diff --git a/litellm/main.py b/litellm/main.py
index 8ee102f5d07..f455b6181fe 100644
--- a/litellm/main.py
+++ b/litellm/main.py
@@ -8648,6 +8648,7 @@ def stream_chunk_builder(
                 or delta.get("audio") is not None
                 or delta.get("images") is not None
                 or delta.get("provider_specific_fields") is not None
+                or delta.get("reasoning_items") is not None
             ):
                 is_simple_text_stream = False
                 break
@@ -8800,6 +8801,22 @@ def stream_chunk_builder(
                 all_images.extend(chunk["choices"][0]["delta"]["images"])
             response["choices"][0]["message"]["images"] = all_images
 
+        # Reasoning items carry the provider's encrypted reasoning state, which the
+        # Responses API bridge reads back off the assembled assistant message.
+        reasoning_item_chunks: Final = [  # mutable-ok: local filter over chunks, never escapes this function
+            chunk
+            for chunk in chunks
+            if len(chunk["choices"]) > 0
+            and "reasoning_items" in chunk["choices"][0]["delta"]
+            and chunk["choices"][0]["delta"]["reasoning_items"] is not None
+        ]
+
+        if len(reasoning_item_chunks) > 0:
+            all_reasoning_items: Final[list] = []  # mutable-ok: accumulator, extended in the loop below
+            for chunk in reasoning_item_chunks:
+                all_reasoning_items.extend(chunk["choices"][0]["delta"]["reasoning_items"])
+            response["choices"][0]["message"]["reasoning_items"] = all_reasoning_items
+
         # Combine provider_specific_fields from streaming chunks (e.g., web_search_results, citations)
         # See: https://github.com/BerriAI/litellm/issues/17737
         provider_specific_chunks: Final = [
diff --git a/tests/test_litellm/test_stream_chunk_builder_reasoning_items.py b/tests/test_litellm/test_stream_chunk_builder_reasoning_items.py
new file mode 100644
index 00000000000..a14ce73a2f8
--- /dev/null
+++ b/tests/test_litellm/test_stream_chunk_builder_reasoning_items.py
@@ -0,0 +1,94 @@
+"""
+Tests for stream_chunk_builder reasoning item reassembly.
+
+Previously, stream_chunk_builder dropped delta.reasoning_items entirely: the
+simple-text fast path returned before any aggregation ran, and the full path
+had no reasoning_items branch. The encrypted reasoning state a provider sends
+back was therefore lost from the assembled message, which is what gets cached
+and what the Responses API bridge reads to rebuild its input.
+"""
+
+from litellm import stream_chunk_builder
+from litellm.types.utils import Delta, ModelResponseStream, StreamingChoices
+
+MESSAGES = [{"role": "user", "content": "hi"}]
+
+
+def _chunk(**delta_kwargs) -> ModelResponseStream:
+    return ModelResponseStream(
+        id="chatcmpl-test",
+        created=1700000000,
+        model="test-model",
+        object="chat.completion.chunk",
+        choices=[
+            StreamingChoices(
+                finish_reason=None,
+                index=0,
+                delta=Delta(**delta_kwargs),
+            )
+        ],
+    )
+
+
+def test_reasoning_items_survive_a_text_only_stream():
+    """
+    A stream carrying only content and reasoning_items must keep the reasoning
+    items. This is the fast path, which used to return before aggregating.
+    """
+    reasoning_item = {
+        "id": "rs_abc123",
+        "type": "reasoning",
+        "encrypted_content": "ENCRYPTED-REASONING-BLOB",
+        "summary": [{"type": "summary_text", "text": "Thinking"}],
+    }
+
+    chunks = [
+        _chunk(content="Hello", role="assistant"),
+        _chunk(content="", reasoning_items=[reasoning_item]),
+    ]
+
+    response = stream_chunk_builder(chunks, messages=MESSAGES)
+
+    assert response is not None
+    message = response.choices[0].message
+    assert message.content == "Hello"
+    assert getattr(message, "reasoning_items", None) == [reasoning_item]
+
+
+def test_reasoning_items_are_merged_across_chunks():
+    """
+    Reasoning items arriving in more than one chunk are concatenated, matching
+    how annotations and images are merged. thinking_blocks here also forces the
+    full aggregation path rather than the text-only fast path.
+    """
+    item_a = {"id": "rs_a", "type": "reasoning", "encrypted_content": "BLOB-A"}
+    item_b = {"id": "rs_b", "type": "reasoning", "encrypted_content": "BLOB-B"}
+
+    chunks = [
+        _chunk(
+            content="Part one. ",
+            role="assistant",
+            thinking_blocks=[{"type": "thinking", "thinking": "step 1", "signature": "sig"}],
+            reasoning_items=[item_a],
+        ),
+        _chunk(content="Part two.", reasoning_items=[item_b]),
+    ]
+
+    response = stream_chunk_builder(chunks, messages=MESSAGES)
+
+    assert response is not None
+    message = response.choices[0].message
+    assert message.content == "Part one. Part two."
+    assert getattr(message, "reasoning_items", None) == [item_a, item_b]
+
+
+def test_no_reasoning_items_leaves_the_message_alone():
+    """A stream without reasoning items must not grow a reasoning_items field."""
+    chunks = [_chunk(content="Hello", role="assistant"), _chunk(content=" there")]
+
+    response = stream_chunk_builder(chunks, messages=MESSAGES)
+
+    assert response is not None
+    message = response.choices[0].message
+    assert message.content == "Hello there"
+    assert getattr(message, "reasoning_items", None) is None