feat: Add support for query rewrite in vector_store.search (#4171)

# What does this PR do?

Actualize query rewrite in search API, add
`default_query_expansion_model` and `query_expansion_prompt` in
`VectorStoresConfig`.

Makes `rewrite_query` parameter functional in vector store search.
  - `rewrite_query=false` (default): Use original query
- `rewrite_query=true`: Expand query via LLM, or fail gracefully if no
LLM available

Adds 4 parameters to`VectorStoresConfig`:
- `default_query_expansion_model`: LLM model for query expansion
(optional)
- `query_expansion_prompt`: Custom prompt template (optional, uses
built-in default)
- `query_expansion_max_tokens`: Configurable token limit (default: 100)
- `query_expansion_temperature`: Configurable temperature (default: 0.3)

Enabled `run.yaml`:
```yaml
  vector_stores:
    rewrite_query_params:
      model:
        provider_id: "ollama"
        model_id: "llama3.2:3b-instruct-fp16"
      # prompt defaults to built-in
      # max_tokens defaults to 100
      # temperature defaults to 0.3
```

  Fully customized `run.yaml`:
```yaml
  vector_stores:
    default_provider_id: faiss
    default_embedding_model:
      provider_id: sentence-transformers
      model_id: nomic-ai/nomic-embed-text-v1.5
    rewrite_query_params:
      model:
        provider_id: ollama
        model_id: llama3.2:3b-instruct-fp16
      prompt: "Rewrite this search query to improve retrieval results by expanding it with relevant synonyms and related terms: {query}"
      max_tokens: 100
      temperature: 0.3
```

## Test Plan
Added test and recording

Example script as well:

```python
import asyncio
from llama_stack_client import LlamaStackClient
from io import BytesIO

def gen_file(client, text: str=""):
    file_buffer = BytesIO(text.encode('utf-8'))
    file_buffer.name = "my_file.txt"

    uploaded_file = client.files.create(
        file=file_buffer,
        purpose="assistants"
    )
    return uploaded_file

async def test_query_rewriting():
    client = LlamaStackClient(base_url="http://0.0.0.0:8321/")
    uploaded_file = gen_file(client, "banana banana apple")
    uploaded_file2 = gen_file(client, "orange orange kiwi")

    vs = client.vector_stores.create()
    xf_vs = client.vector_stores.files.create(vector_store_id=vs.id, file_id=uploaded_file.id)
    xf_vs1 = client.vector_stores.files.create(vector_store_id=vs.id, file_id=uploaded_file2.id)
    response1 = client.vector_stores.search(
                vector_store_id=vs.id,
                query="apple",
                max_num_results=3,
                rewrite_query=False
            )
    response2 = client.vector_stores.search(
                vector_store_id=vs.id,
                query="kiwi",
                max_num_results=3,
                rewrite_query=True,
            )

    print(f"\n🔵 Response 1 (rewrite_query=False):\n\033[94m{response1}\033[0m")
    print(f"\n🟢 Response 2 (rewrite_query=True):\n\033[92m{response2}\033[0m")

    for f in [uploaded_file.id, uploaded_file2.id]:
        client.files.delete(file_id=f)
    client.vector_stores.delete(vector_store_id=vs.id)

if __name__ == "__main__":
    asyncio.run(test_query_rewriting())
```

And see the screen shot of the server logs showing it worked. 
<img width="1111" height="826" alt="Screenshot 2025-11-19 at 1 16 03 PM"
src="https://github.com/user-attachments/assets/2d188b44-1fef-4df5-b465-2d6728ca49ce"
/>

Notice the log:
```bash
 Query rewritten:
         'kiwi' → 'kiwi, a small brown or green fruit native to New Zealand, or a person having a fuzzy brown outer skin similar in appearance.'
```
So `kiwi` was expanded.

---------

Signed-off-by: Francisco Javier Arceo <farceo@redhat.com>
Co-authored-by: Matthew Farrellee <matt@cs.wisc.edu>

tests/unit/core/routers/test_vector_io.py

@@ -153,3 +153,156 @@ async def test_create_vector_store_with_wrong_model_type_raises_error():
 
     with pytest.raises(ModelTypeError, match="Model 'text-model' is of type"):
         await router.openai_create_vector_store(request)
+
+
+async def test_query_rewrite_functionality():
+    """Test query rewriting at the router level."""
+    from unittest.mock import MagicMock
+
+    from llama_stack.core.datatypes import QualifiedModel, RewriteQueryParams, VectorStoresConfig
+    from llama_stack.providers.utils.memory.constants import DEFAULT_QUERY_REWRITE_PROMPT
+    from llama_stack_api import VectorStoreSearchResponsePage
+
+    mock_routing_table = Mock()
+
+    # Mock provider that returns search results
+    mock_provider = Mock()
+    mock_search_response = VectorStoreSearchResponsePage(search_query=["rewritten test query"], data=[], has_more=False)
+    mock_provider.openai_search_vector_store = AsyncMock(return_value=mock_search_response)
+    mock_routing_table.get_provider_impl = AsyncMock(return_value=mock_provider)
+
+    # Mock inference API for query rewriting
+    mock_inference_api = Mock()
+    mock_inference_api.openai_chat_completion = AsyncMock(
+        return_value=MagicMock(choices=[MagicMock(message=MagicMock(content="rewritten test query"))])
+    )
+
+    # Create config with rewrite params
+    vector_stores_config = VectorStoresConfig(
+        rewrite_query_params=RewriteQueryParams(
+            model=QualifiedModel(provider_id="test", model_id="llama"),
+            max_tokens=100,
+            temperature=0.3,
+        )
+    )
+
+    router = VectorIORouter(mock_routing_table, vector_stores_config, mock_inference_api)
+
+    # Test query rewrite with rewrite_query=True
+    result = await router.openai_search_vector_store(
+        vector_store_id="vs_123",
+        query="test query",
+        rewrite_query=True,
+        max_num_results=5,
+    )
+
+    # Verify chat completion was called for query rewriting
+    assert mock_inference_api.openai_chat_completion.called
+    chat_call_args = mock_inference_api.openai_chat_completion.call_args[0][0]
+    assert chat_call_args.model == "test/llama"
+
+    # Verify default prompt is used
+    prompt_text = chat_call_args.messages[0].content
+    expected_prompt = DEFAULT_QUERY_REWRITE_PROMPT.format(query="test query")
+    assert prompt_text == expected_prompt
+
+    # Verify provider was called with rewritten query and rewrite_query=False
+    mock_provider.openai_search_vector_store.assert_called_once()
+    call_kwargs = mock_provider.openai_search_vector_store.call_args.kwargs
+    assert call_kwargs["query"] == "rewritten test query"
+    assert call_kwargs["rewrite_query"] is False  # Should be False since router handled it
+
+    assert result is not None
+
+
+async def test_query_rewrite_error_when_not_configured():
+    """Test that query rewriting fails with proper error when not configured."""
+    mock_routing_table = Mock()
+    mock_provider = Mock()
+    mock_routing_table.get_provider_impl = AsyncMock(return_value=mock_provider)
+
+    # No config or inference API
+    router = VectorIORouter(mock_routing_table)
+
+    with pytest.raises(ValueError, match="Query rewriting is not available"):
+        await router.openai_search_vector_store(
+            vector_store_id="vs_123",
+            query="test query",
+            rewrite_query=True,
+            max_num_results=5,
+        )
+
+
+async def test_query_rewrite_with_custom_prompt():
+    """Test query rewriting with custom prompt."""
+    from unittest.mock import MagicMock
+
+    from llama_stack.core.datatypes import QualifiedModel, RewriteQueryParams, VectorStoresConfig
+    from llama_stack_api import VectorStoreSearchResponsePage
+
+    mock_routing_table = Mock()
+
+    mock_provider = Mock()
+    mock_search_response = VectorStoreSearchResponsePage(search_query=["custom rewrite"], data=[], has_more=False)
+    mock_provider.openai_search_vector_store = AsyncMock(return_value=mock_search_response)
+    mock_routing_table.get_provider_impl = AsyncMock(return_value=mock_provider)
+
+    mock_inference_api = Mock()
+    mock_inference_api.openai_chat_completion = AsyncMock(
+        return_value=MagicMock(choices=[MagicMock(message=MagicMock(content="custom rewrite"))])
+    )
+
+    vector_stores_config = VectorStoresConfig(
+        rewrite_query_params=RewriteQueryParams(
+            model=QualifiedModel(provider_id="test", model_id="llama"),
+            prompt="Custom prompt: {query}",
+            max_tokens=150,
+            temperature=0.7,
+        )
+    )
+
+    router = VectorIORouter(mock_routing_table, vector_stores_config, mock_inference_api)
+
+    await router.openai_search_vector_store(
+        vector_store_id="vs_123",
+        query="test query",
+        rewrite_query=True,
+        max_num_results=5,
+    )
+
+    # Verify custom prompt was used
+    chat_call_args = mock_inference_api.openai_chat_completion.call_args[0][0]
+    assert chat_call_args.messages[0].content == "Custom prompt: test query"
+    assert chat_call_args.max_tokens == 150
+    assert chat_call_args.temperature == 0.7
+
+
+async def test_search_without_rewrite():
+    """Test that search without rewrite_query doesn't call inference API."""
+    from llama_stack_api import VectorStoreSearchResponsePage
+
+    mock_routing_table = Mock()
+
+    mock_provider = Mock()
+    mock_search_response = VectorStoreSearchResponsePage(search_query=["test query"], data=[], has_more=False)
+    mock_provider.openai_search_vector_store = AsyncMock(return_value=mock_search_response)
+    mock_routing_table.get_provider_impl = AsyncMock(return_value=mock_provider)
+
+    mock_inference_api = Mock()
+    mock_inference_api.openai_chat_completion = AsyncMock()
+
+    router = VectorIORouter(mock_routing_table, inference_api=mock_inference_api)
+
+    await router.openai_search_vector_store(
+        vector_store_id="vs_123",
+        query="test query",
+        rewrite_query=False,
+        max_num_results=5,
+    )
+
+    # Verify inference API was NOT called
+    assert not mock_inference_api.openai_chat_completion.called
+
+    # Verify provider was called with original query
+    call_kwargs = mock_provider.openai_search_vector_store.call_args.kwargs
+    assert call_kwargs["query"] == "test query"

tests/unit/core/test_stack_validation.py

@@ -10,7 +10,13 @@ from unittest.mock import AsyncMock
 
 import pytest
 
-from llama_stack.core.datatypes import QualifiedModel, SafetyConfig, StackConfig, VectorStoresConfig
+from llama_stack.core.datatypes import (
+    QualifiedModel,
+    RewriteQueryParams,
+    SafetyConfig,
+    StackConfig,
+    VectorStoresConfig,
+)
 from llama_stack.core.stack import validate_safety_config, validate_vector_stores_config
 from llama_stack.core.storage.datatypes import ServerStoresConfig, StorageConfig
 from llama_stack_api import Api, ListModelsResponse, ListShieldsResponse, Model, ModelType, Shield
@@ -82,6 +88,17 @@ class TestVectorStoresValidation:
 
         await validate_vector_stores_config(run_config.vector_stores, {Api.models: mock_models})
 
+    async def test_validate_rewrite_query_prompt_missing_placeholder(self):
+        """Test validation fails when prompt template is missing {query} placeholder."""
+        config = VectorStoresConfig(
+            rewrite_query_params=RewriteQueryParams(
+                prompt="This prompt has no placeholder",
+            ),
+        )
+
+        with pytest.raises(ValueError, match="'\\{query\\}' placeholder is required"):
+            await validate_vector_stores_config(config, {})
+
 
 class TestSafetyConfigValidation:
     async def test_validate_success(self):