fix(inference): enable routing of models with provider_data alone (#3928)

This PR enables routing of fully qualified model IDs of the form
`provider_id/model_id` even when the models are not registered with the
Stack.

Here's the situation: assume a remote inference provider which works
only when users provide their own API keys via
`X-LlamaStack-Provider-Data` header. By definition, we cannot list
models and hence update our routing registry. But because we _require_ a
provider ID in the models now, we can identify which provider to route
to and let that provider decide.

Note that we still try to look up our registry since it may have a
pre-registered alias. Just that we don't outright fail when we are not
able to look it up.

Also, updated inference router so that the responses have the _exact_
model that the request had.

## Test Plan

Added an integration test

Closes #3929

---------

Co-authored-by: ehhuang <ehhuang@users.noreply.github.com>

tests/integration/inference/test_openai_embeddings.py

@@ -161,8 +161,7 @@ def test_openai_embeddings_single_string(compat_client, client_with_models, embe
 
     assert response.object == "list"
 
-    # Handle provider-scoped model identifiers (e.g., sentence-transformers/nomic-ai/nomic-embed-text-v1.5)
-    assert response.model == embedding_model_id or response.model.endswith(f"/{embedding_model_id}")
+    assert response.model == embedding_model_id
     assert len(response.data) == 1
     assert response.data[0].object == "embedding"
     assert response.data[0].index == 0
@@ -186,8 +185,7 @@ def test_openai_embeddings_multiple_strings(compat_client, client_with_models, e
 
     assert response.object == "list"
 
-    # Handle provider-scoped model identifiers (e.g., sentence-transformers/nomic-ai/nomic-embed-text-v1.5)
-    assert response.model == embedding_model_id or response.model.endswith(f"/{embedding_model_id}")
+    assert response.model == embedding_model_id
     assert len(response.data) == len(input_texts)
 
     for i, embedding_data in enumerate(response.data):
@@ -365,8 +363,7 @@ def test_openai_embeddings_base64_batch_processing(compat_client, client_with_mo
     # Validate response structure
     assert response.object == "list"
 
-    # Handle provider-scoped model identifiers (e.g., sentence-transformers/nomic-ai/nomic-embed-text-v1.5)
-    assert response.model == embedding_model_id or response.model.endswith(f"/{embedding_model_id}")
+    assert response.model == embedding_model_id
     assert len(response.data) == len(input_texts)
 
     # Validate each embedding in the batch

tests/integration/inference/test_provider_data_routing.py

@@ -0,0 +1,133 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+"""
+Test that models can be routed using provider_id/model_id format
+when the provider is configured but the specific model is not registered.
+
+This test validates the fix in src/llama_stack/core/routers/inference.py
+that enables routing based on provider_data alone.
+"""
+
+from unittest.mock import AsyncMock, patch
+
+import pytest
+
+from llama_stack import LlamaStackAsLibraryClient
+from llama_stack.apis.datatypes import Api
+from llama_stack.apis.inference.inference import (
+    OpenAIAssistantMessageParam,
+    OpenAIChatCompletion,
+    OpenAIChatCompletionUsage,
+    OpenAIChoice,
+)
+from llama_stack.core.telemetry.telemetry import MetricEvent
+
+
+class OpenAIChatCompletionWithMetrics(OpenAIChatCompletion):
+    metrics: list[MetricEvent] | None = None
+
+
+def test_unregistered_model_routing_with_provider_data(client_with_models):
+    """
+    Test that a model can be routed using provider_id/model_id format
+    even when the model is not explicitly registered, as long as the provider
+    is available.
+
+    This validates the fix where the router:
+    1. Tries to lookup model in routing table
+    2. If not found, splits model_id by "/" to extract provider_id and provider_resource_id
+    3. Routes directly to the provider with the provider_resource_id
+
+    Without the fix, this would raise ModelNotFoundError immediately.
+    With the fix, the routing succeeds and the request reaches the provider.
+    """
+    if not isinstance(client_with_models, LlamaStackAsLibraryClient):
+        pytest.skip("Test requires library client for provider-level patching")
+
+    client = client_with_models
+
+    # Use a model format that follows provider_id/model_id convention
+    # We'll use anthropic as an example since it's a remote provider that
+    # benefits from this pattern
+    test_model_id = "anthropic/claude-3-5-sonnet-20241022"
+
+    # First, verify the model is NOT registered
+    registered_models = {m.identifier for m in client.models.list()}
+    assert test_model_id not in registered_models, f"Model {test_model_id} should not be pre-registered for this test"
+
+    # Check if anthropic provider is available in ci-tests
+    providers = {p.provider_id: p for p in client.providers.list()}
+    if "anthropic" not in providers:
+        pytest.skip("Anthropic provider not configured in ci-tests - cannot test unregistered model routing")
+
+    # Get the actual provider implementation from the library client's stack
+    inference_router = client.async_client.impls.get(Api.inference)
+    if not inference_router:
+        raise RuntimeError("No inference router found")
+
+    # The inference router's routing_table.impls_by_provider_id should have anthropic
+    # Let's patch the anthropic provider's openai_chat_completion method
+    # to avoid making real API calls
+    mock_response = OpenAIChatCompletionWithMetrics(
+        id="chatcmpl-test-123",
+        created=1234567890,
+        model="claude-3-5-sonnet-20241022",
+        choices=[
+            OpenAIChoice(
+                index=0,
+                finish_reason="stop",
+                message=OpenAIAssistantMessageParam(
+                    content="Mocked response to test routing",
+                ),
+            )
+        ],
+        usage=OpenAIChatCompletionUsage(
+            prompt_tokens=5,
+            completion_tokens=10,
+            total_tokens=15,
+        ),
+    )
+
+    # Get the routing table from the inference router
+    routing_table = inference_router.routing_table
+
+    # Patch the anthropic provider's openai_chat_completion method
+    anthropic_provider = routing_table.impls_by_provider_id.get("anthropic")
+    if not anthropic_provider:
+        raise RuntimeError("Anthropic provider not found in routing table even though it's in providers list")
+
+    with patch.object(
+        anthropic_provider,
+        "openai_chat_completion",
+        new_callable=AsyncMock,
+        return_value=mock_response,
+    ) as mock_method:
+        # Make the request with the unregistered model
+        response = client.chat.completions.create(
+            model=test_model_id,
+            messages=[
+                {
+                    "role": "user",
+                    "content": "Test message for unregistered model routing",
+                }
+            ],
+            stream=False,
+        )
+
+        # Verify the provider's method was called
+        assert mock_method.called, "Provider's openai_chat_completion should have been called"
+
+        # Verify the response came through
+        assert response.choices[0].message.content == "Mocked response to test routing"
+
+        # Verify that the router passed the correct model to the provider
+        # (without the "anthropic/" prefix)
+        call_args = mock_method.call_args
+        params = call_args[0][0]  # First positional argument is the params object
+        assert params.model == "claude-3-5-sonnet-20241022", (
+            f"Provider should receive model without provider prefix, got {params.model}"
+        )