diff --git a/litellm/router.py b/litellm/router.py
index 2d3d6971e3b..bb8485e0ef5 100644
--- a/litellm/router.py
+++ b/litellm/router.py
@@ -475,6 +475,7 @@ _NO_SESSION_KWARGS: Final[Mapping[str, Mapping[str, object]]] = MappingProxyType
 _SESSION_ADAPTER: Final = TypeAdapter(Mapping[str, object])
 _UNCONSTRAINED_RESPONSE_FORMAT: Final[Mapping[str, str]] = MappingProxyType({"type": "text"})
 _SILENT_MODEL_ADAPTER: Final = TypeAdapter(str | list[str])
+_CONTENT_BLOCKS_ADAPTER: Final = TypeAdapter(list[dict[str, object]])
 
 
 def _as_retry_skipped_deployment_ids(value: object) -> tuple[str, ...]:
@@ -2997,12 +2998,13 @@ class Router:
                             getattr(complete_response_object_usage, "completion_tokens", 0) or 0
                         )
                         reduced_ceilings: Final = self._continuation_output_ceilings(initial_kwargs, emitted_tokens)
-                        if reduced_ceilings is None:
-                            self._raise_original_mid_stream_error(e)
-                        initial_kwargs.update(reduced_ceilings)
-                        initial_kwargs["messages"] = self._build_completion_continuation_input(
+                        continuation_messages: Final = self._build_completion_continuation_input(
                             messages, e.generated_content
                         )
+                        if reduced_ceilings is None or continuation_messages is None:
+                            self._raise_original_mid_stream_error(e)
+                        initial_kwargs.update(reduced_ceilings)
+                        initial_kwargs["messages"] = continuation_messages
                         initial_kwargs[MID_STREAM_CONTINUATION_KWARG] = MID_STREAM_CONTINUATION_MARKER
                     else:
                         initial_kwargs["messages"] = messages
@@ -3243,17 +3245,23 @@ class Router:
 
     @staticmethod
     def _build_completion_continuation_input(
-        messages: Sequence[Mapping[str, str]],
+        messages: Sequence[Mapping[str, object]],
         generated_content: str,
-    ) -> Sequence[Mapping[str, object]]:
+    ) -> Sequence[Mapping[str, object]] | None:
         """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."""
+        request never ends in two assistant messages. None when that turn's content has a shape this cannot extend."""
         last: Final = messages[-1] if messages else None
-        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]
+        if last is None or last.get("role") != "assistant":
+            return [*messages, {"role": "assistant", "content": generated_content, "prefix": True}]
+        content: Final = last.get("content")
+        if content is None or isinstance(content, str):
+            return [*messages[:-1], {**last, "content": f"{content or ''}{generated_content}", "prefix": True}]
+        try:
+            blocks: Final = _CONTENT_BLOCKS_ADAPTER.validate_python(content)
+        except ValidationError:
+            return None
+        extended: Final = [*blocks, {"type": "text", "text": generated_content}]
+        return [*messages[:-1], {**last, "content": extended, "prefix": True}]
 
     @staticmethod
     def _build_responses_continuation_input(
diff --git a/tests/unit/test_router/test_router.py b/tests/unit/test_router/test_router.py
index d59674f1c57..3d592737e1e 100644
--- a/tests/unit/test_router/test_router.py
+++ b/tests/unit/test_router/test_router.py
@@ -2901,6 +2901,21 @@ def test_build_completion_continuation_input_folds_into_trailing_plain_assistant
     assert built[-1] == {"role": "assistant", "content": "Sure, here it is", "prefix": True}
 
 
+def test_build_completion_continuation_input_keeps_structured_assistant_content():
+    """Content blocks on a trailing assistant turn stay blocks: the partial lands as one more text
+    block instead of a stringified list, and a content shape that cannot be extended declines."""
+    messages = [{"role": "user", "content": "hi"}, {"role": "assistant", "content": [{"type": "text", "text": "Sure, "}]}]
+    built = litellm.Router._build_completion_continuation_input(messages, "here it is")
+    assert built is not None
+    assert [m["role"] for m in built] == ["user", "assistant"]
+    assert built[-1] == {
+        "role": "assistant",
+        "content": [{"type": "text", "text": "Sure, "}, {"type": "text", "text": "here it is"}],
+        "prefix": True,
+    }
+    assert litellm.Router._build_completion_continuation_input([{"role": "assistant", "content": 42}], "x") is None
+
+
 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."""