fix(bridge): keep the provider's own model prefix on chat-to-responses calls

completion() strips the litellm routing prefix before it dispatches to the
responses bridge, but responses() runs get_llm_provider() again, so a model id
that itself starts with the provider name lost a second prefix and reached the
provider as a name it does not know. Handing responses() the prefixed model
back makes its own resolve a no-op: across the 3061 cost map entries, 76 reach
the responses bridge and only the four perplexity Agent API models change.

litellm/completion_extras/litellm_responses_transformation/handler.py

@@ -25,6 +25,11 @@ class ResponsesToCompletionBridgeHandlerInputKwargs(TypedDict):
     encoding: object
 
 
+def _restore_routing_prefix(model: str, custom_llm_provider: str) -> str:
+    """`responses()` runs `get_llm_provider()` itself, so hand back the prefixed model `completion()` started from."""
+    return f"{custom_llm_provider}/{model}"
+
+
 class ResponsesToCompletionBridgeHandler:
     def __init__(self):
         from .transformation import LiteLLMResponsesTransformationHandler
@@ -184,14 +189,11 @@ class ResponsesToCompletionBridgeHandler:
             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.
+        # Set on request_data rather than passed as explicit kwargs: the spread of
+        # `sanitized_litellm_params` already carries both, so passing them again
+        # would raise a duplicate-keyword TypeError.
         request_data["custom_llm_provider"] = custom_llm_provider
+        request_data["model"] = _restore_routing_prefix(model, custom_llm_provider)
         result: Final = responses(
             **request_data,
         )
@@ -282,13 +284,11 @@ class ResponsesToCompletionBridgeHandler:
         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`.
+        # Set on request_data rather than passed as explicit kwargs: the spread of
+        # `sanitized_litellm_params` already carries both, so passing them again
+        # would raise a duplicate-keyword TypeError.
         request_data["custom_llm_provider"] = custom_llm_provider
+        request_data["model"] = _restore_routing_prefix(model, custom_llm_provider)
         result: Final = await aresponses(
             **request_data,
             aresponses=True,

tests/test_litellm/completion_extras/litellm_responses_transformation/test_completion_extras_litellm_responses_transformation_handler.py

@@ -7,11 +7,13 @@ import pytest
 
 sys.path.insert(0, os.path.abspath("../../.."))
 
+import litellm
 from litellm.completion_extras.litellm_responses_transformation.handler import (
     ResponsesToCompletionBridgeHandler,
 )
 from litellm.litellm_core_utils.litellm_logging import Logging as LiteLLMLogging
 from litellm.litellm_core_utils.streaming_handler import CustomStreamWrapper
+from litellm.types.router import GenericLiteLLMParams
 from litellm.types.utils import ModelResponse
 
 
@@ -265,3 +267,74 @@ def test_completion_streams_completed_model_response():
     assert "".join(chunk.choices[0].delta.content or "" for chunk in chunks) == "pong", (
         f"completed response did not stream its content: {chunks}"
     )
+
+
+_PROVIDER_NATIVE_MODEL_CASES = [
+    ("perplexity", "perplexity/kimi-k3", "perplexity/kimi-k3"),
+    ("perplexity", "openai/gpt-5.2", "openai/gpt-5.2"),
+    ("openai", "gpt-5.4", "gpt-5.4"),
+]
+
+
+def _upstream_model_for(handed_model: str, custom_llm_provider: str) -> str:
+    upstream_model, _, _, _ = litellm.get_llm_provider(
+        model=handed_model,
+        litellm_params=GenericLiteLLMParams(custom_llm_provider=custom_llm_provider),
+    )
+    return upstream_model
+
+
+@pytest.mark.parametrize(
+    "custom_llm_provider, bridge_model, expected_upstream_model",
+    _PROVIDER_NATIVE_MODEL_CASES,
+)
+def test_completion_keeps_provider_native_model_id_through_responses(
+    custom_llm_provider, bridge_model, expected_upstream_model
+):
+    """responses() resolves the provider itself, so the bridge must not hand it an already-stripped model."""
+    cached = ModelResponse(id="chatcmpl-cached", model=bridge_model)
+    bridge = ResponsesToCompletionBridgeHandler()
+    kwargs = _bridge_kwargs(stream=False)
+    kwargs["model"] = bridge_model
+    kwargs["custom_llm_provider"] = custom_llm_provider
+
+    with (
+        patch.object(
+            bridge.transformation_handler,
+            "transform_request",
+            return_value={"model": bridge_model, "input": "hi"},
+        ),
+        patch("litellm.responses", return_value=cached) as responses_call,
+    ):
+        bridge.completion(**kwargs)
+
+    handed_model = responses_call.call_args.kwargs["model"]
+    assert _upstream_model_for(handed_model, custom_llm_provider) == expected_upstream_model
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    "custom_llm_provider, bridge_model, expected_upstream_model",
+    _PROVIDER_NATIVE_MODEL_CASES,
+)
+async def test_acompletion_keeps_provider_native_model_id_through_responses(
+    custom_llm_provider, bridge_model, expected_upstream_model
+):
+    cached = ModelResponse(id="chatcmpl-cached-async", model=bridge_model)
+    bridge = ResponsesToCompletionBridgeHandler()
+    kwargs = _bridge_kwargs(stream=False)
+    kwargs["model"] = bridge_model
+    kwargs["custom_llm_provider"] = custom_llm_provider
+
+    with (
+        patch.object(
+            bridge.transformation_handler,
+            "transform_request",
+            return_value={"model": bridge_model, "input": "hi"},
+        ),
+        patch("litellm.aresponses", new=AsyncMock(return_value=cached)) as responses_call,
+    ):
+        await bridge.acompletion(**kwargs)
+
+    handed_model = responses_call.call_args.kwargs["model"]
+    assert _upstream_model_for(handed_model, custom_llm_provider) == expected_upstream_model