fix: respect drop_params when mapping metadata.user_id to user in Responses adapter

When litellm.drop_params is True, the Anthropic→Responses API adapter
was still unconditionally setting responses_kwargs["user"] from
metadata.user_id, causing 400 errors on strict gateways that reject
the user field.

Guard the assignment behind a drop_params check so the field is omitted
when drop_params is enabled.

Fixes #26241

litellm/llms/anthropic/experimental_pass_through/responses_adapters/transformation.py

@@ -8,6 +8,8 @@ path used for OpenAI and Azure models.
 import json
 from typing import Any, Dict, List, Optional, Union, cast
 
+import litellm
+
 from litellm.llms.anthropic.experimental_pass_through.utils import (
     is_reasoning_auto_summary_enabled,
 )
@@ -384,7 +386,8 @@ class LiteLLMAnthropicToResponsesAPIAdapter:
         # metadata user_id -> user
         metadata = anthropic_request.get("metadata")
         if isinstance(metadata, dict) and "user_id" in metadata:
-            responses_kwargs["user"] = str(metadata["user_id"])[:64]
+            if not litellm.drop_params:
+                responses_kwargs["user"] = str(metadata["user_id"])[:64]
 
         return responses_kwargs
 

tests/test_litellm/llms/anthropic/experimental_pass_through/responses_adapters/test_responses_adapters_transformation.py

@@ -1042,4 +1042,51 @@ class TestTranslateResponse:
         assert "thinking" in types
         assert "text" in types
         assert "tool_use" in types
-        assert result["stop_reason"] == "tool_use"
+
+
+# ---------------------------------------------------------------------------
+# drop_params: metadata.user_id -> user field
+# ---------------------------------------------------------------------------
+
+
+class TestDropParamsMetadataUserId:
+    """litellm.drop_params must suppress the user field mapped from metadata.user_id."""
+
+    def test_user_field_set_when_drop_params_false(self):
+        """user is included when drop_params is False (default)."""
+        import litellm
+
+        req = _make_request(metadata={"user_id": "alice"})
+        original = litellm.drop_params
+        try:
+            litellm.drop_params = False
+            kwargs = _ADAPTER.translate_request(req)
+        finally:
+            litellm.drop_params = original
+        assert kwargs.get("user") == "alice"
+
+    def test_user_field_omitted_when_drop_params_true(self):
+        """user is omitted when drop_params is True (issue #26241)."""
+        import litellm
+
+        req = _make_request(metadata={"user_id": "alice"})
+        original = litellm.drop_params
+        try:
+            litellm.drop_params = True
+            kwargs = _ADAPTER.translate_request(req)
+        finally:
+            litellm.drop_params = original
+        assert "user" not in kwargs
+
+    def test_no_metadata_no_user_field(self):
+        """No metadata means no user field regardless of drop_params."""
+        import litellm
+
+        req = _make_request()
+        original = litellm.drop_params
+        try:
+            litellm.drop_params = False
+            kwargs = _ADAPTER.translate_request(req)
+        finally:
+            litellm.drop_params = original
+        assert "user" not in kwargs