diff --git a/src/anthropic/lib/streaming/_beta_messages.py b/src/anthropic/lib/streaming/_beta_messages.py
index 0348875b..428e4f17 100644
--- a/src/anthropic/lib/streaming/_beta_messages.py
+++ b/src/anthropic/lib/streaming/_beta_messages.py
@@ -475,7 +475,12 @@ def accumulate_event(
         raise RuntimeError(f'Unexpected event order, got {event.type} before "message_start"')
 
     if event.type == "content_block_start":
-        # TODO: check index
+        if event.index != len(current_snapshot.content):
+            raise RuntimeError(
+                f'Unexpected "content_block_start" index, got {event.index} '
+                f"but expected {len(current_snapshot.content)}"
+            )
+
         current_snapshot.content.append(
             cast(
                 Any,  # Pydantic does not support generic unions at runtime
diff --git a/src/anthropic/lib/streaming/_messages.py b/src/anthropic/lib/streaming/_messages.py
index 5c0da999..cc450903 100644
--- a/src/anthropic/lib/streaming/_messages.py
+++ b/src/anthropic/lib/streaming/_messages.py
@@ -454,7 +454,12 @@ def accumulate_event(
         raise RuntimeError(f'Unexpected event order, got {event.type} before "message_start"')
 
     if event.type == "content_block_start":
-        # TODO: check index
+        if event.index != len(current_snapshot.content):
+            raise RuntimeError(
+                f'Unexpected "content_block_start" index, got {event.index} '
+                f"but expected {len(current_snapshot.content)}"
+            )
+
         current_snapshot.content.append(
             cast(
                 Any,  # Pydantic does not support generic unions at runtime
diff --git a/tests/lib/streaming/test_content_block_index.py b/tests/lib/streaming/test_content_block_index.py
new file mode 100644
index 00000000..ed0a4e6b
--- /dev/null
+++ b/tests/lib/streaming/test_content_block_index.py
@@ -0,0 +1,91 @@
+from __future__ import annotations
+
+import httpx
+import pytest
+
+from anthropic.types.usage import Usage
+from anthropic.types.text_block import TextBlock
+from anthropic.types.parsed_message import ParsedMessage
+from anthropic.types.beta.beta_usage import BetaUsage
+from anthropic.lib.streaming._messages import accumulate_event
+from anthropic.types.beta.beta_text_block import BetaTextBlock
+from anthropic.lib.streaming._beta_messages import accumulate_event as accumulate_beta_event
+from anthropic.types.beta.parsed_beta_message import ParsedBetaMessage
+from anthropic.types.raw_content_block_start_event import RawContentBlockStartEvent
+from anthropic.types.beta.beta_raw_content_block_start_event import BetaRawContentBlockStartEvent
+
+
+def _snapshot() -> ParsedMessage[None]:
+    return ParsedMessage(
+        id="msg_123",
+        type="message",
+        role="assistant",
+        content=[],
+        model="claude-sonnet-4-5",
+        stop_reason=None,
+        stop_sequence=None,
+        usage=Usage(input_tokens=10, output_tokens=10),
+    )
+
+
+def _start_event(index: int) -> RawContentBlockStartEvent:
+    return RawContentBlockStartEvent(
+        type="content_block_start",
+        index=index,
+        content_block=TextBlock(type="text", text=""),
+    )
+
+
+def _beta_snapshot() -> ParsedBetaMessage[None]:
+    return ParsedBetaMessage(
+        id="msg_123",
+        type="message",
+        role="assistant",
+        content=[],
+        model="claude-sonnet-4-5",
+        stop_reason=None,
+        stop_sequence=None,
+        usage=BetaUsage(input_tokens=10, output_tokens=10),
+    )
+
+
+def _beta_start_event(index: int) -> BetaRawContentBlockStartEvent:
+    return BetaRawContentBlockStartEvent(
+        type="content_block_start",
+        index=index,
+        content_block=BetaTextBlock(type="text", text=""),
+    )
+
+
+class TestContentBlockIndex:
+    def test_sequential_indices_append(self) -> None:
+        snapshot = _snapshot()
+
+        snapshot = accumulate_event(event=_start_event(0), current_snapshot=snapshot)
+        assert len(snapshot.content) == 1
+
+        snapshot = accumulate_event(event=_start_event(1), current_snapshot=snapshot)
+        assert len(snapshot.content) == 2
+
+    def test_unexpected_index_raises(self) -> None:
+        # snapshot has no content yet, so the only valid index is 0
+        with pytest.raises(RuntimeError, match='Unexpected "content_block_start" index, got 5 but expected 0'):
+            accumulate_event(event=_start_event(5), current_snapshot=_snapshot())
+
+    def test_beta_sequential_indices_append(self) -> None:
+        headers = httpx.Headers({"some-header": "value"})
+        snapshot = _beta_snapshot()
+
+        snapshot = accumulate_beta_event(event=_beta_start_event(0), current_snapshot=snapshot, request_headers=headers)
+        assert len(snapshot.content) == 1
+
+        snapshot = accumulate_beta_event(event=_beta_start_event(1), current_snapshot=snapshot, request_headers=headers)
+        assert len(snapshot.content) == 2
+
+    def test_beta_unexpected_index_raises(self) -> None:
+        with pytest.raises(RuntimeError, match='Unexpected "content_block_start" index, got 5 but expected 0'):
+            accumulate_beta_event(
+                event=_beta_start_event(5),
+                current_snapshot=_beta_snapshot(),
+                request_headers=httpx.Headers({"some-header": "value"}),
+            )