microsoft/semantic-kernel · error · VectorStoreOperationException

Filter expression exceeds the maximum allowed complexity.

Error message

Filter expression exceeds the maximum allowed complexity.

What it means

Thrown by _parse_and_validate_filter (in_memory.py:770-771) when the number of AST nodes in the filter exceeds max_filter_ast_node_count (default 128). It guards against denial-of-service via enormous or deeply nested expressions and is checked during the allowlist walk.

Source

Thrown at python/semantic_kernel/connectors/in_memory.py:771

        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."
                )

            # For Attribute nodes, validate that dangerous dunder attributes are not accessed
            if isinstance(node, ast.Attribute) and node.attr in self.blocked_filter_attributes:
                raise VectorStoreOperationException(
                    f"Access to attribute '{node.attr}' is not allowed in filter expressions. "
                    "This attribute could be used to escape the filter sandbox."
                )

            # For Name nodes, only allow the lambda parameter
            if isinstance(node, ast.Name) and node.id not in lambda_param_names:

View on GitHub (pinned to c028a0c7dc)

Solutions

1. Simplify the filter expression.
2. Split one large filter into a list of filters (options.filter accepts a list, OR-semantics).
3. Move large constant data out of the filter and pass it as a callable closure.
4. Raise max_filter_ast_node_count if the complexity is legitimate.

Example fix

# before
opts = VectorSearchOptions(filter="lambda x: " + " or ".join(f"x.id == {i}" for i in range(200)))
# after
ids = set(range(200))
opts = VectorSearchOptions(filter=lambda x: x.id in ids)

import ast
def node_count_within(expr: str, cap: int = 128) -> bool:
    tree = ast.parse(expr, mode="eval")
    return sum(1 for _ in ast.walk(tree)) <= cap

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: A filter with deeply nested boolean logic, very long chained comparisons, or huge literal collections that expand the node count past 128.

Common situations: Auto-generating filters from large rule sets; combining many conditions with and/or; lowering the node cap.

Related errors

• Sequence repetition in filter expressions exceeds the maximu
• Collection literals in filter expressions exceed the maximum
• Sequence operations in filter expressions exceed the maximum
• Filter string exceeds the maximum allowed length.
• Attribute '{node.func.attr}' is not callable in filter expre

AI-assisted analysis of microsoft/semantic-kernel@c028a0c7dc (2026-08-13). Data as JSON: /api/errors/2e3d6393d231a585. Report an issue: GitHub.