fix(websearch): prevent internal _websearch_interception flags from leaking to provider API

The websearch interception deployment hook sets
kwargs["_websearch_interception_converted_stream"] = True to track stream
conversion. This internal flag was leaking into extra_body via
add_provider_specific_params_to_optional_params, causing OpenAI to reject
requests with "Unknown parameter: '_websearch_interception_converted_stream'".

Skip underscore-prefixed keys when building extra_body, since they are
internal flags and should never be forwarded to providers.

Fixes #28458

litellm/utils.py

@@ -4839,7 +4839,11 @@ def add_provider_specific_params_to_optional_params(
         ):
             extra_body = passed_params.pop("extra_body", None) or {}
             for k in passed_params.keys():
-                if k not in openai_params and passed_params[k] is not None:
+                if (
+                    k not in openai_params
+                    and passed_params[k] is not None
+                    and not k.startswith("_")
+                ):
                     extra_body[k] = passed_params[k]
             if not isinstance(optional_params.get("extra_body"), dict):
                 optional_params["extra_body"] = {}

tests/test_litellm/test_websearch_interception_stream_param.py

@@ -0,0 +1,123 @@
+"""
+Unit tests for fix #28458: Internal _websearch_interception_converted_stream flag
+must NOT leak into extra_body when building provider-specific params.
+
+The websearch interception deployment hook sets
+kwargs["_websearch_interception_converted_stream"] = True to track that stream
+was converted from True to False. This internal flag was leaking through
+add_provider_specific_params_to_optional_params into extra_body, causing
+OpenAI to reject the request with "Unknown parameter:
+'_websearch_interception_converted_stream'".
+
+The fix filters out any key starting with '_' from being added to extra_body,
+since underscore-prefixed keys are internal flags, not provider params.
+"""
+
+import pytest
+
+from litellm.utils import add_provider_specific_params_to_optional_params
+
+
+class TestInternalFlagsNotInExtraBody:
+    """Verify internal _-prefixed flags are excluded from extra_body."""
+
+    def test_websearch_flag_excluded_from_extra_body(self):
+        """_websearch_interception_converted_stream must not appear in extra_body."""
+        passed_params = {
+            "temperature": 0.7,
+            "_websearch_interception_converted_stream": True,
+        }
+        optional_params = {}
+        openai_params = ["temperature", "model", "messages"]
+
+        result = add_provider_specific_params_to_optional_params(
+            optional_params=optional_params,
+            passed_params=passed_params,
+            custom_llm_provider="openai",
+            openai_params=openai_params,
+        )
+
+        extra_body = result.get("extra_body", {})
+        assert "_websearch_interception_converted_stream" not in extra_body
+
+    def test_regular_non_openai_params_still_in_extra_body(self):
+        """Non-underscore, non-OpenAI params should still go to extra_body."""
+        passed_params = {
+            "temperature": 0.7,
+            "custom_provider_param": "value",
+        }
+        optional_params = {}
+        openai_params = ["temperature", "model", "messages"]
+
+        result = add_provider_specific_params_to_optional_params(
+            optional_params=optional_params,
+            passed_params=passed_params,
+            custom_llm_provider="openai",
+            openai_params=openai_params,
+        )
+
+        extra_body = result.get("extra_body", {})
+        assert extra_body.get("custom_provider_param") == "value"
+
+    def test_all_underscore_prefixed_flags_excluded(self):
+        """Any key starting with _ should be excluded from extra_body."""
+        passed_params = {
+            "_websearch_interception_converted_stream": True,
+            "_websearch_interception_other_flag": "test",
+            "_some_other_internal_flag": 42,
+            "valid_param": "keep",
+        }
+        optional_params = {}
+        openai_params = ["model", "messages"]
+
+        result = add_provider_specific_params_to_optional_params(
+            optional_params=optional_params,
+            passed_params=passed_params,
+            custom_llm_provider="openai",
+            openai_params=openai_params,
+        )
+
+        extra_body = result.get("extra_body", {})
+        assert "_websearch_interception_converted_stream" not in extra_body
+        assert "_websearch_interception_other_flag" not in extra_body
+        assert "_some_other_internal_flag" not in extra_body
+        assert extra_body.get("valid_param") == "keep"
+
+    def test_openai_compatible_providers_also_filtered(self):
+        """Internal flags should also be filtered for openai-compatible providers."""
+        passed_params = {
+            "_websearch_interception_converted_stream": True,
+            "custom_param": "value",
+        }
+        optional_params = {}
+        openai_params = ["model", "messages"]
+
+        result = add_provider_specific_params_to_optional_params(
+            optional_params=optional_params,
+            passed_params=passed_params,
+            custom_llm_provider="azure",
+            openai_params=openai_params,
+        )
+
+        extra_body = result.get("extra_body", {})
+        assert "_websearch_interception_converted_stream" not in extra_body
+        assert extra_body.get("custom_param") == "value"
+
+    def test_no_extra_body_when_only_internal_flags(self):
+        """If only internal flags exist (besides openai params), extra_body should be empty."""
+        passed_params = {
+            "temperature": 0.7,
+            "_websearch_interception_converted_stream": True,
+        }
+        optional_params = {}
+        openai_params = ["temperature", "model", "messages"]
+
+        result = add_provider_specific_params_to_optional_params(
+            optional_params=optional_params,
+            passed_params=passed_params,
+            custom_llm_provider="openai",
+            openai_params=openai_params,
+        )
+
+        extra_body = result.get("extra_body", {})
+        assert len(extra_body) == 0