microsoft/semantic-kernel · warning · VectorStoreInitializationException

Distance function {field.distance_function} is not supported

Error message

Distance function {field.distance_function} is not supported for index kind {field.index_kind}.

What it means

A VectorStoreInitializationException raised by the inner match in _create_index() for the FLAT/DEFAULT index_kind branch when the distance_function is supported by the outer map but has no case in the inner match (the inner match only handles EUCLIDEAN_SQUARED_DISTANCE/DEFAULT -> IndexFlatL2 and DOT_PROD -> IndexFlatIP, with a '_' fallback). Because the outer DISTANCE_FUNCTION_MAP and the inner match share the same supported set, this branch is effectively a secondary guard that fires only if the two sets ever diverge.

Source

Thrown at python/semantic_kernel/connectors/faiss.py:59

    IndexKind.DEFAULT: True,
}


def _create_index(field: VectorStoreField) -> faiss.Index:
    """Create a Faiss index."""
    if field.index_kind not in INDEX_KIND_MAP:
        raise VectorStoreInitializationException(f"Index kind {field.index_kind} is not supported.")
    if field.distance_function not in DISTANCE_FUNCTION_MAP:
        raise VectorStoreInitializationException(f"Distance function {field.distance_function} is not supported.")
    match field.index_kind:
        case IndexKind.FLAT | IndexKind.DEFAULT:
            match field.distance_function:
                case DistanceFunction.EUCLIDEAN_SQUARED_DISTANCE | DistanceFunction.DEFAULT:
                    return faiss.IndexFlatL2(field.dimensions)
                case DistanceFunction.DOT_PROD:
                    return faiss.IndexFlatIP(field.dimensions)
                case _:
                    raise VectorStoreInitializationException(
                        f"Distance function {field.distance_function} is "
                        f"not supported for index kind {field.index_kind}."
                    )
        case _:
            raise VectorStoreInitializationException(f"Index with {field.index_kind} is not supported.")


class FaissCollection(InMemoryCollection[TKey, TModel], Generic[TKey, TModel]):
    """Create a Faiss collection.

    The Faiss Collection builds on the InMemoryVectorCollection,
    it maintains indexes and mappings for each vector field.
    """

    indexes: MutableMapping[str, faiss.Index] = Field(default_factory=dict)
    indexes_key_map: MutableMapping[str, MutableMapping[TKey, int]] = Field(default_factory=dict)

    def __init__(

View on GitHub (pinned to c028a0c7dc)

Solutions

Check the library version / file an issue — the inner and outer distance-function maps should stay consistent.
As a workaround, supply a pre-built faiss.Index via the 'index'/'indexes' parameter to bypass _create_index entirely.

Defensive patterns

Strategy: try-catch

Validate before calling

from semantic_kernel.connectors.faiss import DISTANCE_FUNCTION_MAP
SUPPORTED_INNER = {DistanceFunction.EUCLIDEAN_SQUARED_DISTANCE, DistanceFunction.DEFAULT, DistanceFunction.DOT_PROD}
assert set(DISTANCE_FUNCTION_MAP) == SUPPORTED_INNER, "Faiss distance maps are inconsistent; report a library bug"

Try / catch

from semantic_kernel.exceptions import VectorStoreInitializationException
try:
    FaissCollection(record_type=Doc)
except VectorStoreInitializationException as e:
    if "not supported for index kind" in str(e):
        # supply a pre-built faiss.Index to bypass _create_index
        ...

Prevention

Pin a known-good library version.
When in doubt, pass a pre-built faiss.Index instead of relying on auto-creation.

When it happens

Trigger: Reachable only if a distance_function is present in DISTANCE_FUNCTION_MAP (passes the outer check at faiss.py:49-50) but lacks a case in the inner match. With the current maps these sets are identical, so it is defensive; would fire if the maps were edited inconsistently.

Common situations: Not encountered in normal usage. Could surface after a library upgrade that adds a distance function to the outer map but forgets the inner match.

Related errors

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/777145651d0370ac. Report an issue: GitHub.