fix: handle list-content messages in thinking block interleaving

When assistant content is already a list containing thinking blocks
inline (not str/None), SEQUENTIAL MODE was still prepending all
thinking_blocks from provider_specific_fields, causing duplication
and breaking Anthropic's position-dependent signature verification.

Now detects if the content list already has thinking blocks and skips
the extend(thinking_blocks) to preserve the original interleaved order.

Addresses the correctness gap identified by Greptile review where
list-content messages bypass INTERLEAVED MODE.

Fixes: https://github.com/BerriAI/litellm/issues/23047

litellm/litellm_core_utils/prompt_templates/factory.py

@@ -2614,13 +2614,28 @@ def anthropic_messages_pt(  # noqa: PLR0915
             else:
                 # SEQUENTIAL MODE: No server tool calls, or no thinking blocks,
                 # or content is a list. Use the original sequential approach.
+
+                # When content is a list, check if it already contains thinking
+                # blocks inline. If so, skip prepending thinking_blocks to avoid
+                # duplication and preserve the original interleaved order.
+                # Fixes the gap where list-content messages bypass INTERLEAVED
+                # MODE and still get thinking blocks prepended out of order.
+                _content_is_list = "content" in assistant_content_block and isinstance(
+                    assistant_content_block["content"], list
+                )
+                _list_has_thinking = False
+                if _content_is_list:
+                    for _item in assistant_content_block["content"]:
+                        if isinstance(_item, dict) and _item.get("type") == "thinking":
+                            _list_has_thinking = True
+                            break
+
                 if (
                     thinking_blocks is not None
+                    and not _list_has_thinking
                 ):  # IMPORTANT: ADD THIS FIRST, ELSE ANTHROPIC WILL RAISE AN ERROR
                     assistant_content.extend(thinking_blocks)
-                if "content" in assistant_content_block and isinstance(
-                    assistant_content_block["content"], list
-                ):
+                if _content_is_list:
                     for m in assistant_content_block["content"]:
                         # handle thinking blocks
                         thinking_block = cast(str, m.get("thinking", ""))

tests/llm_translation/test_prompt_factory.py

@@ -1847,3 +1847,100 @@ def test_anthropic_messages_pt_interleave_more_thinking_than_tool_groups():
         "thinking",            # extra - before text
         "text",
     ], f"Expected order but got: {types}"
+
+
+def test_anthropic_messages_pt_list_content_with_thinking_preserves_order():
+    """
+    Test that when assistant content is already a list containing interleaved
+    thinking blocks and server tool blocks, the thinking_blocks from
+    provider_specific_fields are NOT duplicated/prepended.
+
+    This covers the gap identified by Greptile where list-content messages
+    bypass INTERLEAVED MODE and fall into SEQUENTIAL MODE, which previously
+    would prepend all thinking_blocks again, causing duplication and
+    breaking Anthropic's position-dependent signature verification.
+
+    Fixes: https://github.com/BerriAI/litellm/issues/23047
+    """
+    messages = [
+        {"role": "user", "content": "Search for AI news"},
+        {
+            "role": "assistant",
+            # Content is already a list with interleaved thinking + server tool blocks
+            "content": [
+                {
+                    "type": "thinking",
+                    "thinking": "Let me search for AI news.",
+                    "signature": "sig_1",
+                },
+                {
+                    "type": "server_tool_use",
+                    "id": "srvtoolu_01SEARCH1",
+                    "name": "web_search",
+                    "input": {"query": "AI news"},
+                },
+                {
+                    "type": "web_search_tool_result",
+                    "tool_use_id": "srvtoolu_01SEARCH1",
+                    "content": [
+                        {
+                            "type": "web_search_result",
+                            "url": "https://example.com",
+                            "title": "AI News",
+                            "snippet": "Latest AI news",
+                        }
+                    ],
+                },
+                {
+                    "type": "thinking",
+                    "thinking": "Now let me summarize.",
+                    "signature": "sig_2",
+                },
+                {
+                    "type": "text",
+                    "text": "Here is the AI news summary.",
+                },
+            ],
+            # thinking_blocks also present in provider_specific_fields
+            "thinking_blocks": [
+                {
+                    "type": "thinking",
+                    "thinking": "Let me search for AI news.",
+                    "signature": "sig_1",
+                },
+                {
+                    "type": "thinking",
+                    "thinking": "Now let me summarize.",
+                    "signature": "sig_2",
+                },
+            ],
+        },
+        {"role": "user", "content": "Tell me more"},
+    ]
+
+    result = anthropic_messages_pt(
+        messages, model="claude-sonnet-4-5", llm_provider="anthropic"
+    )
+
+    assistant_msg = next(m for m in result if m["role"] == "assistant")
+    content = assistant_msg["content"]
+    types = [c.get("type") for c in content]
+
+    # The list content already has the correct interleaved order.
+    # thinking_blocks should NOT be prepended again (which would cause
+    # duplication and break signature verification).
+    assert types == [
+        "thinking",
+        "server_tool_use",
+        "web_search_tool_result",
+        "thinking",
+        "text",
+    ], f"Expected preserved list order without duplicate thinking blocks, but got: {types}"
+
+    # Verify no duplicate thinking blocks
+    thinking_count = sum(1 for t in types if t == "thinking")
+    assert thinking_count == 2, f"Expected 2 thinking blocks, got {thinking_count} (duplication detected)"
+
+    # Verify signatures preserved in correct positions
+    assert content[0]["signature"] == "sig_1"
+    assert content[3]["signature"] == "sig_2"