From 4eabe4367e5bdef8089fc32da11286be4d4c0f1d Mon Sep 17 00:00:00 2001
From: Gennaro Malafronte <8798987+malafronte@users.noreply.github.com>
Date: Thu, 23 Apr 2026 01:13:49 +0200
Subject: [PATCH] fix(anthropic): drop empty reasoning sentinel

---
 .../adapters/transformation.py                |  4 ++-
 ...al_pass_through_adapters_transformation.py | 28 +++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py b/litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py
index e3c62dc9ea7..5c8e26b8855 100644
--- a/litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py
+++ b/litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py
@@ -664,7 +664,9 @@ class LiteLLMAnthropicMessagesAdapter:
                 if len(tool_calls) > 0:
                     assistant_message["tool_calls"] = tool_calls  # type: ignore
                     assistant_message["reasoning_content"] = (
-                        "".join(reasoning_content_parts) if len(reasoning_content_parts) > 0 else " "
+                            "".join(reasoning_content_parts)
+                            if len(reasoning_content_parts) > 0
+                            else None
                     )  # type: ignore
                 if len(thinking_blocks) > 0:
                     assistant_message["thinking_blocks"] = thinking_blocks  # type: ignore
diff --git a/tests/test_litellm/llms/anthropic/experimental_pass_through/adapters/test_anthropic_experimental_pass_through_adapters_transformation.py b/tests/test_litellm/llms/anthropic/experimental_pass_through/adapters/test_anthropic_experimental_pass_through_adapters_transformation.py
index 8b7e5c3f05b..7556d842f7f 100644
--- a/tests/test_litellm/llms/anthropic/experimental_pass_through/adapters/test_anthropic_experimental_pass_through_adapters_transformation.py
+++ b/tests/test_litellm/llms/anthropic/experimental_pass_through/adapters/test_anthropic_experimental_pass_through_adapters_transformation.py
@@ -1007,6 +1007,34 @@ def test_translate_anthropic_messages_to_openai_tool_use_with_signature():
     )
 
 
+def test_translate_anthropic_messages_to_openai_tool_use_without_thinking_blocks():
+    anthropic_messages = [
+        AnthropicMessagesUserMessageParam(
+            role="user",
+            content=[{"type": "text", "text": "Use the weather tool."}],
+        ),
+        AnthopicMessagesAssistantMessageParam(
+            role="assistant",
+            content=[
+                {
+                    "type": "tool_use",
+                    "id": "toolu_01234",
+                    "name": "get_weather",
+                    "input": {"location": "Boston"},
+                }
+            ],
+        ),
+    ]
+
+    adapter = LiteLLMAnthropicMessagesAdapter()
+    result = adapter.translate_anthropic_messages_to_openai(messages=anthropic_messages)
+
+    assert len(result) == 2
+    assert result[1]["role"] == "assistant"
+    assert len(result[1]["tool_calls"]) == 1
+    assert result[1].get("reasoning_content") is None
+
+
 def test_translate_anthropic_messages_to_openai_tool_result_with_multiple_content_items():
     """
     Test that tool_result with multiple content items creates a single tool message