diff --git a/litellm/completion_extras/litellm_responses_transformation/handler.py b/litellm/completion_extras/litellm_responses_transformation/handler.py
index 2de7bda6467..87c26b776e8 100644
--- a/litellm/completion_extras/litellm_responses_transformation/handler.py
+++ b/litellm/completion_extras/litellm_responses_transformation/handler.py
@@ -182,6 +182,14 @@ def completion(self, *args, **kwargs) -> Union[
             client=kwargs.get("client"),
         )
 
+        # Pin the resolved provider so `responses()` doesn't re-run
+        # `get_llm_provider()` on the model string and strip a second
+        # provider prefix (see GitHub issue #28505).  request_data already
+        # carries `custom_llm_provider` via the spread of
+        # `sanitized_litellm_params`; overwriting it on the dict (rather
+        # than adding an explicit kwarg) avoids the duplicate-keyword
+        # TypeError that would otherwise fire on the real bridge path.
+        request_data["custom_llm_provider"] = custom_llm_provider
         result = responses(
             **request_data,
         )
@@ -268,6 +276,13 @@ async def acompletion(
         except Exception as e:
             raise e
 
+        # Pin the resolved provider so `aresponses()` doesn't re-run
+        # `get_llm_provider()` on the model string and strip a second
+        # provider prefix (see GitHub issue #28505).  Set on request_data
+        # rather than passed as a separate kwarg to avoid the duplicate-
+        # keyword TypeError when `sanitized_litellm_params` already
+        # carries `custom_llm_provider`.
+        request_data["custom_llm_provider"] = custom_llm_provider
         result = await aresponses(
             **request_data,
             aresponses=True,
diff --git a/tests/test_litellm/completion_extras/__init__.py b/tests/test_litellm/completion_extras/__init__.py
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/tests/test_litellm/completion_extras/test_responses_bridge_provider_propagation.py b/tests/test_litellm/completion_extras/test_responses_bridge_provider_propagation.py
new file mode 100644
index 00000000000..b41dbd54b85
--- /dev/null
+++ b/tests/test_litellm/completion_extras/test_responses_bridge_provider_propagation.py
@@ -0,0 +1,116 @@
+"""
+Regression test for https://github.com/BerriAI/litellm/issues/28505 -
+the Responses API bridge double-strips the provider prefix from the
+model name when a Chat Completions request has both `tools` and
+`reasoning_effort`.
+
+Root cause: the bridge handler called `litellm.responses()` /
+`litellm.aresponses()` without passing the already-resolved
+`custom_llm_provider`. The downstream call then re-invoked
+`get_llm_provider()` with `custom_llm_provider=None`, which stripped
+a second provider prefix from a `provider/provider/model` deployment
+string.
+
+This test pins both the sync and async bridge handler call sites:
+the resolved `custom_llm_provider` must be forwarded to the underlying
+`responses` / `aresponses` call so the provider isn't re-detected.
+"""
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from litellm.completion_extras.litellm_responses_transformation.handler import (
+    ResponsesToCompletionBridgeHandler,
+)
+
+
+def _validated_kwargs():
+    return {
+        "model": "openai/openai/openai/gpt-5.5",
+        "messages": [{"role": "user", "content": "hi"}],
+        "optional_params": {},
+        "litellm_params": {},
+        "headers": {},
+        "model_response": MagicMock(),
+        "logging_obj": MagicMock(),
+        "custom_llm_provider": "openai",
+    }
+
+
+def test_sync_completion_forwards_custom_llm_provider():
+    handler = ResponsesToCompletionBridgeHandler()
+    handler.transformation_handler = MagicMock()
+    handler.transformation_handler.transform_request.return_value = {
+        "model": "openai/openai/openai/gpt-5.5",
+        "input": [],
+        # `_build_sanitized_litellm_params` spreads `custom_llm_provider` from
+        # `litellm_params` into request_data on the real bridge path.  Seed
+        # it here so the test exercises the overwrite (not an explicit kwarg
+        # that would TypeError against an already-present key).
+        "custom_llm_provider": "should-be-overwritten",
+    }
+    handler.transformation_handler.transform_response.return_value = (
+        _validated_kwargs()["model_response"]
+    )
+    with (
+        patch.object(
+            handler, "validate_input_kwargs", return_value=_validated_kwargs()
+        ),
+        patch(
+            "litellm.responses",
+            return_value=MagicMock(spec=[]),
+        ) as mock_responses,
+    ):
+        # The handler routes ResponsesAPIResponse through transform_response.
+        # We just want to verify the kwargs going INTO responses().
+        try:
+            handler.completion(acompletion=False)
+        except Exception:
+            # Downstream handling (transform_response, type checks) is not
+            # the subject of this test.
+            pass
+        assert mock_responses.called
+        kwargs = mock_responses.call_args.kwargs
+        assert kwargs.get("custom_llm_provider") == "openai", (
+            "sync bridge must forward custom_llm_provider to litellm.responses() "
+            "so the downstream get_llm_provider() call does not re-strip the "
+            "provider prefix on a provider/provider/model deployment string"
+        )
+
+
+@pytest.mark.asyncio
+async def test_async_completion_forwards_custom_llm_provider():
+    handler = ResponsesToCompletionBridgeHandler()
+    handler.transformation_handler = MagicMock()
+    handler.transformation_handler.transform_request.return_value = {
+        "model": "openai/openai/openai/gpt-5.5",
+        "input": [],
+        # `_build_sanitized_litellm_params` spreads `custom_llm_provider` from
+        # `litellm_params` into request_data on the real bridge path.  Seed
+        # it here so the test exercises the overwrite (not an explicit kwarg
+        # that would TypeError against an already-present key).
+        "custom_llm_provider": "should-be-overwritten",
+    }
+
+    async def _fake_aresponses(**kwargs):
+        _fake_aresponses.kwargs = kwargs
+        return MagicMock(spec=[])
+
+    _fake_aresponses.kwargs = {}
+
+    with (
+        patch.object(
+            handler, "validate_input_kwargs", return_value=_validated_kwargs()
+        ),
+        patch("litellm.aresponses", _fake_aresponses),
+    ):
+        try:
+            await handler.acompletion()
+        except Exception:
+            pass
+        assert _fake_aresponses.kwargs.get("custom_llm_provider") == "openai", (
+            "async bridge must forward custom_llm_provider to litellm.aresponses() "
+            "so the downstream get_llm_provider() call does not re-strip the "
+            "provider prefix on a provider/provider/model deployment string"
+        )