fix: handle missing usage metadata on premature Anthropic stream termination

src/strands/models/anthropic.py

@@ -396,6 +396,7 @@ async def stream(
         Raises:
             ContextWindowOverflowException: If the input exceeds the model's context window.
             ModelThrottledException: If the request is throttled by Anthropic.
+            RuntimeError: If the stream ends before final usage metadata is available.
         """
         logger.debug("formatting request")
         request = self.format_request(messages, tool_specs, system_prompt, tool_choice)
@@ -405,11 +406,18 @@ async def stream(
         try:
             async with self.client.messages.stream(**request) as stream:
                 logger.debug("got response from model")
+                event = None
                 async for event in stream:
                     if event.type in AnthropicModel.EVENT_TYPES:
                         yield self.format_chunk(event.model_dump())
 
-                usage = event.message.usage  # type: ignore
+                if event is None:
+                    raise RuntimeError("Anthropic stream terminated before receiving any events")
+
+                usage = getattr(getattr(event, "message", None), "usage", None)
+                if usage is None:
+                    raise RuntimeError("Anthropic stream ended without usage metadata")
+
                 yield self.format_chunk({"type": "metadata", "usage": usage.model_dump()})
 
         except anthropic.RateLimitError as error:

tests/strands/models/test_anthropic.py

@@ -738,6 +738,52 @@ async def test_stream(anthropic_client, model, agenerator, alist):
     anthropic_client.messages.stream.assert_called_once_with(**expected_request)
 
 
+@pytest.mark.asyncio
+async def test_stream_premature_termination(anthropic_client, model, agenerator, alist):
+    """Test that stream fails clearly on premature termination.
+
+    When the Anthropic API stream ends before message_stop (e.g. network
+    timeout), the request should fail with a clear error instead of crashing
+    with AttributeError.
+
+    Regression test for #1868.
+    """
+    mock_event_1 = unittest.mock.Mock(
+        type="message_start",
+        model_dump=lambda: {"type": "message_start"},
+    )
+    # Last event has no .message attribute (simulating premature termination)
+    mock_event_2 = unittest.mock.Mock(
+        type="content_block_stop",
+        model_dump=lambda: {"type": "content_block_stop", "index": 0},
+        spec=["type", "model_dump"],
+    )
+
+    mock_context = unittest.mock.AsyncMock()
+    mock_context.__aenter__.return_value = agenerator([mock_event_1, mock_event_2])
+    anthropic_client.messages.stream.return_value = mock_context
+
+    messages = [{"role": "user", "content": [{"text": "hello"}]}]
+    response = model.stream(messages, None, None)
+
+    with pytest.raises(RuntimeError, match="without usage metadata"):
+        await alist(response)
+
+
+@pytest.mark.asyncio
+async def test_stream_empty_no_events(anthropic_client, model, agenerator, alist):
+    """Test that an empty stream fails clearly."""
+    mock_context = unittest.mock.AsyncMock()
+    mock_context.__aenter__.return_value = agenerator([])
+    anthropic_client.messages.stream.return_value = mock_context
+
+    messages = [{"role": "user", "content": [{"text": "hello"}]}]
+    response = model.stream(messages, None, None)
+
+    with pytest.raises(RuntimeError, match="before receiving any events"):
+        await alist(response)
+
+
 @pytest.mark.asyncio
 async def test_stream_rate_limit_error(anthropic_client, model, alist):
     anthropic_client.messages.stream.side_effect = anthropic.RateLimitError(