fix(anthropic): keep the request out of deliberate translation 400s on Claude routes

A BadRequestError raised by the shared message translation already names its problem, so it no longer carries every message as Received Messages. A 700 KB text attachment with an unsupported charset otherwise answered a 700 KB 400, and redacting that text held the event loop for about 40 s through the proxy.

litellm/llms/anthropic/chat/transformation.py

@@ -2384,6 +2384,8 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
                 llm_provider=self._resolved_provider,
                 drop_params=drop_params,
             )
+        except litellm.BadRequestError as e:
+            raise AnthropicError(status_code=400, message=str(e))
         except Exception as e:
             raise AnthropicError(
                 status_code=400,

tests/unit/llms/anthropic/chat/test_anthropic_chat_transformation.py

@@ -1,3 +1,4 @@
+import base64
 import copy
 import json
 import logging
@@ -10103,3 +10104,27 @@ def test_claude_cache_control_cap_does_not_count_a_message_level_hint_that_list_
 
     assert sorted(_cached_texts(body)) == ["system prompt", "turn 1", "turn 2", "turn 3"]
     assert json.dumps(body).count('"cache_control"') == 4
+
+
+def test_completion_rejecting_a_text_attachment_does_not_echo_the_request_back(respx_mock: respx.MockRouter):
+    route: Final = respx_mock.post("https://api.anthropic.com/v1/messages").mock(return_value=httpx.Response(500))
+    attachment_data: Final = base64.b64encode(b"attachment body that stays out of the error " * 64).decode()
+
+    with pytest.raises(litellm.BadRequestError) as exc_info:
+        litellm.completion(
+            model="anthropic/claude-sonnet-4-5-20250929",
+            api_key="mock_api_key",
+            messages=[
+                {
+                    "role": "user",
+                    "content": [
+                        {"type": "text", "text": "Summarize this file"},
+                        {"type": "file", "file": {"file_data": f"data:text/plain;charset=punycode;base64,{attachment_data}"}},
+                    ],
+                }
+            ],
+        )
+
+    assert "unsupported encoding: punycode" in str(exc_info.value)
+    assert attachment_data not in str(exc_info.value)
+    assert route.call_count == 0