fix(anthropic): preserve reasoning fallback

litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py

@@ -1407,8 +1407,8 @@ class LiteLLMAnthropicMessagesAdapter:
                 return "tool_use", cast("ContentBlockContentBlockDict", tool_block)
             elif choice.delta.content is not None and len(choice.delta.content) > 0:
                 return "text", TextBlock(type="text", text="")
-            elif isinstance(choice, StreamingChoices) and hasattr(choice.delta, "thinking_blocks"):
-                thinking_blocks = choice.delta.thinking_blocks or []
+            elif isinstance(choice, StreamingChoices):
+                thinking_blocks = getattr(choice.delta, "thinking_blocks", None) or []
                 if len(thinking_blocks) > 0:
                     thinking_block = thinking_blocks[0]
                     if thinking_block["type"] == "thinking":
@@ -1421,13 +1421,8 @@ class LiteLLMAnthropicMessagesAdapter:
                         return "thinking", ChatCompletionThinkingBlock(
                             type="thinking", thinking=thinking, signature=signature
                         )
-            # OpenAI-compatible reasoning backends (e.g. vLLM/SGLang reasoning
-            # parsers) populate ``reasoning_content`` without ``thinking_blocks``.
-            # ``Delta`` deletes the ``thinking_blocks`` attribute when unset, so the
-            # branch above is skipped entirely; open a ``thinking`` block here so the
-            # matching ``thinking_delta`` stream is not emitted into a text block.
-            elif isinstance(choice, StreamingChoices) and getattr(choice.delta, "reasoning_content", None):
-                return "thinking", ChatCompletionThinkingBlock(type="thinking", thinking="", signature="")
+                if getattr(choice.delta, "reasoning_content", None):
+                    return "thinking", ChatCompletionThinkingBlock(type="thinking", thinking="", signature="")
 
         return "text", TextBlock(type="text", text="")
 
@@ -1454,8 +1449,10 @@ class LiteLLMAnthropicMessagesAdapter:
                 for tool in choice.delta.tool_calls:
                     if tool.function is not None and tool.function.arguments is not None:
                         partial_json = (partial_json or "") + tool.function.arguments
-            elif isinstance(choice, StreamingChoices) and hasattr(choice.delta, "thinking_blocks"):
-                thinking_blocks = choice.delta.thinking_blocks or []
+            elif isinstance(choice, StreamingChoices):
+                thinking_blocks = getattr(choice.delta, "thinking_blocks", None) or []
+                choice_reasoning_content = ""
+                choice_reasoning_signature = ""
                 if len(thinking_blocks) > 0:
                     for thinking_block in thinking_blocks:
                         if thinking_block["type"] == "thinking":
@@ -1465,13 +1462,14 @@ class LiteLLMAnthropicMessagesAdapter:
                             assert isinstance(thinking, str)
                             assert isinstance(signature, str)
 
-                            reasoning_content += thinking
-                            reasoning_signature += signature
-            # Handle reasoning_content when thinking_blocks is not present
-            # This handles providers like OpenRouter that return reasoning_content
-            elif isinstance(choice, StreamingChoices) and hasattr(choice.delta, "reasoning_content"):
-                if choice.delta.reasoning_content is not None:
-                    reasoning_content += choice.delta.reasoning_content
+                            choice_reasoning_content += thinking
+                            choice_reasoning_signature += signature
+                if choice_reasoning_content or choice_reasoning_signature:
+                    reasoning_content += choice_reasoning_content
+                    reasoning_signature += choice_reasoning_signature
+                elif hasattr(choice.delta, "reasoning_content"):
+                    if choice.delta.reasoning_content is not None:
+                        reasoning_content += choice.delta.reasoning_content
 
         if partial_json is not None:
             return "input_json_delta", ContentJsonBlockDelta(type="input_json_delta", partial_json=partial_json)

tests/test_litellm/llms/anthropic/experimental_pass_through/adapters/test_streaming_iterator_first_delta.py

@@ -576,6 +576,43 @@ def test_reasoning_content_first_stream_opens_thinking_block_at_index_zero_sync(
     _assert_thinking_first_block_opens_at_index_zero(_drain_sync(wrapper))
 
 
+@pytest.mark.parametrize(
+    "thinking_blocks",
+    [
+        pytest.param([], id="empty"),
+        pytest.param(
+            [{"type": "redacted_thinking", "data": "redacted"}],
+            id="redacted-only",
+        ),
+    ],
+)
+def test_reasoning_content_falls_back_without_usable_thinking_blocks_sync(
+    thinking_blocks,
+):
+    chunks = [
+        _make_chunk(
+            Delta(
+                content=None,
+                reasoning_content="fallback thought",
+                thinking_blocks=thinking_blocks,
+            )
+        ),
+        _make_chunk(Delta(content=None), finish_reason="stop"),
+    ]
+    wrapper = AnthropicStreamWrapper(completion_stream=iter(chunks), model="claude-x")
+    events = _drain_sync(wrapper)
+
+    starts = [
+        (event["index"], event["content_block"]["type"])
+        for event in events
+        if event.get("type") == "content_block_start"
+    ]
+    assert starts == [(0, "thinking")]
+    assert _thinking_deltas(events) == ["fallback thought"]
+    assert _text_deltas(events) == []
+    _assert_deltas_match_their_block_type(events)
+
+
 def _blank_lead_chunks() -> List[MagicMock]:
     return [
         _make_chunk(Delta(content=None)),