @@ async def _handle_model_execution(
                 else:
                     tool_specs = agent.tool_registry.get_all_tool_specs()
 
-                async for event in stream_messages(
-                    agent.model,
-                    agent.system_prompt,
-                    agent.messages,
-                    tool_specs,
-                    system_prompt_content=agent._system_prompt_content,
-                    tool_choice=structured_output_context.tool_choice,
-                    invocation_state=invocation_state,
-                    model_state=agent._model_state,
-                    cancel_signal=agent._cancel_signal,
-                ):
-                    yield event
+                try:
+                    streamed_events = stream_messages(
+                        agent.model,
+                        agent.system_prompt,
+                        agent.messages,
+                        tool_specs,
+                        system_prompt_content=agent._system_prompt_content,
+                        tool_choice=structured_output_context.tool_choice,
+                        invocation_state=invocation_state,
+                        model_state=agent._model_state,
+                        cancel_signal=agent._cancel_signal,
+                    )
+                    async for event in streamed_events:
+                        yield event
+                finally:
+                    await streamed_events.aclose()
 
                 stop_reason, message, usage, metrics = event["stop"]
                 invocation_state.setdefault("request_state", {})
@@ -410,6 +425,9 @@ async def _handle_model_execution(
                 # No retry requested, raise the exception
                 yield ForceStopEvent(reason=e)
                 raise e
+            except BaseException as e:
+                tracer.end_span_with_error(model_invoke_span, str(e), e)
+                raise
 
     try:
         # Add message in trace and mark the end of the stream messages trace

src/strands/telemetry/tracer.py

@@ -184,7 +184,7 @@ def _end_span(
         self,
         span: Span,
         attributes: dict[str, AttributeValue] | None = None,
-        error: Exception | None = None,
+        error: BaseException | None = None,
         error_message: str | None = None,
     ) -> None:
         """Generic helper method to end a span.
@@ -224,7 +224,7 @@ def _end_span(
                 except Exception as e:
                     logger.warning("error=<%s> | failed to force flush tracer provider", e)
 
-    def end_span_with_error(self, span: Span, error_message: str, exception: Exception | None = None) -> None:
+    def end_span_with_error(self, span: Span, error_message: str, exception: BaseException | None = None) -> None:
         """End a span with error status.
 
         Args:
@@ -450,7 +450,9 @@ def start_tool_call_span(
 
         return span
 
-    def end_tool_call_span(self, span: Span, tool_result: ToolResult | None, error: Exception | None = None) -> None:
+    def end_tool_call_span(
+        self, span: Span, tool_result: ToolResult | None, error: BaseException | None = None
+    ) -> None:
         """End a tool call span with results.
 
         Args:
@@ -650,7 +652,7 @@ def end_agent_span(
         self,
         span: Span,
         response: AgentResult | None = None,
-        error: Exception | None = None,
+        error: BaseException | None = None,
     ) -> None:
         """End an agent span with results and metrics.
 

tests/strands/agent/test_agent.py

@@ -1423,6 +1423,27 @@ async def test_agent_stream_async_creates_and_ends_span_on_exception(mock_get_tr
     mock_tracer.end_agent_span.assert_called_once_with(span=mock_span, error=test_exception)
 
 
+@pytest.mark.asyncio
+@unittest.mock.patch("strands.agent.agent.get_tracer")
+async def test_agent_stream_async_creates_and_ends_span_on_base_exception(mock_get_tracer, mock_model, alist):
+    """Test that stream_async ends the agent span when a BaseException occurs."""
+    mock_tracer = unittest.mock.MagicMock()
+    mock_span = unittest.mock.MagicMock()
+    mock_tracer.start_agent_span.return_value = mock_span
+    mock_get_tracer.return_value = mock_tracer
+
+    test_exception = KeyboardInterrupt("stop now")
+    mock_model.mock_stream.side_effect = test_exception
+
+    agent = Agent(model=mock_model)
+
+    with pytest.raises(KeyboardInterrupt, match="stop now"):
+        stream = agent.stream_async("test prompt")
+        await alist(stream)
+
+    mock_tracer.end_agent_span.assert_called_once_with(span=mock_span, error=test_exception)
+
+
 def test_agent_init_with_state_object():
     agent = Agent(state=AgentState({"foo": "bar"}))
     assert agent.state.get("foo") == "bar"

tests/strands/event_loop/test_event_loop.py

@@ -685,6 +685,121 @@ async def test_event_loop_tracing_with_tool_execution(
     assert mock_tracer.end_model_invoke_span.call_count == 2
 
 
+@patch("strands.event_loop.event_loop.get_tracer")
+@pytest.mark.asyncio
+async def test_event_loop_cycle_closes_spans_on_stream_aclose(
+    mock_get_tracer,
+    agent,
+    model,
+    mock_tracer,
+):
+    mock_get_tracer.return_value = mock_tracer
+    cycle_span = MagicMock()
+    mock_tracer.start_event_loop_cycle_span.return_value = cycle_span
+    model_span = MagicMock()
+    mock_tracer.start_model_invoke_span.return_value = model_span
+
+    async def interrupted_stream():
+        yield {"contentBlockDelta": {"delta": {"text": "test text"}}}
+        await asyncio.sleep(10)
+        yield {"contentBlockStop": {}}
+
+    model.stream.return_value = interrupted_stream()
+
+    stream = strands.event_loop.event_loop.event_loop_cycle(
+        agent=agent,
+        invocation_state={},
+    )
+    await anext(stream)
+    await anext(stream)
+    await anext(stream)
+    await stream.aclose()
+
+    assert [call.args[0] for call in mock_tracer.end_span_with_error.call_args_list] == [model_span, cycle_span]
+    assert [call.args[1] for call in mock_tracer.end_span_with_error.call_args_list] == [
+        "",
+        "",
+    ]
+
+
+@patch("strands.event_loop.event_loop.get_tracer")
+@pytest.mark.asyncio
+async def test_event_loop_cycle_closes_spans_on_task_cancellation(
+    mock_get_tracer,
+    agent,
+    model,
+    mock_tracer,
+):
+    mock_get_tracer.return_value = mock_tracer
+    cycle_span = MagicMock()
+    mock_tracer.start_event_loop_cycle_span.return_value = cycle_span
+    model_span = MagicMock()
+    mock_tracer.start_model_invoke_span.return_value = model_span
+
+    blocked_on_stream = asyncio.Event()
+    release_stream = asyncio.Event()
+
+    async def interrupted_stream():
+        yield {"contentBlockDelta": {"delta": {"text": "test text"}}}
+        blocked_on_stream.set()
+        await release_stream.wait()
+        yield {"contentBlockStop": {}}
+
+    model.stream.return_value = interrupted_stream()
+
+    async def consume() -> None:
+        stream = strands.event_loop.event_loop.event_loop_cycle(
+            agent=agent,
+            invocation_state={},
+        )
+        async for _ in stream:
+            pass
+
+    task = asyncio.create_task(consume())
+    await blocked_on_stream.wait()
+    task.cancel()
+
+    with pytest.raises(asyncio.CancelledError):
+        await task
+
+    assert [call.args[0] for call in mock_tracer.end_span_with_error.call_args_list] == [model_span, cycle_span]
+    assert [call.args[1] for call in mock_tracer.end_span_with_error.call_args_list] == [
+        "",
+        "",
+    ]
+
+
+@patch("strands.event_loop.event_loop.get_tracer")
+@pytest.mark.asyncio
+async def test_event_loop_cycle_closes_spans_on_keyboard_interrupt(
+    mock_get_tracer,
+    agent,
+    model,
+    mock_tracer,
+    alist,
+):
+    mock_get_tracer.return_value = mock_tracer
+    cycle_span = MagicMock()
+    mock_tracer.start_event_loop_cycle_span.return_value = cycle_span
+    model_span = MagicMock()
+    mock_tracer.start_model_invoke_span.return_value = model_span
+
+    test_exception = KeyboardInterrupt("stop now")
+    model.stream.side_effect = test_exception
+
+    with pytest.raises(KeyboardInterrupt, match="stop now"):
+        stream = strands.event_loop.event_loop.event_loop_cycle(
+            agent=agent,
+            invocation_state={},
+        )
+        await alist(stream)
+
+    assert mock_tracer.end_span_with_error.call_args_list == [
+        call(model_span, "stop now", test_exception),
+        call(cycle_span, "stop now", test_exception),
+    ]
+
+
 @pytest.mark.asyncio
 async def test_event_loop_cycle_closes_cycle_span_before_recursive_cycle(
     agent,

tests/strands/telemetry/test_tracer.py

@@ -140,6 +140,18 @@ def test_end_span_with_empty_exception_message_uses_exception_name(mock_span):
     mock_span.end.assert_called_once()
 
 
+def test_end_span_with_empty_base_exception_message_uses_exception_name(mock_span):
+    """Test that empty BaseException messages fall back to the exception type name."""
+    tracer = Tracer()
+    error = KeyboardInterrupt()
+
+    tracer.end_span_with_error(mock_span, "", error)
+
+    mock_span.set_status.assert_called_once_with(StatusCode.ERROR, "KeyboardInterrupt")
+    mock_span.record_exception.assert_called_once_with(error)
+    mock_span.end.assert_called_once()
+
+
 def test_end_span_with_error_prefers_explicit_message(mock_span):
     """Test that an explicit error message takes precedence over the exception text."""
     tracer = Tracer()
@@ -1162,6 +1174,30 @@ def test_force_flush_with_error(mock_span, mock_get_tracer_provider):
     mock_tracer_provider.force_flush.assert_called_once()
 
 
+def test_end_agent_span_with_empty_error_message_uses_exception_name(mock_span):
+    """Test that agent spans fall back to the exception type name for empty errors."""
+    tracer = Tracer()
+    error = Exception()
+
+    tracer.end_agent_span(mock_span, error=error)
+
+    mock_span.set_status.assert_called_once_with(StatusCode.ERROR, "Exception")
+    mock_span.record_exception.assert_called_once_with(error)
+    mock_span.end.assert_called_once()
+
+
+def test_end_tool_call_span_with_empty_error_message_uses_exception_name(mock_span):
+    """Test that tool call spans fall back to the exception type name for empty errors."""
+    tracer = Tracer()
+    error = Exception()
+
+    tracer.end_tool_call_span(mock_span, None, error=error)
+
+    mock_span.set_status.assert_called_once_with(StatusCode.ERROR, "Exception")
+    mock_span.record_exception.assert_called_once_with(error)
+    mock_span.end.assert_called_once()
+
+
 def test_end_tool_call_span_with_none(mock_span):
     """Test ending a tool call span with None result."""
     tracer = Tracer()