fix: handle premature stream termination for Anthropic (#1868)

Author: gsird

src/strands/models/anthropic.py

@@ -409,8 +409,12 @@ async def stream(
                     if event.type in AnthropicModel.EVENT_TYPES:
                         yield self.format_chunk(event.model_dump())
 
-                usage = event.message.usage  # type: ignore
-                yield self.format_chunk({"type": "metadata", "usage": usage.model_dump()})
+                try:
+                    message_snapshot = await stream.get_final_message()
+                except AssertionError as e:
+                    logger.warning("error=<%s> | failed to retrieve message snapshot, usage metadata unavailable", e)
+                else:
+                    yield self.format_chunk({"type": "metadata", "usage": message_snapshot.usage.model_dump()})
 
         except anthropic.RateLimitError as error:
             raise ModelThrottledException(str(error)) from error

tests/strands/models/test_anthropic.py

@@ -52,6 +52,24 @@ class TestOutputModel(pydantic.BaseModel):
     return TestOutputModel
 
 
+def generate_mock_stream_context(events, final_message=None):
+    mock_stream = unittest.mock.AsyncMock()
+
+    async def mock_aiter(self):
+        for event in events:
+            yield event
+
+    mock_stream.__aiter__ = mock_aiter
+    if isinstance(final_message, Exception):
+        mock_stream.get_final_message.side_effect = final_message
+    elif final_message:
+        mock_stream.get_final_message.return_value = final_message
+
+    mock_context = unittest.mock.AsyncMock()
+    mock_context.__aenter__.return_value = mock_stream
+    return mock_context
+
+
 def test__init__model_configs(anthropic_client, model_id, max_tokens):
     _ = anthropic_client
 
@@ -692,7 +710,7 @@ def test_format_chunk_unknown(model):
 
 
 @pytest.mark.asyncio
-async def test_stream(anthropic_client, model, agenerator, alist):
+async def test_stream(anthropic_client, model, alist):
     mock_event_1 = unittest.mock.Mock(
         type="message_start",
         dict=lambda: {"type": "message_start"},
@@ -713,9 +731,14 @@ async def test_stream(anthropic_client, model, agenerator, alist):
         ),
     )
 
-    mock_context = unittest.mock.AsyncMock()
-    mock_context.__aenter__.return_value = agenerator([mock_event_1, mock_event_2, mock_event_3])
-    anthropic_client.messages.stream.return_value = mock_context
+    anthropic_client.messages.stream.return_value = generate_mock_stream_context(
+        [mock_event_1, mock_event_2, mock_event_3],
+        final_message=unittest.mock.Mock(
+            usage=unittest.mock.Mock(
+                model_dump=lambda: {"input_tokens": 1, "output_tokens": 2},
+            )
+        ),
+    )
 
     messages = [{"role": "user", "content": [{"text": "hello"}]}]
     response = model.stream(messages, None, None)
@@ -738,6 +761,42 @@ 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_early_termination(anthropic_client, model, alist, caplog):
+    caplog.set_level(logging.WARNING, logger="strands.models.anthropic")
+    mock_event = unittest.mock.Mock(
+        type="message_start",
+        model_dump=lambda: {"type": "message_start"},
+    )
+
+    anthropic_client.messages.stream.return_value = generate_mock_stream_context(
+        [mock_event],
+        final_message=AssertionError("message snapshot is not available"),
+    )
+
+    messages = [{"role": "user", "content": [{"text": "hello"}]}]
+    tru_events = await alist(model.stream(messages, None, None))
+
+    assert len(tru_events) == 1
+    assert "messageStart" in tru_events[0]
+    assert "failed to retrieve message snapshot, usage metadata unavailable" in caplog.text
+
+
+@pytest.mark.asyncio
+async def test_stream_empty(anthropic_client, model, alist, caplog):
+    caplog.set_level(logging.WARNING, logger="strands.models.anthropic")
+    anthropic_client.messages.stream.return_value = generate_mock_stream_context(
+        [],
+        final_message=AssertionError("message snapshot is not available"),
+    )
+
+    messages = [{"role": "user", "content": [{"text": "hello"}]}]
+    tru_events = await alist(model.stream(messages, None, None))
+
+    assert tru_events == []
+    assert "failed to retrieve message snapshot, usage metadata unavailable" in caplog.text
+
+
 @pytest.mark.asyncio
 async def test_stream_rate_limit_error(anthropic_client, model, alist):
     anthropic_client.messages.stream.side_effect = anthropic.RateLimitError(
@@ -780,7 +839,7 @@ async def test_stream_bad_request_error(anthropic_client, model):
 
 
 @pytest.mark.asyncio
-async def test_structured_output(anthropic_client, model, test_output_model_cls, agenerator, alist):
+async def test_structured_output(anthropic_client, model, test_output_model_cls, alist):
     messages = [{"role": "user", "content": [{"text": "Generate a person"}]}]
 
     events = [
@@ -815,18 +874,16 @@ async def test_structured_output(anthropic_client, model, test_output_model_cls,
                 return_value={"type": "message_stop", "message": {"stop_reason": "tool_use"}}
             ),
         ),
-        unittest.mock.Mock(
-            message=unittest.mock.Mock(
-                usage=unittest.mock.Mock(
-                    model_dump=unittest.mock.Mock(return_value={"input_tokens": 0, "output_tokens": 0})
-                ),
-            ),
-        ),
     ]
 
-    mock_context = unittest.mock.AsyncMock()
-    mock_context.__aenter__.return_value = agenerator(events)
-    anthropic_client.messages.stream.return_value = mock_context
+    anthropic_client.messages.stream.return_value = generate_mock_stream_context(
+        events,
+        final_message=unittest.mock.Mock(
+            usage=unittest.mock.Mock(
+                model_dump=unittest.mock.Mock(return_value={"input_tokens": 0, "output_tokens": 0})
+            ),
+        ),
+    )
 
     stream = model.structured_output(test_output_model_cls, messages)
     events = await alist(stream)