fix(responses): trim thinking blocks on a reasoning turn that starts with web searches

When Claude searches before writing any text, the reasoning item is followed directly by web_search_call items. Their text was merged onto the reasoning message, so it no longer looked reasoning-only and kept every thinking block side by side. Every replayed assistant message now keeps only its newest block.

litellm/responses/litellm_completion_transformation/transformation.py

@@ -1633,7 +1633,8 @@ class LiteLLMCompletionResponsesConfig:
         immediately following assistant message.
 
         Signed ``thinking_blocks`` decoded from ``encrypted_content`` travel the
-        same way, but only the newest one is replayed. A reasoning item does not
+        same way, but each assistant message keeps only the newest one, including
+        a reasoning message that hosted search text was merged onto. A reasoning item does not
         record where each block sat between the turn's server tool steps and
         text, and Claude rejects a latest assistant message whose thinking
         blocks sit side by side when it wrote them apart.
@@ -1692,8 +1693,9 @@ class LiteLLMCompletionResponsesConfig:
                     cast(dict[str, object], msg)["reasoning_content"] = combined  # cast-ok: mutable reasoning carrier
                 else:
                     setattr(msg, "reasoning_content", combined)  # noqa: B010  # attribute name is fixed, not dynamic
-            if pending_blocks:
-                replayed: Final = list((pending_blocks + (_thinking_blocks(msg) or ()))[-1:])
+            own_blocks: Final = _thinking_blocks(msg) or ()
+            if pending_blocks or len(own_blocks) > 1:
+                replayed: Final = list((pending_blocks + own_blocks)[-1:])
                 if isinstance(msg, dict):
                     cast(dict[str, object], msg)["thinking_blocks"] = replayed  # cast-ok: mutable reasoning carrier
                 else:
@@ -1736,7 +1738,7 @@ class LiteLLMCompletionResponsesConfig:
                 pending.append((_reasoning_text(msg), _thinking_blocks(msg)))
                 continue
 
-            if pending and _role(msg) == "assistant":
+            if _role(msg) == "assistant":
                 _apply_pending(msg, pending)
                 pending = []
             elif pending:

tests/unit/responses/litellm_completion_transformation/test_reasoning_input_item_preservation.py

@@ -14,6 +14,7 @@ JSON array of thinking blocks on the response side.
 """
 
 import json
+from itertools import chain
 
 import pytest
 
@@ -253,43 +254,46 @@ class TestEncryptedReasoningRoundTrip:
         assert len(messages) == 1
         assert [block["thinking"] for block in messages[0]["thinking_blocks"]] == ["newer"]
 
-    def test_web_search_turn_replays_only_its_last_thinking_block(self):
+    @pytest.mark.parametrize("answer_first", [True, False], ids=["answer_before_searches", "searches_before_answer"])
+    def test_web_search_turn_replays_only_its_last_thinking_block(self, answer_first):
         """Claude rejects a follow-up whose replay puts blocks it wrote apart side by side."""
         blocks = [
             {"type": "thinking", "thinking": "", "signature": "sig-before-search-1"},
             {"type": "thinking", "thinking": "", "signature": "sig-before-search-2"},
             {"type": "thinking", "thinking": "", "signature": "sig-before-answer"},
         ]
+        answer = {
+            "type": "message",
+            "id": "msg_1",
+            "role": "assistant",
+            "status": "completed",
+            "content": [{"type": "output_text", "text": "Paul Haggis.", "annotations": []}],
+        }
+        searches = [
+            {
+                "type": "web_search_call",
+                "id": "ws_1",
+                "status": "completed",
+                "action": {"type": "search", "query": "2006 Best Picture"},
+            },
+            {
+                "type": "web_search_call",
+                "id": "ws_2",
+                "status": "completed",
+                "action": {"type": "search", "query": "Crash 2004 film director"},
+            },
+        ]
         messages = _transform_input(
             [
                 {"role": "user", "content": "Who directed the 2006 Best Picture winner?"},
                 {"type": "reasoning", "id": "rs_1", "summary": [], "encrypted_content": json.dumps(blocks)},
-                {
-                    "type": "message",
-                    "id": "msg_1",
-                    "role": "assistant",
-                    "status": "completed",
-                    "content": [{"type": "output_text", "text": "Paul Haggis.", "annotations": []}],
-                },
-                {
-                    "type": "web_search_call",
-                    "id": "ws_1",
-                    "status": "completed",
-                    "action": {"type": "search", "query": "2006 Best Picture"},
-                },
-                {
-                    "type": "web_search_call",
-                    "id": "ws_2",
-                    "status": "completed",
-                    "action": {"type": "search", "query": "Crash 2004 film director"},
-                },
+                *([answer, *searches] if answer_first else [*searches, answer]),
                 {"role": "user", "content": "Summarize that in one sentence."},
             ]
         )
         assistants = [message for message in messages if message["role"] == "assistant"]
-        assert len(assistants) == 1
-        assert assistants[0]["thinking_blocks"] == [blocks[-1]]
-        assert "Paul Haggis." in json.dumps(assistants[0]["content"])
+        assert list(chain.from_iterable(message.get("thinking_blocks") or [] for message in assistants)) == [blocks[-1]]
+        assert "Paul Haggis." in json.dumps([message["content"] for message in assistants])
         assert messages[-1]["role"] == "user"
 
     def test_reasoning_before_a_user_turn_replays_one_block_in_one_message(self):