fix(anthropic): tighten thinking-signature retry (Greptile)

- Omit messages whose list content is empty after stripping thinking blocks
- Retry only on HTTP 400 plus invalid-signature body match
- Return response inline from retry loop; drop unreachable None guard
- Tests: thinking-only turn dropped, non-400 no retry

Made-with: Cursor

litellm/llms/anthropic/common_utils.py

@@ -760,6 +760,9 @@ def strip_thinking_blocks_from_anthropic_messages(messages: List[Any]) -> List[A
     """
     Return a new message list with thinking / redacted_thinking content blocks removed
     from each message. Used to recover from invalid thinking signatures on retry.
+
+    Messages whose content is a list and becomes empty after stripping are omitted,
+    since Anthropic rejects empty content arrays.
     """
     out: List[Any] = []
     for m in messages:
@@ -769,7 +772,7 @@ def strip_thinking_blocks_from_anthropic_messages(messages: List[Any]) -> List[A
         mm = copy.deepcopy(m)
         content = mm.get("content")
         if isinstance(content, list):
-            mm["content"] = [
+            filtered = [
                 b
                 for b in content
                 if not (
@@ -777,6 +780,9 @@ def strip_thinking_blocks_from_anthropic_messages(messages: List[Any]) -> List[A
                     and b.get("type") in ("thinking", "redacted_thinking")
                 )
             ]
+            if not filtered:
+                continue
+            mm["content"] = filtered
         out.append(mm)
     return out
 

litellm/llms/base_llm/anthropic_messages/transformation.py

@@ -141,7 +141,9 @@ class BaseAnthropicMessagesConfig(ABC):
             is_anthropic_invalid_thinking_signature_error,
         )
 
-        return is_anthropic_invalid_thinking_signature_error(e.response.text)
+        return e.response.status_code == 400 and is_anthropic_invalid_thinking_signature_error(
+            e.response.text
+        )
 
     def transform_anthropic_messages_request_on_http_error(
         self, e: httpx.HTTPStatusError, request_data: dict
@@ -154,6 +156,8 @@ class BaseAnthropicMessagesConfig(ABC):
             strip_thinking_blocks_from_anthropic_messages_request_dict,
         )
 
-        if is_anthropic_invalid_thinking_signature_error(e.response.text):
+        if e.response.status_code == 400 and is_anthropic_invalid_thinking_signature_error(
+            e.response.text
+        ):
             strip_thinking_blocks_from_anthropic_messages_request_dict(request_data)
         return request_data

litellm/llms/custom_httpx/llm_http_handler.py

@@ -1833,7 +1833,6 @@ class BaseLLMHTTPHandler:
         max_attempts = max(provider_config.max_retry_on_anthropic_messages_http_error, 1)
         litellm_params_dict = dict(litellm_params)
         optional_params_dict = dict(litellm_params)
-        response: Optional[httpx.Response] = None
         for attempt_idx in range(max_attempts):
             try:
                 response = await async_httpx_client.post(
@@ -1844,6 +1843,7 @@ class BaseLLMHTTPHandler:
                     logging_obj=logging_obj,
                 )
                 response.raise_for_status()
+                return response
             except httpx.HTTPStatusError as e:
                 hit_max_attempt = attempt_idx + 1 == max_attempts
                 should_retry = provider_config.should_retry_anthropic_messages_on_http_error(
@@ -1874,14 +1874,10 @@ class BaseLLMHTTPHandler:
                 raise self._handle_error(e=e, provider_config=provider_config)
             except Exception as e:
                 raise self._handle_error(e=e, provider_config=provider_config)
-            break
 
-        if response is None:
-            raise self._handle_error(
-                e=ValueError("No response from Anthropic /v1/messages"),
-                provider_config=provider_config,
-            )
-        return response
+        raise RuntimeError(
+            "unreachable: anthropic messages HTTP retry loop exited without return"
+        )
 
     async def async_anthropic_messages_handler(
         self,

tests/test_litellm/llms/anthropic/test_anthropic_common_utils.py

@@ -1181,6 +1181,24 @@ class TestAnthropicThinkingSignatureSelfHeal:
         assert out[1]["content"][0]["type"] == "text"
         assert messages[1]["content"][0]["type"] == "thinking"
 
+    def test_strip_thinking_blocks_drops_message_when_only_thinking_blocks(self):
+        from litellm.llms.anthropic.common_utils import (
+            strip_thinking_blocks_from_anthropic_messages,
+        )
+
+        messages = [
+            {"role": "user", "content": "hi"},
+            {
+                "role": "assistant",
+                "content": [
+                    {"type": "thinking", "thinking": "plan", "signature": "sig"},
+                ],
+            },
+        ]
+        out = strip_thinking_blocks_from_anthropic_messages(messages)
+        assert len(out) == 1
+        assert out[0]["role"] == "user"
+
     def test_strip_thinking_blocks_from_anthropic_messages_request_dict(self):
         from litellm.llms.anthropic.common_utils import (
             strip_thinking_blocks_from_anthropic_messages_request_dict,
@@ -1204,7 +1222,7 @@ class TestAnthropicThinkingSignatureSelfHeal:
         }
         strip_thinking_blocks_from_anthropic_messages_request_dict(data)
         assert "thinking" not in data
-        assert data["messages"][0]["content"] == []
+        assert data["messages"] == []
 
     def test_anthropic_messages_config_http_retry_helpers(self):
         import httpx
@@ -1230,6 +1248,10 @@ class TestAnthropicThinkingSignatureSelfHeal:
         err_bad = httpx.HTTPStatusError("bad", request=req, response=resp_bad)
         assert config.should_retry_anthropic_messages_on_http_error(err_bad, {}) is False
 
+        resp_500 = httpx.Response(500, request=req, text=err_text)
+        err_500 = httpx.HTTPStatusError("bad", request=req, response=resp_500)
+        assert config.should_retry_anthropic_messages_on_http_error(err_500, {}) is False
+
         data = {
             "model": "claude-sonnet-4-20250514",
             "messages": [
@@ -1248,4 +1270,4 @@ class TestAnthropicThinkingSignatureSelfHeal:
         }
         config.transform_anthropic_messages_request_on_http_error(err, data)
         assert "thinking" not in data
-        assert data["messages"][0]["content"] == []
+        assert data["messages"] == []