fix(router): fold the continuation prefill into a trailing assistant turn

A request that already ends in an assistant message took the partial output as a second assistant turn, which prefill providers reject. The partial now extends that turn in place, with prefix set, so the continuation request never ends in two assistant messages.

litellm/router.py

@@ -3246,10 +3246,11 @@ class Router:
         messages: Sequence[Mapping[str, str]],
         generated_content: str,
     ) -> Sequence[Mapping[str, object]]:
-        """Append the partial output as an assistant prefill, extending an existing trailing prefill in place."""
+        """Append the partial output as an assistant prefill, or extend a trailing assistant turn in place so the
+        request never ends in two assistant messages."""
         last: Final = messages[-1] if messages else None
-        if last is not None and last.get("role") == "assistant" and last.get("prefix"):
-            merged: Final = {**last, "content": str(last.get("content") or "") + generated_content}
+        if last is not None and last.get("role") == "assistant":
+            merged: Final = {**last, "content": str(last.get("content") or "") + generated_content, "prefix": True}
             return [*messages[:-1], merged]
         prefill: Final = {"role": "assistant", "content": generated_content, "prefix": True}
         return [*messages, prefill]

tests/unit/test_router/test_router.py

@@ -2892,6 +2892,15 @@ def test_build_completion_continuation_input_folds_into_existing_prefill():
     assert twice[-1] == {"role": "assistant", "content": "part one part two", "prefix": True}
 
 
+def test_build_completion_continuation_input_folds_into_trailing_plain_assistant_turn():
+    """A request that already ends in an assistant message takes the partial as its prefill
+    instead of gaining a second assistant turn, which prefill providers reject."""
+    messages = [{"role": "user", "content": "hi"}, {"role": "assistant", "content": "Sure, "}]
+    built = litellm.Router._build_completion_continuation_input(messages, "here it is")
+    assert [m["role"] for m in built] == ["user", "assistant"]
+    assert built[-1] == {"role": "assistant", "content": "Sure, here it is", "prefix": True}
+
+
 def test_continuation_output_ceilings_reduces_by_emitted_tokens():
     """A continuation must complete within the caller's original allowance, so each
     output ceiling is reduced by the tokens already emitted."""