fix(anthropic): split refusal off a combined finish_reason chunk

A fake-streamed provider hands the adapter one chunk carrying both the
delta payload and the finish_reason, which is exactly what the combined
chunk splitter exists for, but its content check never listed the refusal.
The translation short-circuits on finish_reason, so that refusal text was
dropped and the client got `stop_reason: refusal` over an empty content
array, the symptom this PR set out to fix.

Both refusal accumulators also drop their `mutable-ok` lists for a plain
string attribute

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

@@ -101,6 +101,10 @@ class _CombinedChunkSplitter:
     @staticmethod
     def _is_combined(chunk: "ModelResponseStream") -> bool:
         """True if ``chunk`` carries response content AND a finish_reason."""
+        from litellm.llms.anthropic.experimental_pass_through.messages.utils import (
+            openai_chat_refusal_text,
+        )
+
         choices: Final = _optional_attr_sequence(chunk, "choices")
         if not choices:
             return False
@@ -115,6 +119,7 @@ class _CombinedChunkSplitter:
             or _optional_attr(delta, "tool_calls")
             or _optional_attr(delta, "reasoning_content")
             or _optional_attr(delta, "thinking_blocks")
+            or openai_chat_refusal_text(delta)
         )
 
     _PAYLOAD_FIELD_GROUPS: "tuple[tuple[str, ...], ...]" = (
@@ -306,7 +311,7 @@ class AnthropicStreamWrapper(AdapterCompletionStreamWrapper):
         # Synthesized compaction block from compact_20260112 polyfill (streaming).
         self.compaction_block = compaction_block
         self.iterations_usage = iterations_usage
-        self._refusal_text_parts: list[str] = []  # mutable-ok: accumulates streamed refusal delta text across chunks
+        self._refusal_text: str = ""
         self.sent_compaction_block: bool = False
         # Per-phase flags so the compaction block's start/delta/stop events
         # are emitted (and the public state machine is advanced) in
@@ -1001,7 +1006,7 @@ class AnthropicStreamWrapper(AdapterCompletionStreamWrapper):
         self,
         processed_chunk: ContentBlockDelta | MessageBlockDelta,
     ) -> ContentBlockDelta | MessageBlockDelta:
-        if processed_chunk.get("type") != "message_delta" or not self._refusal_text_parts:
+        if processed_chunk.get("type") != "message_delta" or not self._refusal_text:
             return processed_chunk
         delta: Final = cast(Mapping[str, object], processed_chunk["delta"])  # cast-ok: keys checked before use
         if delta.get("stop_reason") == "max_tokens":
@@ -1017,7 +1022,7 @@ class AnthropicStreamWrapper(AdapterCompletionStreamWrapper):
                 "delta": {  # mutable-ok: fresh message_delta payload; never mutated after construction
                     **delta,
                     "stop_reason": "refusal",
-                    "stop_details": refusal_stop_details("".join(self._refusal_text_parts)),
+                    "stop_details": refusal_stop_details(self._refusal_text),
                 },
             },
         )
@@ -1107,13 +1112,13 @@ class AnthropicStreamWrapper(AdapterCompletionStreamWrapper):
 
         from .transformation import LiteLLMAnthropicMessagesAdapter
 
-        refusal_text: Final = openai_chat_refusal_text(chunk.choices[0].delta)
-        if refusal_text is not None:
-            self._refusal_text_parts.append(refusal_text)
-
         if chunk.choices[0].finish_reason is not None:
             return False
 
+        refusal_text: Final = openai_chat_refusal_text(chunk.choices[0].delta)
+        if refusal_text is not None:
+            self._refusal_text = self._refusal_text + refusal_text
+
         (
             block_type,
             content_block_start,

litellm/llms/anthropic/experimental_pass_through/responses_adapters/streaming_iterator.py

@@ -54,7 +54,7 @@ class AnthropicResponsesStreamWrapper:
         self._sent_message_start = False
         self._sent_message_stop = False
         self._chunk_queue: deque[dict[str, object]] = deque()
-        self._refusal_text_parts: list[str] = []  # mutable-ok: accumulates streamed refusal delta text across chunks
+        self._refusal_text: str = ""
         self._sync_responses_iterator: Iterator[object] | None = None
 
     def _make_message_start(self) -> dict[str, object]:
@@ -142,7 +142,7 @@ class AnthropicResponsesStreamWrapper:
             delta = getattr(event, "delta", "") or (event.get("delta", "") if isinstance(event, dict) else "")
             if not isinstance(delta, str) or not delta:
                 return
-            self._refusal_text_parts.append(delta)
+            self._refusal_text = self._refusal_text + delta
             item_id = getattr(event, "item_id", None) or (event.get("item_id") if isinstance(event, dict) else None)
             block_idx = self._item_id_to_block_index.get(item_id, -1) if item_id else self._current_block_index
             if block_idx < 0:
@@ -241,7 +241,7 @@ class AnthropicResponsesStreamWrapper:
                 event.get("response") if isinstance(event, dict) else None
             )
             output: Final = (getattr(response_obj, "output", None) or ()) if response_obj is not None else ()
-            refusal_text: Final = responses_output_refusal_text(output) or ("".join(self._refusal_text_parts) or None)
+            refusal_text: Final = responses_output_refusal_text(output) or (self._refusal_text or None)
             status: Final = getattr(response_obj, "status", None) if response_obj is not None else None
             has_tool_call: Final = any(
                 getattr(item, "type", None) == "function_call"

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

@@ -183,6 +183,10 @@ async def test_streaming_chat_refusal_parked_in_provider_specific_fields_is_emit
 
 
 def test_streaming_chat_combined_refusal_and_finish_reason_is_preserved():
+    """Fake-streamed responses arrive as one chunk carrying both the delta and the
+    finish_reason. The refusal has to be split off and streamed as text, or the
+    client gets ``stop_reason: refusal`` over an empty content array.
+    """
     chunks = [
         _make_chunk(
             Delta(content=None, refusal="I cannot fulfill this request."),
@@ -193,6 +197,7 @@ def test_streaming_chat_combined_refusal_and_finish_reason_is_preserved():
 
     events = _drain_sync(wrapper)
 
+    assert _text_deltas(events) == ["I cannot fulfill this request."]
     message_delta = next(event for event in events if event["type"] == "message_delta")
     assert message_delta["delta"]["stop_reason"] == "refusal"
     assert message_delta["delta"]["stop_details"]["explanation"] == "I cannot fulfill this request."
@@ -202,7 +207,7 @@ def test_streaming_chat_combined_refusal_and_finish_reason_is_preserved():
 async def test_streaming_chat_combined_refusal_and_finish_reason_is_preserved_async():
     chunks = [
         _make_chunk(
-            Delta(content=None, refusal="I cannot fulfill this request."),
+            Delta(content=None, provider_specific_fields={"refusal": "I cannot fulfill this request."}),
             finish_reason="stop",
         )
     ]
@@ -210,6 +215,7 @@ async def test_streaming_chat_combined_refusal_and_finish_reason_is_preserved_as
 
     events = await _drain_async(wrapper)
 
+    assert _text_deltas(events) == ["I cannot fulfill this request."]
     message_delta = next(event for event in events if event["type"] == "message_delta")
     assert message_delta["delta"]["stop_reason"] == "refusal"
     assert message_delta["delta"]["stop_details"]["explanation"] == "I cannot fulfill this request."