microsoft/semantic-kernel · error · VectorStoreOperationException

Filter string must be a lambda expression, e.g. 'lambda x: x

Error message

Filter string must be a lambda expression, e.g. 'lambda x: x.key == 1'

What it means

Thrown by _parse_and_validate_filter (in_memory.py:759-761) when the string parses successfully but the top-level node is not an ast.Lambda (e.g. a bare expression like '1+1', a function call, an assignment, or an import attempt). Only lambda expressions are accepted as the filter root.

Source

Thrown at python/semantic_kernel/connectors/in_memory.py:760

    def _parse_and_validate_filter(self, filter_str: str) -> Callable:
        """Parse and validate a string filter as a lambda expression, then return the callable.

        Uses an allowlist approach - only explicitly permitted AST node types and function names
        are allowed. This can be customized by overriding `allowed_filter_ast_nodes` and
        `allowed_filter_functions` class attributes.
        """
        if len(filter_str) > self.max_filter_source_length:
            raise VectorStoreOperationException("Filter string exceeds the maximum allowed length.")

        try:
            tree = ast.parse(filter_str, mode="eval")
        except SyntaxError as e:
            raise VectorStoreOperationException(f"Filter string is not valid Python: {e}") from e

        # Only allow lambda expressions at the top level
        if not (isinstance(tree, ast.Expression) and isinstance(tree.body, ast.Lambda)):
            raise VectorStoreOperationException(
                "Filter string must be a lambda expression, e.g. 'lambda x: x.key == 1'"
            )

        # Get the lambda parameter name(s) to allow them as valid Name nodes
        lambda_node = tree.body
        lambda_param_names = {arg.arg for arg in lambda_node.args.args}
        lambda_param_order = [arg.arg for arg in lambda_node.args.args]
        # Walk the AST to validate all nodes against the allowlist
        for node_count, node in enumerate(ast.walk(tree), start=1):
            if node_count > self.max_filter_ast_node_count:
                raise VectorStoreOperationException("Filter expression exceeds the maximum allowed complexity.")

            node_type = type(node)

            # Check if the node type is allowed
            if node_type not in self.allowed_filter_ast_nodes:
                raise VectorStoreOperationException(
                    f"AST node type '{node_type.__name__}' is not allowed in filter expressions."

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Wrap the expression in a lambda, e.g. 'lambda x: x.id == 1'.
2. Use the lambda parameter (x) to reference the record in attribute or subscript form.
3. For non-lambda filter languages, translate them to a Python lambda or pass a callable.

Example fix

# before
VectorSearchOptions(filter="x.id == 1")          # not a lambda
VectorSearchOptions(filter="id eq 1")             # OData, not python
# after
VectorSearchOptions(filter="lambda x: x.id == 1")

import ast
def is_lambda_expr(expr: str) -> bool:
    tree = ast.parse(expr, mode="eval")
    return isinstance(tree, ast.Expression) and isinstance(tree.body, ast.Lambda)

try:
    results = await collection.search(search_type=SearchType.VECTOR, options=opts)
except VectorStoreOperationException as e:
    logger.warning("filter rejected: %s", e.__cause__ or e)
    results = None

When it happens

Trigger: Passing 'x.id == 1' (no lambda wrapper), a bare expression, a statement, or an attempt like "__import__('os')" whose top level is a Call rather than a Lambda.

Common situations: Forgetting the 'lambda x:' prefix; copying an expression from another store's filter syntax; passing SQL-like or OData strings.

Related errors

• AST node type '{node_type.__name__}' is not allowed in filte
• Call target node type '{type(node.func).__name__}' is not al
• Access to attribute '{node.attr}' is not allowed in filter e
• Access to attribute '{node.attr}' is not allowed in filter e
• Dictionary unpacking is not allowed in filter expressions.

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/6051b4410c7a45e9. Report an issue: GitHub.