diff --git a/core/src/util/supported_functions.rs b/core/src/util/supported_functions.rs
index c6338cd6..cec63873 100644
--- a/core/src/util/supported_functions.rs
+++ b/core/src/util/supported_functions.rs
@@ -28,20 +28,24 @@ pub fn contains_unsupported_functions(
     plan: &LogicalPlan,
     sup: &FunctionSupport,
 ) -> Result<bool, DataFusionError> {
-    plan.exists(|plan| {
-        Ok(plan.expressions().into_iter().any(|expr| {
-            let mut found_unsupported = false;
-            let _ = expr.apply(|expr| {
+    let mut found_unsupported = false;
+    plan.apply_with_subqueries(|plan| {
+        for expr in plan.expressions() {
+            expr.apply(|expr| {
                 if sup.supports(expr) {
                     Ok(TreeNodeRecursion::Continue)
                 } else {
                     found_unsupported = true;
                     Ok(TreeNodeRecursion::Stop)
                 }
-            });
-            found_unsupported
-        }))
-    })
+            })?;
+            if found_unsupported {
+                return Ok(TreeNodeRecursion::Stop);
+            }
+        }
+        Ok(TreeNodeRecursion::Continue)
+    })?;
+    Ok(found_unsupported)
 }
 
 #[derive(Clone, Debug)]
@@ -163,3 +167,193 @@ impl FunctionRestriction {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use datafusion::arrow::datatypes::{DataType, Field, Schema};
+    use datafusion::logical_expr::builder::LogicalTableSource;
+    use datafusion::logical_expr::expr::ScalarFunction;
+    use datafusion::logical_expr::{create_udf, ColumnarValue, LogicalPlanBuilder, Subquery};
+    use datafusion::prelude::col;
+    use std::sync::Arc;
+
+    fn stub_udf(name: &str) -> Arc<ScalarUDF> {
+        Arc::new(create_udf(
+            name,
+            vec![DataType::Utf8],
+            DataType::Utf8,
+            datafusion::logical_expr::Volatility::Immutable,
+            Arc::new(|args: &[ColumnarValue]| Ok(args[0].clone())),
+        ))
+    }
+
+    fn deny_support(names: &[&str]) -> FunctionSupport {
+        FunctionSupport::new(
+            Some(FunctionRestriction::Deny(
+                names.iter().map(|s| s.to_string()).collect(),
+            )),
+            None,
+            None,
+        )
+    }
+
+    fn scan_plan(table: &str) -> LogicalPlan {
+        let schema = Arc::new(Schema::new(vec![
+            Field::new("id", DataType::Int32, false),
+            Field::new("val", DataType::Utf8, true),
+        ]));
+        let source = Arc::new(LogicalTableSource::new(schema))
+            as Arc<dyn datafusion::logical_expr::TableSource>;
+        LogicalPlanBuilder::scan(table, source, None)
+            .expect("scan")
+            .build()
+            .expect("build")
+    }
+
+    #[test]
+    fn detects_denied_function_in_top_level_projection() {
+        let udf = stub_udf("denied_fn");
+        let plan = LogicalPlanBuilder::from(scan_plan("t"))
+            .project(vec![Expr::ScalarFunction(ScalarFunction::new_udf(
+                udf,
+                vec![col("val")],
+            ))])
+            .expect("project")
+            .build()
+            .expect("build");
+
+        let sup = deny_support(&["denied_fn"]);
+        assert!(
+            contains_unsupported_functions(&plan, &sup).expect("check"),
+            "should detect denied function in top-level projection"
+        );
+    }
+
+    #[test]
+    fn allows_plan_without_denied_functions() {
+        let udf = stub_udf("allowed_fn");
+        let plan = LogicalPlanBuilder::from(scan_plan("t"))
+            .project(vec![Expr::ScalarFunction(ScalarFunction::new_udf(
+                udf,
+                vec![col("val")],
+            ))])
+            .expect("project")
+            .build()
+            .expect("build");
+
+        let sup = deny_support(&["denied_fn"]);
+        assert!(
+            !contains_unsupported_functions(&plan, &sup).expect("check"),
+            "should allow plan with only non-denied functions"
+        );
+    }
+
+    #[test]
+    fn detects_denied_function_inside_in_subquery() {
+        let udf = stub_udf("denied_fn");
+
+        // Build subquery: SELECT denied_fn(val) FROM inner_t
+        let subquery_plan = LogicalPlanBuilder::from(scan_plan("inner_t"))
+            .project(vec![Expr::ScalarFunction(ScalarFunction::new_udf(
+                udf,
+                vec![col("val")],
+            ))
+            .alias("result")])
+            .expect("project")
+            .build()
+            .expect("build");
+
+        // Build outer: SELECT id FROM t WHERE id IN (subquery)
+        let outer = LogicalPlanBuilder::from(scan_plan("t"))
+            .filter(Expr::InSubquery(
+                datafusion::logical_expr::expr::InSubquery::new(
+                    Box::new(col("id")),
+                    Subquery {
+                        subquery: Arc::new(subquery_plan),
+                        outer_ref_columns: vec![],
+                        spans: Default::default(),
+                    },
+                    false,
+                ),
+            ))
+            .expect("filter")
+            .build()
+            .expect("build");
+
+        let sup = deny_support(&["denied_fn"]);
+        assert!(
+            contains_unsupported_functions(&outer, &sup).expect("check"),
+            "should detect denied function inside IN subquery"
+        );
+    }
+
+    #[test]
+    fn detects_denied_function_inside_scalar_subquery() {
+        let udf = stub_udf("denied_fn");
+
+        // Build scalar subquery: SELECT denied_fn(val) FROM inner_t
+        let subquery_plan = LogicalPlanBuilder::from(scan_plan("inner_t"))
+            .project(vec![Expr::ScalarFunction(ScalarFunction::new_udf(
+                udf,
+                vec![col("val")],
+            ))
+            .alias("result")])
+            .expect("project")
+            .build()
+            .expect("build");
+
+        // Build outer: SELECT id FROM t WHERE id = (scalar subquery)
+        let outer = LogicalPlanBuilder::from(scan_plan("t"))
+            .filter(col("id").eq(Expr::ScalarSubquery(Subquery {
+                subquery: Arc::new(subquery_plan),
+                outer_ref_columns: vec![],
+                spans: Default::default(),
+            })))
+            .expect("filter")
+            .build()
+            .expect("build");
+
+        let sup = deny_support(&["denied_fn"]);
+        assert!(
+            contains_unsupported_functions(&outer, &sup).expect("check"),
+            "should detect denied function inside scalar subquery"
+        );
+    }
+
+    #[test]
+    fn detects_denied_function_inside_exists_subquery() {
+        let udf = stub_udf("denied_fn");
+
+        // Build subquery: SELECT denied_fn(val) FROM inner_t
+        let subquery_plan = LogicalPlanBuilder::from(scan_plan("inner_t"))
+            .project(vec![Expr::ScalarFunction(ScalarFunction::new_udf(
+                udf,
+                vec![col("val")],
+            ))
+            .alias("result")])
+            .expect("project")
+            .build()
+            .expect("build");
+
+        // Build outer: SELECT id FROM t WHERE EXISTS (subquery)
+        let outer = LogicalPlanBuilder::from(scan_plan("t"))
+            .filter(Expr::Exists(datafusion::logical_expr::expr::Exists::new(
+                Subquery {
+                    subquery: Arc::new(subquery_plan),
+                    outer_ref_columns: vec![],
+                    spans: Default::default(),
+                },
+                false,
+            )))
+            .expect("filter")
+            .build()
+            .expect("build");
+
+        let sup = deny_support(&["denied_fn"]);
+        assert!(
+            contains_unsupported_functions(&outer, &sup).expect("check"),
+            "should detect denied function inside EXISTS subquery"
+        );
+    }
+}