diff --git a/litellm/llms/anthropic/experimental_pass_through/responses_adapters/streaming_iterator.py b/litellm/llms/anthropic/experimental_pass_through/responses_adapters/streaming_iterator.py
index 94c5200be64..5f1362e259f 100644
--- a/litellm/llms/anthropic/experimental_pass_through/responses_adapters/streaming_iterator.py
+++ b/litellm/llms/anthropic/experimental_pass_through/responses_adapters/streaming_iterator.py
@@ -155,10 +155,24 @@ class AnthropicResponsesStreamWrapper:
                 event.get("delta", "") if isinstance(event, dict) else ""
             )
             block_idx = (
-                self._item_id_to_block_index.get(item_id, self._current_block_index)
+                self._item_id_to_block_index.get(item_id, -1)
                 if item_id
                 else self._current_block_index
             )
+            if block_idx < 0:
+                # Some providers (e.g. LMStudio) skip response.output_item.added,
+                # so no text block is open yet; synthesize content_block_start
+                # instead of emitting a delta with index -1
+                block_idx = self._next_block_index()
+                if item_id:
+                    self._item_id_to_block_index[item_id] = block_idx
+                self._chunk_queue.append(
+                    {
+                        "type": "content_block_start",
+                        "index": block_idx,
+                        "content_block": {"type": "text", "text": ""},
+                    }
+                )
             self._chunk_queue.append(
                 {
                     "type": "content_block_delta",
diff --git a/tests/test_litellm/llms/anthropic/experimental_pass_through/responses_adapters/test_responses_adapters_streaming_iterator.py b/tests/test_litellm/llms/anthropic/experimental_pass_through/responses_adapters/test_responses_adapters_streaming_iterator.py
new file mode 100644
index 00000000000..450f69fb87c
--- /dev/null
+++ b/tests/test_litellm/llms/anthropic/experimental_pass_through/responses_adapters/test_responses_adapters_streaming_iterator.py
@@ -0,0 +1,79 @@
+"""
+Tests for AnthropicResponsesStreamWrapper
+(litellm/llms/anthropic/experimental_pass_through/responses_adapters/streaming_iterator.py)
+"""
+
+import os
+import sys
+
+sys.path.insert(
+    0, os.path.abspath(os.path.join(os.path.dirname(__file__), "../../../../../.."))
+)
+
+from litellm.llms.anthropic.experimental_pass_through.responses_adapters.streaming_iterator import (
+    AnthropicResponsesStreamWrapper,
+)
+
+
+def _process_all(events: list) -> list:
+    wrapper = AnthropicResponsesStreamWrapper(responses_stream=None, model="m")
+    for event in events:
+        wrapper._process_event(event)
+    return list(wrapper._chunk_queue)
+
+
+class TestProcessEventTextDeltaWithoutOutputItemAdded:
+    """Streams that skip response.output_item.added (e.g. LMStudio) must still
+    open a text block before any delta and never emit index -1."""
+
+    def test_process_event_synthesizes_content_block_start_before_delta(self):
+        chunks = _process_all(
+            [
+                {"type": "response.output_text.delta", "item_id": "i1", "delta": "Hel"},
+                {"type": "response.output_text.delta", "item_id": "i1", "delta": "lo"},
+            ]
+        )
+        assert [c["type"] for c in chunks] == [
+            "content_block_start",
+            "content_block_delta",
+            "content_block_delta",
+        ]
+        assert chunks[0]["content_block"] == {"type": "text", "text": ""}
+        assert [c["index"] for c in chunks] == [0, 0, 0]
+        assert chunks[1]["delta"] == {"type": "text_delta", "text": "Hel"}
+
+    def test_process_event_delta_without_item_id_never_yields_negative_index(self):
+        chunks = _process_all([{"type": "response.output_text.delta", "delta": "Hi"}])
+        assert [(c["type"], c["index"]) for c in chunks] == [
+            ("content_block_start", 0),
+            ("content_block_delta", 0),
+        ]
+
+    def test_process_event_unregistered_item_id_opens_new_text_block(self):
+        chunks = _process_all(
+            [
+                {
+                    "type": "response.output_item.added",
+                    "item": {"type": "reasoning", "id": "rs_1"},
+                },
+                {"type": "response.output_text.delta", "item_id": "m1", "delta": "Hi"},
+            ]
+        )
+        assert chunks[1]["type"] == "content_block_start"
+        assert chunks[1]["content_block"] == {"type": "text", "text": ""}
+        assert [c["index"] for c in chunks[1:]] == [1, 1]
+
+    def test_process_event_registered_item_id_does_not_synthesize_start(self):
+        chunks = _process_all(
+            [
+                {
+                    "type": "response.output_item.added",
+                    "item": {"type": "message", "id": "m1"},
+                },
+                {"type": "response.output_text.delta", "item_id": "m1", "delta": "Hi"},
+            ]
+        )
+        assert [(c["type"], c["index"]) for c in chunks] == [
+            ("content_block_start", 0),
+            ("content_block_delta", 0),
+        ]