Merge 54f15b9a53 into 44d84360fb

litellm/litellm_core_utils/streaming_chunk_builder_utils.py

@@ -324,7 +324,15 @@ class ChunkProcessor:
         system_fingerprint: Final = chunk.get("system_fingerprint", None)
 
         first_chunk_with_choices: Final = next((c for c in chunks if c.get("choices")), chunk)
-        role: Final = first_chunk_with_choices["choices"][0]["delta"]["role"]
+        if (
+            first_chunk_with_choices.get("choices")
+            and len(first_chunk_with_choices["choices"]) > 0
+            and first_chunk_with_choices["choices"][0].get("delta")
+            and "role" in first_chunk_with_choices["choices"][0]["delta"]
+        ):
+            role: Final = first_chunk_with_choices["choices"][0]["delta"]["role"]
+        else:
+            role = "assistant"
         finish_reason = "stop"
         for chunk in chunks:
             if "choices" in chunk and len(chunk["choices"]) > 0:

tests/test_litellm/litellm_core_utils/test_streaming_chunk_builder_utils_empty_choices.py

@@ -0,0 +1,66 @@
+import os
+import sys
+
+sys.path.insert(
+    0, os.path.abspath("../../..")
+)  # Adds the parent directory to the system path
+
+from litellm.litellm_core_utils.streaming_chunk_builder_utils import ChunkProcessor
+
+
+def test_build_base_response_all_chunks_empty_choices():
+    """
+    Regression test for the IndexError in ChunkProcessor.build_base_response
+    when every chunk has choices=[] (e.g. Anthropic usage-only events).
+    ``next((c for c in chunks if c.get("choices")), chunk)`` skips every chunk
+    (empty list is falsy) and falls back to ``chunk``, whose ``choices`` is also
+    ``[]`` — the old code then did ``choices[0]`` and raised IndexError.
+    build_base_response should default to role='assistant'.
+    """
+    chunks = [
+        {
+            "id": "chatcmpl-all-empty-choices",
+            "object": "chat.completion.chunk",
+            "created": 1234567890,
+            "model": "gpt-4.1-mini",
+            "choices": [],
+        },
+        {
+            "id": "chatcmpl-all-empty-choices",
+            "object": "chat.completion.chunk",
+            "created": 1234567890,
+            "model": "gpt-4.1-mini",
+            "choices": [],
+        },
+    ]
+
+    processor = ChunkProcessor(chunks=chunks)
+    response = processor.build_base_response(chunks)
+
+    assert response.choices[0].message.role == "assistant"
+    assert response.choices[0].message.content == ""
+
+
+def test_build_base_response_falls_back_to_assistant_when_role_missing():
+    """
+    When a chunk has non-empty choices but its delta omits the role key,
+    build_base_response should still produce a valid response and default
+    to 'assistant'.
+    """
+    chunks = [
+        {
+            "id": "chatcmpl-no-role",
+            "object": "chat.completion.chunk",
+            "created": 1234567890,
+            "model": "gpt-4.1-mini",
+            "choices": [
+                {"index": 0, "delta": {"content": "hello"}}
+            ],
+        }
+    ]
+
+    processor = ChunkProcessor(chunks=chunks)
+    response = processor.build_base_response(chunks)
+
+    assert response.choices[0].message.role == "assistant"
+    assert response.choices[0].message.content == ""